How to Do a Roth Conversion (and When It Pays)

A Roth conversion is one of the few moves in personal finance where you volunteer to pay a tax bill early. You take money that has been sitting in a traditional IRA or another pre-tax account, hand a slice of it to the government now, and in return the rest is meant to grow and come out tax-free for the rest of your life. Whether that is a smart trade or an expensive mistake comes down to a single comparison, and the uncomfortable truth is that half of that comparison lives in the future.

This walkthrough covers what a conversion is, how you actually execute one, and the arithmetic that decides whether it pays. It works through the core rate test, the pro-rata rule that ambushes anyone holding both pre-tax and after-tax IRA money, why paying the tax from outside the account matters more than almost anything else, the separate five-year clock on converted amounts, and the windows where conversions are most attractive. Every dollar figure, rate, and band here is illustrative, chosen to show the shape of the math rather than to state a current rule, because brackets, thresholds, surcharge tiers, and required distribution ages are set by the IRS and change regularly. Confirm today’s figures with the IRS and a qualified tax professional before you move a dollar. If you are instead looking at a contribution workaround for high earners, our explainer on the backdoor Roth IRA covers that separate strategy, and the calculator lets you model the growth a conversion is meant to protect.

What a Roth conversion actually is

A Roth conversion takes money that already exists inside a pre-tax retirement account and reclassifies it as Roth money by paying the income tax due on it. The most common version moves dollars from a traditional IRA to a Roth IRA. Some workplace plans also allow an in-plan conversion of pre-tax balances to a Roth source inside the same plan, and rollovers from a former employer plan can be converted on the way into a Roth IRA. In all of these, the account balance does not change on the day of the move. What changes is the tax label attached to it.

That distinction matters because a conversion is often confused with a contribution. A contribution is new money going in and is capped by an annual limit. A conversion is old money changing character and has no dollar limit and no income limit; you can convert as much or as little as you like in a year, whether you earn a lot or nothing at all. The price of that freedom is that the converted amount is added to your ordinary income for the year, which is a real and immediate consequence rather than a paperwork formality. Our overview of retirement account types sets out which accounts sit on which side of the pre-tax line.

Why anyone bothers: what a conversion buys

The appeal of a conversion is that it converts an uncertain future liability into a known present cost. Every dollar in a traditional IRA is really two things: your money and a claim the tax system holds against it. You do not know the size of that claim, because it will be settled at whatever rate applies whenever you withdraw, under whatever tax law exists then. Converting settles the claim today at a rate you can see, which some people value for the certainty alone.

Beyond certainty, a conversion buys three practical things. It removes the converted amount from the balance that will one day drive required minimum distributions, which shrinks a forced income stream later in life. It creates a pool of money you can draw on without adding to taxable income, which is a useful control valve in a year when other income is already high. And it hands your heirs an account that generally comes to them tax-free rather than one that arrives with a tax bill attached. None of these guarantees the trade is worthwhile; they are the benefits sitting on the other side of the tax you volunteer to pay.

The core test: your rate now versus your rate later

Strip away everything else and a conversion passes or fails on one comparison. If the effective tax rate you pay on the conversion today is lower than the rate those same dollars would have met on withdrawal, converting wins. If today’s rate is higher, converting loses. If the two rates are identical and the tax comes out of the converted balance, the two paths land in exactly the same place, because multiplying by the growth factor and by the tax factor gives the same answer in either order.

That last point is worth sitting with, because it explains why so much conversion advice sounds like hand-waving. Take an illustrative $50,000 of pre-tax money growing at an assumed 7 percent for 20 years, which multiplies it by roughly 3.87. Convert now at an illustrative 22 percent, funding the $11,000 tax out of the conversion itself, and $39,000 grows to about $150,900 tax-free. Leave it alone and the full $50,000 grows to about $193,500, from which a 22 percent withdrawal tax leaves about $150,900. Identical. The rate difference is the entire game.

Why that test is a forecast, not a calculation

Here is the part most conversion pitches skip. You can compute today’s rate precisely. You cannot compute the rate you will face in fifteen or twenty-five years, because it depends on your future income, your future filing status, where you live, and tax law that has not been written yet. Anyone presenting a conversion decision as arithmetic is quietly treating a forecast as an input. The math is exact; the number you feed it is a guess.

That does not make the exercise useless. It makes it a matter of reasoning about direction rather than chasing precision. A few things push the forecast one way or the other with some reliability. Pensions, Social Security, and required distributions all tend to add income later, which argues that your rate later may be higher than it looks today. Retiring early with no pension and years before benefits start pushes the other way, creating genuinely low-income years. Losing a spouse can shift a survivor to a less generous filing status on similar income, which raises the rate later. Weigh these as tendencies, not certainties, and expect to be wrong at the margins.

Illustrative net value by the tax rate that applies

The chart below makes the rate sensitivity visible. It starts from the same illustrative $50,000 of pre-tax money, grows it at an assumed 7 percent for 20 years to roughly $193,500, and then applies different tax rates. The top row shows the outcome when no tax is taken out of the retirement money at all, because you paid it from a separate savings account. The rows below show what remains once tax at various illustrative rates is taken, whether that tax is paid now on a conversion or later on a withdrawal.

Two lessons come out of that chart. The first is that the spread between rates is worth real money: the difference between meeting a 12 percent rate and a 32 percent rate on the same illustrative balance is roughly $38,700, which is more than a third of the smaller outcome. The second is that the top row is a different kind of row. It is not a lower tax rate; it is the same tax paid with dollars that were never going to enjoy tax-free growth anyway. That single choice does more for the outcome than any plausible rate difference, which is why it gets its own section below.

How you actually do a conversion

The mechanics are simpler than the decision. If both accounts are at the same institution, a conversion is usually a form or an online request that moves a stated dollar amount, or specific holdings, from your traditional IRA to your Roth IRA. If they sit at different institutions, you request a direct transfer from the receiving side rather than taking a check, because money that passes through your hands introduces withholding and rollover deadlines you do not need. Nothing is bought or sold if you convert in kind, meaning the same shares simply land in the other account.

The order of operations matters more than the paperwork. Open the Roth IRA first if you do not already have one, so the destination exists before you start; our walkthrough on opening a Roth IRA covers that setup. Decide the dollar amount before you sit down at the form, because the form will happily accept whatever you type. Decline withholding on the conversion if you can, and arrange the tax payment separately. Then keep the confirmation and the year-end tax forms together, because the reporting is what keeps the tax system from treating already-taxed dollars as taxable twice.

What the conversion does to your tax return

A conversion shows up as ordinary income, which means it does not simply add a line and a bill. It raises the income figure that other parts of the return look at. That can reduce or eliminate income-tested credits and deductions, change how much of your Social Security is taxable, affect eligibility for health insurance subsidies if you buy coverage on your own, and push you into a higher band for other purposes. The tax on the conversion itself is often the smallest of these effects for someone with a complicated return.

There is also a timing consequence. Because the converted amount is income you did not have withholding against, a large conversion can trigger an underpayment situation unless you make an estimated tax payment or increase withholding elsewhere during the year. People routinely convert in December, feel good about it, and then meet a penalty in April that had nothing to do with the merits of the conversion. The rules for estimated payments and safe harbors are set by the IRS and change, so confirm the current requirements, and consider making the payment in the same quarter you convert rather than waiting.

Paying the tax from outside the account

This is the single most consequential mechanical choice in the whole exercise, and it is easy to get wrong because the convenient option is the bad one. When you convert, the custodian will often offer to withhold tax from the amount being moved. Accepting that offer means fewer dollars arrive in the Roth, and those missing dollars are the ones that would have compounded tax-free for decades. The tax bill is the same size either way; what differs is which pool of money pays it.

Think of it as a question about wrapper space. A dollar inside a Roth grows without any further tax. A dollar in an ordinary savings or brokerage account gets taxed on its interest, dividends, and gains along the way. Using outside dollars to pay the conversion tax effectively moves value from the taxed wrapper into the untaxed one, which is a free improvement that has nothing to do with rates. If you do not have the cash outside to cover the tax, that is a strong signal to convert a smaller amount rather than a signal to withhold.

A worked comparison: outside cash versus withholding

Put numbers on it. Take an illustrative $50,000 conversion at an assumed 22 percent effective rate, producing an $11,000 tax bill, with the converted money growing at an assumed 7 percent for 20 years. If you pay the $11,000 from a checking account, the full $50,000 lands in the Roth and grows to roughly $193,500, all of it available tax-free. If instead you withhold the $11,000 from the conversion, only $39,000 lands in the Roth, and it grows to about $150,900.

The difference is roughly $42,600 of tax-free money, created by nothing more than which account wrote the check. That is 22 percent of the ending balance, gone, on a decision most people make in about four seconds on a web form. And there is a second cost hiding in the withholding path: if you are under 59 and a half, the withheld amount is generally treated as a distribution you took rather than converted, which can attach an early withdrawal penalty. On an illustrative $11,000 withheld, a 10 percent penalty would be another $1,100. Our breakdown of Roth IRA withdrawal rules covers how distributions from these accounts are ordered and treated.

The pro-rata rule when you hold after-tax IRA money

Some people hold after-tax basis inside a traditional IRA, usually from nondeductible contributions made in past years. The instinct is to convert only that after-tax money, since it has already been taxed and would come across for free. You cannot. The pro-rata rule treats all of your traditional, SEP, and SIMPLE IRAs as a single combined pot for this purpose, and every converted dollar is treated as a proportional slice of the whole thing.

An illustration makes the effect concrete. Suppose you hold $180,000 of pre-tax money and $20,000 of after-tax basis across your IRAs, a $200,000 total in which basis is 10 percent. Convert an illustrative $50,000 and the tax system treats $5,000 of it as a tax-free return of basis and $45,000 as taxable, regardless of which account you drew from or what you intended. At an illustrative 22 percent, that is $9,900 of tax on a conversion you may have expected to be free. Some people reduce this by rolling pre-tax IRA money into a current employer plan first, if the plan accepts it, since plan balances are not counted in the calculation. That same rule is the central pitfall in our explainer on the backdoor Roth IRA, where it does the same damage for a different reason.

Partial conversions and bracket filling

The technique that makes conversions manageable is refusing to do them all at once. A partial conversion means moving only as much as fits inside a target tax band in a given year, then stopping. You look at the income you already expect, work out how much room remains before the next band begins, and convert an amount that fills that room without spilling over. Next year you do it again. Over five or ten years, a large pre-tax balance can be worked down in slices, each one taxed at a rate you chose.

This reframes the decision usefully. Instead of asking whether to convert, which feels enormous, you ask how much to convert this year, which is a dial rather than a switch. It also limits the damage from being wrong, because no single year’s guess about future rates carries the whole balance. The practical constraint is that you need to estimate your other income for the year before you know the room available, which is why many people convert late in the year when the picture is clearer, while still leaving time to make the estimated tax payment. The band boundaries and rates are set annually by the IRS, so pull the current figures rather than relying on last year’s.

Where an illustrative conversion falls across tax bands

The chart below shows how a single conversion can touch more than one band. Picture an illustrative $50,000 conversion sitting on top of other income, where the first $4,000 finishes off a band taxed at an illustrative 12 percent, the next $26,000 fills a band taxed at 22 percent, and the last $20,000 spills into a band taxed at 24 percent. The tax comes to $480 plus $5,720 plus $4,800, which is $11,000.

Two things fall out of that split. First, the rate people quote as “my bracket” is the top band they touch, not what they actually pay; the blended 22 percent here is the number that belongs in any comparison against a future rate. Second, the last slice is the expensive one. Stopping at $30,000 rather than $50,000 would have produced $6,200 of tax, a blended rate closer to 20.7 percent, because none of it reached the 24 percent band. Whether that restraint is worth the slower pace depends on how many low-rate years you expect to have, which brings the forecast problem back into the room.

The five-year clock on converted amounts

Roth accounts carry five-year rules, and there are two of them doing different jobs. The first attaches to your Roth IRA overall and governs whether earnings can come out tax-free once you are otherwise eligible. The second attaches to each converted amount individually and governs whether that converted principal can be withdrawn without an early withdrawal penalty if you are under 59 and a half. Satisfying one does not satisfy the other.

The practical consequence is that money you convert this year is not freely spendable this year if you are young, even though it is your own principal. Each conversion starts its own clock in the year it happens, which is why people building an early retirement income bridge convert in a staggered series several years before they intend to spend, so that a matured tranche becomes available each year. Once you reach the age threshold where the early withdrawal penalty stops applying, the per-conversion clock stops mattering for that penalty. The specifics are IRS territory and worth confirming rather than assuming, especially if you are planning withdrawals around a particular date. Our breakdown of Roth IRA withdrawal rules goes further into how these orderings work.

The window: a low-income year

Conversions are worth the most in years when your income dips. A sabbatical, a business loss, a gap between jobs, the first year of self-employment, a year with unusually large deductions, or simply retiring in June all create room in the lower bands that would otherwise go unused. Filling that room with converted income is close to the purest version of the strategy, because you are moving dollars out of a future high band into a present low one.

The trap is recognising the year while it is happening. Low-income years often feel like bad years, and a bad year is not when people go looking for a way to volunteer tax. But the room is genuinely temporary; unused band capacity does not carry forward. It also helps to think of it as a rolling opportunity rather than a single event, since anyone with a large pre-tax balance is likely to see several such years across a working life and a retirement. Model the amount before you act, and confirm your estimated other income, because converting into a year that turns out better than expected is how a low-rate conversion quietly becomes a high-rate one.

The window: early retirement before Social Security and RMDs

For many people the richest window is the stretch between the end of a paycheck and the start of Social Security and required distributions. Income in those years can be genuinely low: you are living off cash and taxable investments, no wages are arriving, benefits have not started, and nothing is being forced out of your retirement accounts yet. That combination creates band room, sometimes a lot of it, for a decade or more.

The window closes from both ends. Claiming Social Security adds taxable income, and the age at which you claim is itself a decision with its own trade-offs, covered in our explainer on full retirement age and our walkthrough on estimating your benefit. Required distributions add more, and once they begin they are not optional. Someone with a large traditional balance who does nothing during this window can find that their taxable income in their seventies is higher than it was while working, which is exactly the outcome a conversion strategy is meant to prevent. Model the window before you enter it, because the years inside it are the ones you cannot get back.

The window: a down market

A market decline creates a smaller, more opportunistic window. If your holdings have fallen in value, converting the same number of shares costs less tax, because tax is charged on the value transferred on the day you convert. The recovery then happens inside the Roth, tax-free, rather than inside the traditional account where it would eventually have been taxed.

Put an illustration on it. Suppose a set of holdings was worth $62,500 and falls 20 percent to $50,000. Converting those same shares now costs an illustrative 22 percent on $50,000, which is $11,000, rather than 22 percent on $62,500, which would have been $13,750. That is $2,750 of tax saved on the same underlying investment, and everything the shares recover afterwards sits on the tax-free side. Two cautions belong with this. Nobody knows whether a decline is finished, so treat the discount as a bonus on a conversion you already wanted to make rather than a reason to make one. And since conversions cannot be undone, a further fall after you convert cannot be reversed by unwinding the transaction.

What conversions do to your future RMDs

Traditional retirement accounts eventually force money out. Once you reach the age set by law, a percentage of the balance must be distributed each year and taxed as income, whether or not you need the cash. The percentage rises with age, so a large untouched balance can produce a growing, mandatory, taxable income stream late in life, precisely when a widowed survivor may also be filing under a less generous status.

Every dollar converted is a dollar that will never be part of that calculation, because Roth IRAs are not subject to lifetime required distributions for the original owner. That is one of the clearest, least speculative benefits of converting: it is not a forecast about rates, it is a structural reduction in forced future income. Two mechanical points matter. Conversions do not count toward satisfying a required distribution, and in a year when one is due, that distribution must generally be taken before any conversion. The age at which distributions begin has been changed by legislation more than once, so confirm the current age rather than relying on a number you remember. Our explainer on required minimum distributions covers the mechanics in detail.

Medicare premium surcharges and the two-year lookback

Medicare charges higher Part B and Part D premiums to people whose income exceeds certain thresholds, and it makes that determination using income reported for a year roughly two years earlier. This produces one of the least intuitive consequences of a conversion: a decision made in one year can raise a monthly premium in a year you have long since stopped thinking about it. People who convert in the run-up to enrolling in Medicare are often the ones caught.

The structure makes it worse than a normal tax effect. The surcharge tiers are cliffs, not slopes. Crossing a threshold by a small margin moves you into the entire higher tier for the year, so the last few hundred dollars of a conversion can carry a surcharge cost far out of proportion to the income they represent. There are two sensible responses: size conversions with the thresholds in view rather than discovering them afterwards, and know that a genuine life-changing event that reduced your income can be reported to ask for a redetermination. The thresholds and surcharge amounts are adjusted regularly, so confirm the current tiers before assuming where you sit.

How a conversion can pull your Social Security into tax

Social Security benefits are not automatically taxable, and they are not automatically fully taxable either. A share of them becomes taxable once a measure of your income, which combines your other income with part of your benefits, rises past certain thresholds. A conversion adds directly to that measure, so a conversion made while you are already collecting benefits can make a portion of those benefits taxable that would not have been otherwise.

The effect can be surprisingly sharp. Because each additional dollar of conversion income can drag more benefit income into the taxable column alongside it, the true marginal cost of converting in those years can run meaningfully above the band rate you were aiming at. This is a strong argument for front-loading conversions into the years before benefits start, which is the same reason the early retirement window matters so much. If you are already claiming, it does not make conversions wrong; it makes the real rate higher than the headline, which is exactly the number the core test needs. Our walkthrough on estimating your Social Security benefit helps you put a figure on the income side of this.

State tax, and the state you plan to live in

Federal tax is only part of the bill. States treat retirement income very differently: some tax it in full, some exempt some or all of it, and some have no income tax at all. A conversion is state-taxable income in the year you convert, in the state where you live at the time, and that can add several percentage points to the cost of a move you evaluated on federal rates alone.

The forward-looking version of this question is the interesting one. If you expect to move to a state with materially different treatment of retirement income, the timing of conversions relative to that move can matter as much as the federal rate comparison. Converting heavily in a high-tax state just before relocating to a low-tax one is the version people regret. The opposite sequence can be genuinely valuable. State rules vary widely and change, and residency for tax purposes is its own technical question, so treat this as an area to raise with a professional who knows both states rather than something to settle from a general article.

Conversions are irreversible, so size them carefully

Treat a conversion as permanent once it is done. The mechanism that once allowed people to reverse a conversion by recharacterizing it back to a traditional account is not available for conversions, so there is no dependable way to unwind the transaction or the tax it produced. If markets fall the week after you convert, if your income turns out higher than you estimated, or if a large unexpected item of income appears in December, the conversion stands.

Everything about how conversions should be executed follows from that. Convert in partial amounts rather than one sweep, so that a single bad estimate touches one slice. Convert later in the year when your income picture is clearer rather than in January on a guess. Leave a cushion below any threshold you are trying to stay under, whether that is a band boundary, a surcharge tier, or a subsidy cliff, because the cost of overshooting is asymmetric. And confirm the current position on what can and cannot be reversed before you rely on any of this, since the rules governing these accounts have been rewritten more than once.

Mistakes that cost people real money

The recurring errors are mechanical rather than strategic. Withholding the tax from the conversion is the most expensive and the most common, worth roughly $42,600 in the illustration above and potentially adding an early withdrawal penalty for anyone under 59 and a half. Forgetting the pro-rata rule is second, because it turns an expected tax-free transfer into a mostly taxable one. Converting before taking a required distribution in a year one is due creates a problem that has to be corrected rather than argued about.

Three more are worth naming. Converting an amount based on last year’s band boundaries, without checking the current figures, quietly pushes income into a higher band. Skipping the estimated tax payment produces an underpayment charge that has nothing to do with whether the conversion was wise. And treating a headline bracket as the rate you pay, rather than the blended rate the conversion actually produces, distorts the core comparison in both directions. None of these are exotic, and all of them are avoidable by slowing down and putting numbers on paper first.

A worked example: three years of partial conversions

Take an illustrative case. Dana retires at 62 with $400,000 in a traditional IRA, no pension, and no plans to claim Social Security for several years. She is living on taxable savings, so her other income is low, and she has cash outside her retirement accounts to pay tax with. She decides to convert $50,000 a year for three years rather than converting a large amount at once.

On the illustrative band structure used earlier, each year’s conversion produces about $11,000 of tax, a blended 22 percent, paid from her savings account rather than withheld. Across three years that is $150,000 converted and about $33,000 of tax settled, leaving roughly $250,000 of pre-tax money plus whatever it has earned. The converted money grows tax-free from there and will never generate a required distribution. Each year she recalculates before converting, because her other income, the band boundaries, and her distance from Medicare enrollment all shift. If a year arrives where her income is higher than expected, she converts less or skips it entirely. That is the whole technique: an annual decision, sized to the year in front of her.

Using a Roth conversion calculator

A conversion calculator earns its keep by making the trade tangible. Enter the amount you are considering, the rate you think applies today, the rate you expect later, the years of growth, and an assumed return, and it will show the two paths side by side: what converting leaves you with, what leaving the money alone leaves you with, and how much paying the tax from outside cash is worth. Seeing the gap move as you change one input is a faster education than any explanation of the underlying algebra. The calculator on this page runs the compound growth engine that sits underneath all of it.

What no calculator can do is supply the future rate honestly, and that is the input the answer is most sensitive to. It also cannot know your state, your Medicare timing, your Social Security situation, or whether a conversion will collide with an income-tested threshold you have not thought about. Treat the output as a way to understand which levers matter, vary the future rate deliberately to see how fragile the conclusion is, and check whether the answer flips within a range of assumptions you consider plausible. If it does, that is genuinely useful information: it means the case is close, and a close case is one to take to a professional rather than settle from a spreadsheet.

Who should talk to a professional first

Some conversion decisions are simple enough to reason through alone. A modest conversion in a clearly low-income year, with no after-tax IRA basis, no Medicare proximity, no benefits being claimed, and cash on hand for the tax, is a decision most people can make with a calculator and an afternoon. Even then, confirming the current bands and thresholds is worth the time.

Others are not. If you hold after-tax basis in any IRA, if you are within a few years of Medicare enrollment, if you are already claiming Social Security, if you are considering a conversion large enough to reach into higher bands, if you buy health insurance on your own and receive income-tested help with the premium, if you are weighing a move between states, or if the amounts involved are large enough that being wrong matters, the interactions get complicated quickly and the cost of a mistake is permanent. That is what a qualified tax professional or advisor is for. Our explainer on fiduciary financial advisors covers what to look for in someone whose advice is not conflicted, and our comparison of Roth and traditional IRAs frames the underlying tax-now-versus-tax-later question the conversion decision inherits.

The bottom line

A Roth conversion moves existing pre-tax money into a Roth by paying the income tax on it now, and whether it pays comes down to whether the rate you pay today is lower than the rate those dollars would have met later. That comparison is half arithmetic and half forecast, so the sensible posture is to reason about direction, convert in partial amounts sized to a target band, and accept that no single year’s decision carries the whole balance. Pay the tax from outside the account: in the illustration used throughout, that one choice was worth roughly $42,600 of tax-free money after 20 years, more than any plausible difference in rates. Watch the pro-rata rule if you hold after-tax IRA basis, respect the separate five-year clock on each converted amount, and remember that a conversion echoes into Medicare premiums two years later, into how much of your Social Security is taxed, and into your state return. Every band, rate, threshold, and age here is a teaching illustration rather than current law, and all of them change, so confirm today’s figures with the IRS and bring your own numbers to a qualified tax professional. Run the growth side of the trade through the calculator to see what the tax-free treatment is actually protecting.

SumLoft publishes educational material about how financial math works, and nothing above is tax advice, investment advice, or a recommendation to convert any amount. Every band, rate, threshold, age, and dollar figure in this walkthrough was chosen to illustrate a mechanism, not to state current law; brackets, surcharge tiers, distribution ages, and the treatment of these accounts are set by the IRS and by legislation and change regularly. A conversion is generally permanent and its effects reach into future years, so verify the present rules with the IRS and work through your own situation with a qualified tax professional or advisor before acting.

Edited by Hamza Hai, MBA · Editor

Hamza Hai is the editor of SumLoft. She holds an MBA and reviews the site's articles against our editorial standards, checking that every figure is labelled for what it is, that nothing is presented as verified fact without a source the reader can check, and that the writing stays useful to a non-specialist.

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