Roth conversion calculator

What is a Roth conversion?

A Roth conversion moves money out of a traditional IRA, SEP IRA, SIMPLE IRA, or 401(k) and into a Roth IRA. The pre-tax portion joins your ordinary income for that year, and in exchange the money grows and comes out tax-free later.⁴

How does a Roth conversion work?

You tell the custodian how much to move, the account type changes, and the taxable amount lands on Form 8606 and your Form 1040. There is no income limit, no dollar cap, and no restriction on how often you do it.

One thing you cannot do is change your mind. The Tax Cuts and Jobs Act ended recharacterization of conversions in 2018, so the tax is locked in for the year you convert.⁴The guide on when a conversion makes sense goes deeper on the decision itself.

How is Roth conversion tax calculated?

The taxable share of an IRA conversion is one minus your basis divided by your total non-Roth IRA balance. Convert $100,000 when 10% of your IRA money is after-tax basis, and $90,000 of it is taxable.

Section 408(d)(2) treats every traditional, SEP, and SIMPLE IRA you own as a single account for this purpose.³ You do not get to convert only the after-tax dollars, no matter which account the money physically comes from.

This is what quietly breaks the backdoor Roth for a lot of people. Make a nondeductible contribution while holding an old rollover IRA and the pro-rata rule pulls that rollover into the calculation, turning what looked like a tax-free move into a mostly taxable one.See the backdoor Roth guide for how that plays out. A 401(k) to Roth conversion is different, because the plan tracks its own after-tax basis and does not aggregate with your IRAs.

The balance that belongs in this calculation is your December 31 total across every traditional, SEP, and SIMPLE IRA, plus whatever you converted or distributed during the year: Form 8606's line 6.³ Rolling a pre-tax 401(k) into a traditional IRA in the same year adds pre-tax dollars to that balance, diluting your basis across a bigger pool and raising your taxable share. This calculator has no separate input for a same-year rollover, so it assumes you made none. If you did, enter your IRA balance as of December 31, after the rollover landed, or the balance you enter will be too low, understating the taxable share and understating your tax.

2026 marginal rate thresholds

Conversion income fills whatever room is left in your current bracket before spilling into the next one.¹

Standard deduction and the two separate senior deductions

There are two age-based deductions for people 65 and older, and they stack. Missing the second one is a common way to overstate a retiree's tax bill.¹

The senior bonus deduction runs for tax years 2025 through 2028 only, is not indexed, and requires married filers to file jointly. It phases out at 6% of MAGI above the threshold.²

Here is the part most sites get wrong. That 6% reduction applies to each person's $6,000 allowance, not to a couple's combined $12,000. On the Schedule 1-A worksheet, line 34 takes 6% of the excess and line 35 subtracts it from $6,000, and only then do the two spouses' amounts get added together.² The deduction therefore reaches zero at $100,000 of excess MAGI in every case.

So a joint return is fully phased out at $250,000 whether one spouse is 65 or both are. Anywhere you see $350,000 quoted for a two-spouse couple, someone has divided $12,000 by 6% and skipped the worksheet.

A large conversion can push you through this range, which adds an effective 6% on top of your bracket while it lasts.

Why paying the tax from outside cash moves the break-even so far

When the tax comes out of the IRA itself, you shrink the sheltered balance, and the break-even lands close to your current bracket. Pay from a taxable brokerage account and something different happens: you have moved money that was paying tax on its dividends and gains every year into an account that never will.

The size of that swing is large. One widely cited industry analysis puts the break-even near 35% when the tax comes from IRA funds and near 14% when it comes from outside cash, holding growth assumptions constant. Treat those as illustrative rather than authoritative: no government source publishes break-even rates, and the number moves with the growth and horizon assumptions behind it.

The calculator runs both versions side by side rather than making you pick one.

Medicare IRMAA, and the two-year lookback

Conversion income raises your modified adjusted gross income, and Medicare reads MAGI from two years back. A conversion in 2026 sets your 2028 premiums.⁵

IRMAA is a cliff, not a ramp. One dollar over a threshold raises your premium for all twelve months of that year.

Your 2026 income sets your 2028 premium, because Medicare works on a two-year lookback.⁵ CMS has not published 2028 brackets or amounts yet, so the table below shows the current 2026 figures as a stand-in. Both the thresholds and the dollar amounts will move before 2028.

Converting before Medicare: the ACA premium tax credit

If you buy your own health coverage on an ACA marketplace before turning 65, a conversion's added MAGI can shrink or eliminate your premium tax credit for that year, raising your real out-of-pocket cost beyond the federal tax shown here. This calculator does not model the premium tax credit or its phase-out. If you receive one, check the effect separately before converting.

Social Security taxation and capital gains stacking

Conversion income raises provisional income, which can pull up to 85% of your Social Security benefit into taxable territory.

Long-term capital gains and qualified dividends sit on top of ordinary income, so a conversion can shove gains that would have been taxed at 0% into the 15% band. Both effects mean the real marginal cost of a converted dollar can be higher than the bracket you are nominally in.

The 3.8% net investment income tax

The conversion itself is not subject to the net investment income tax. Distributions from qualified plans and IRAs are specifically excluded from net investment income under section 1411.⁴

What it can do is push your other investment income over the line. The statutory thresholds under section 1411 are $200,000 for single filers and $250,000 for joint returns, and they have never been indexed for inflation, so more people cross them every year.⁴

RMDs, and why conversions happen in the years before they start

In any year you owe a required minimum distribution, the RMD has to be satisfied before you convert anything, and the RMD itself can never be converted.

That is why the gap between retiring and your first required distribution gets so much attention. Income is usually at its lowest and nothing is being forced out yet. RMDs begin at 73 for people born 1951 through 1958 and at 75 for those born in 1960 or later; the 1959 birth year is still unresolved in the regulations.

The Tweed RMD calculator shows when yours starts, including how the 1959 gap is currently handled, and what the first distribution looks like.