Roth Conversion Calculator 2026: Rules and Real Tax Cost

A Roth conversion moves money from a traditional IRA to a Roth IRA. You pay income tax on the amount now, and qualified withdrawals later are tax-free. The question is never just "what bracket am I in?" For retirees, each converted dollar can also make more Social Security taxable, shrink the new senior deduction, push capital gains out of the 0% rate, and raise Medicare premiums two years later. So a conversion "in the 12% bracket" can cost 22% or more. Below: the IRS rules, each of those side effects with the math, and a calculator that shows the real rate for your 2026 numbers.

Table of Contents

The Short Answer

Anyone can convert. The old $100,000 income limit was repealed for tax years beginning after December 31, 2009.
The converted amount is taxable income in the year the money leaves the traditional IRA, so a 2026 conversion has to happen by December 31, 2026. There is no April extension like there is for contributions.
No undo. Conversions made after 2017 cannot be recharacterized back to a traditional IRA.
Your real rate can be well above your bracket. In the 12% bracket, a retiree whose Social Security is partly taxable can pay 22.2% on each extra dollar (12% x 1.85). A couple both 65 or older in the senior deduction phase-out pays 24.64% in the 22% bracket (22% x 1.12).
Watch the cliffs. Medicare IRMAA uses your income from two years earlier, and the ACA premium tax credit disappears above 400% of the poverty line. One dollar over either line costs the whole step.

Roth Conversion Rules

These come from IRS Publications 590-A and 590-B, the Form 8606 instructions and the Internal Revenue Code.

Taxed as income, no 10% penalty on the conversion itself. The taxable part of the amount converted goes on your return for the year you convert. The conversion is not an early distribution, so the 10% additional tax does not apply to it, even before age 59½.
A separate five-year clock for each conversion. If you are under 59½ and withdraw converted money within five years, you may owe the 10% additional tax on the part that was taxable when you converted. Each clock starts on January 1 of the year of that conversion. Withdrawals come out in a set order: your own contributions first, then conversions oldest first, then earnings.
The pro-rata rule. If you ever made nondeductible (after-tax) IRA contributions, you cannot convert only the after-tax dollars. Form 8606 adds up the value of all your traditional, SEP and SIMPLE IRAs on December 31 of the conversion year, plus outstanding rollovers, and treats every conversion as a mix of taxable and nontaxable money in that proportion. Balances in a 401(k) or other employer plan are not IRAs and are not counted.
RMDs come first. You cannot convert a required minimum distribution. In a year you owe an RMD, the first dollars out of the IRA count toward it, so take the RMD and then convert.
No lifetime RMDs on the Roth side. Roth IRAs have no required distributions while the owner is alive, and since 2024 neither do Roth 401(k) and Roth 403(b) accounts.

Why Your Real Rate Can Beat Your Bracket

Your bracket is the rate on one more dollar of taxable income when nothing else moves. In retirement, a conversion moves several other numbers at the same time. These are the five that matter, using 2026 federal rules.

1. The Social Security tax torpedo

How much of your Social Security is taxable depends on your "provisional income": other income plus tax-exempt interest plus half your benefits. Above $25,000 ($32,000 married filing jointly), up to 50% of benefits become taxable. Above $34,000 ($44,000 joint), up to 85%. These amounts are written into IRC section 86 and have never been indexed for inflation, so more retirees cross them every year.

In the 85% zone, each converted dollar adds itself plus 85 cents of benefits to taxable income. In the 12% bracket that is 12% x 1.85 = 22.2%. In the 22% bracket it would be 40.7%. The effect stops once 85% of your benefits are taxable, the maximum, and from there you pay the plain bracket rate again.

2. The senior deduction phase-out

From 2025 through 2028, each person 65 or older gets an extra $6,000 deduction on top of the regular standard deduction. Each $6,000 shrinks by 6% of modified AGI above $75,000 ($150,000 joint). On a joint return where both spouses are 65 or older, each spouse's $6,000 is reduced separately, so the couple loses 12 cents of deduction for every dollar of income above $150,000, until it is gone at $250,000. In the 22% bracket that makes the real rate 22% x 1.12 = 24.64% for the couple, or 22% x 1.06 = 23.32% for a single filer between $75,000 and $175,000.

3. Capital gains pushed out of the 0% rate

Long-term gains and qualified dividends are taxed at 0% while total taxable income stays under $49,450 (single) or $98,900 (joint) in 2026. They sit on top of ordinary income, so each converted dollar can push a dollar of gains from 0% to 15%. For someone in the 12% bracket with gains in that range, the conversion costs 12% plus 15% = 27%.

4. Medicare IRMAA, two years later

Medicare's income-related surcharges are set from your tax return two years back, so a 2026 conversion sets your 2028 premiums. The 2026 tiers start above $109,000 of MAGI for single filers and $218,000 joint, where MAGI is AGI plus tax-exempt interest. The first tier adds $81.20 a month to Part B and $14.50 to Part D, per person: $1,148.40 a year each. IRMAA works in steps, so one dollar over a line costs the full step. The 2028 thresholds will be adjusted for inflation; see the full IRMAA table.

5. The ACA subsidy cliff (before Medicare)

If you buy Marketplace health insurance before 65, a conversion counts as income for the premium tax credit. For 2026 coverage, the credit ends completely above 400% of the 2025 poverty line: $62,600 for one person and $84,600 for two in the 48 contiguous states and DC. For an early retiree, a conversion that crosses this line can cost more in lost subsidy than in tax. Details in our ACA subsidy cliff guide.

Why this matters for planning: the side effects are why the "fill up the 12% bracket" rule of thumb often gives the wrong answer for retirees. The calculator below adds all five up.

Worked Example: A Retired Couple

Take a hypothetical married couple, both 67, filing jointly in 2026. They have $45,000 of pension income and $48,000 of Social Security, and no other income. Before any conversion, $27,250 of their benefits is taxable, their deduction is $47,500 (the $32,200 standard deduction, $1,650 each for being 65 or older, and the $6,000 senior deduction each), and their federal tax is $2,475.

2026 conversion	Extra federal tax	Effective rate on the conversion	Rate on the next dollar	What is happening
$10,000	$2,219	22.2%	22.2%	Every dollar also makes 85 cents of benefits taxable
$20,000	$4,025	20.1%	12.0%	85% of benefits now taxable; the torpedo is over
$40,000	$6,425	16.1%	12.0%	Plain 12% bracket
$60,000	$8,825	14.7%	12.0%	Still 12%; the bracket ends at a $62,500 conversion
$80,000	$13,392	16.7%	24.6%	22% bracket plus the senior deduction phase-out
$100,000	$18,320	18.3%	24.6%	$4,296 of the senior deduction lost so far

Hypothetical. 2026 federal income tax only, from IRS Rev. Proc. 2025-32, IRC section 86 and P.L. 119-21 section 70103. Our arithmetic, rounded to the dollar. No state tax.

The first $16,000 or so of conversion is the most expensive per dollar, not the cheapest. Once 85% of their benefits are taxable, the remaining room up to a $62,500 conversion (about $46,500) is taxed at a flat 12%. For a household like this, spreading conversions thinly over many years pays the 22.2% torpedo rate again every year, so fewer, larger conversions can cost less, the opposite of the usual advice. Their MAGI stays under the $218,000 joint IRMAA line until a conversion of about $132,200, using 2026 tiers.

Roth Conversion Calculator

Enter your expected 2026 income and the amount you are thinking of converting. The calculator runs your federal return twice, with and without the conversion, and shows the extra tax, the effective rate, the rate on the next $1,000, and how much you can convert before you reach the top of each bracket. Open "More options" to add capital gains, state tax, Medicare (IRMAA) and an ACA Marketplace plan.

When a Conversion Pays, and When It Does Not

A conversion is a bet on tax rates. You pay tax now at your rate this year to avoid tax later at whatever rate applies when the money comes out. If both rates are the same and you pay the tax from the IRA itself, the result is the same either way. What changes the answer:

Converting tends to help when: your rate now is lower than you expect later, which is common in the years between retirement and the start of Social Security or RMDs (RMDs start at 73, or 75 for people born in 1960 or later); you can pay the tax from a taxable account rather than from the IRA; you expect large RMDs that will push you into higher brackets or IRMAA tiers; your heirs are likely to be in a higher bracket than you, since most non-spouse heirs must empty an inherited traditional IRA within 10 years and pay tax as they do; or you expect to be a widow or widower filing single, with narrower brackets on similar income.
Converting tends to cost you when: your rate now is higher than your expected rate in retirement; the tax would come out of the IRA itself, which shrinks the amount that gets to grow tax-free; the conversion would cross an IRMAA or ACA line; you might need the money within five years and are under 59½; or you plan to leave the IRA to charity, which pays no income tax on it.

Run your own numbers for several years in the calculator. The amount that keeps you just under the next bracket, IRMAA tier or ACA line is usually where to stop.

Paying the Tax Without a Penalty

A conversion adds income that no employer withholds on. To avoid an underpayment penalty, your withholding plus timely estimated payments for 2026 generally need to reach the smaller of 90% of your 2026 tax or 100% of your 2025 tax. If your 2025 AGI was over $150,000 ($75,000 married filing separately), that second figure is 110%. There is no penalty if you owe less than $1,000 after withholding and credits.

Two practical points. Federal tax withheld counts as if it were paid evenly through the year, even when it is withheld in December, while estimated payments count only from the date you make them. And tax withheld from the conversion itself never reaches the Roth IRA: it is an ordinary distribution, and under 59½ it can face the 10% additional tax. Paying from outside money avoids both problems.

How to Do It Before December 31

Estimate your 2026 income from pensions, Social Security, interest, dividends and any gains, and run it through the calculator to choose an amount.
Take any 2026 RMD first. It cannot be converted.
Ask your IRA custodian for a conversion, usually an online form. A same-firm transfer from the traditional IRA to a Roth IRA is simplest; open the Roth IRA first if you do not have one. You can convert cash or move investments in kind.
Decide how to pay the tax: an estimated payment, or extra withholding from a pension or another IRA distribution.
Finish by December 31. Custodians can set earlier year-end processing cutoffs, so do not wait until the last days.
Report it. You will get Form 1099-R in January. Form 8606 Part II reports the conversion and applies the pro-rata rule if you have after-tax money in any IRA.

Sources & Methodology

IRS Publication 590-A and Publication 590-B: conversion taxation, RMDs, the five-year rule, ordering rules, no Roth RMDs.
Instructions for Form 8606: the pro-rata calculation and December 31 values.
IRS Publication 915 and IRC section 86: taxable Social Security (Worksheet 1).
IRS Rev. Proc. 2025-32: 2026 brackets, standard deduction, additional deduction for age 65+, capital gains thresholds.
Public Law 119-21, section 70103, and IRS Schedule 1-A: the senior deduction and its phase-out.
Public Law 109-222, section 512 (income limit repeal) and Public Law 115-97, section 13611 (end of recharacterization).
Instructions for Form 2210: underpayment safe harbors and withholding timing.
CMS, 2026 Medicare Parts A & B Premiums and Deductibles: IRMAA tiers; SSA POMS HI 01101.010 for the two-year lookback and MAGI.
IRS Rev. Proc. 2025-25 and the 2025 HHS poverty guidelines: the 2026 ACA cliff.

Method notes. The worked example and the calculator are our arithmetic from these sources: the federal return is computed twice, with and without the conversion, following the IRS tax rate schedules, the Qualified Dividends and Capital Gain Tax Worksheet, Pub 915 Worksheet 1 and Schedule 1-A. They leave out the alternative minimum tax, credits, state rules other than a flat rate you enter, and after-tax IRA basis. 2027 brackets and the 2028 IRMAA tiers had not been published as of October 12, 2026.

This article is educational and is not tax, legal or investment advice. Figures were checked against the IRS, the U.S. Code, Public Laws on govinfo.gov, CMS and SSA on 2026-10-12. The examples are hypothetical. A conversion cannot be undone, so consider speaking to a tax professional before you convert.

FAQ

Is there an income limit for Roth conversions?
No. Anyone with a traditional IRA can convert, whatever their income. The limit was repealed starting in 2010. Income limits still apply to direct Roth IRA contributions, which is why some people use the backdoor Roth.

What is the deadline for a 2026 Roth conversion?
December 31, 2026. A conversion counts in the year the money is distributed from the traditional IRA. The April deadline applies only to contributions.

Can I undo a Roth conversion?
No. Recharacterizing a conversion back to a traditional IRA has not been allowed for conversions after 2017.

Does a Roth conversion make my Social Security taxable?
It can. The converted amount counts in the provisional income formula that decides how much of your benefits are taxed, up to 85%. That is the "tax torpedo" above.

Does a Roth conversion affect Medicare premiums?
Yes, two years later. A 2026 conversion raises the MAGI used for your 2028 Part B and Part D premiums. If it crosses an IRMAA tier, each person on Medicare pays the surcharge for that year.

How much should I convert each year?
There is no single right amount. A common approach is to convert up to the top of a bracket, but for retirees, the Social Security, senior deduction, IRMAA and ACA lines often matter more. Use the calculator to find the amount where your rate on the next dollar jumps.

Do I pay the 10% early withdrawal penalty on a conversion if I am under 59½?
Not on the conversion itself. It can apply if you withdraw the converted money within five years, or to any tax withheld from the conversion.

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"Roth Conversion Calculator 2026: The Rules, the Real Tax Rate, and When Converting Pays." Wealthy Pot, 2026. https://wealthypot.com/roth-conversion/