The pro rata rule for Roth IRA conversions is the IRS formula that decides how much of your conversion is taxable when your traditional IRAs hold a mix of pre-tax and after-tax money. You don’t get to convert only the after-tax portion. Instead, every dollar you move to a Roth is treated as a proportional slice of all your non-Roth IRA money combined, and the taxable share is added to your ordinary income for the year. The rule sits in 26 U.S.C. §408(d)(2), which requires all your individual retirement plans to be treated as a single contract for distribution purposes.1Office of the Law Revision Counsel. 26 U.S. Code 408 – Individual Retirement Accounts
Which Accounts Get Aggregated
The denominator in the formula isn’t one account. It’s the combined balance of every traditional IRA, SEP IRA, and SIMPLE IRA you own, no matter how many custodians hold them.2Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
Several account types sit outside that aggregation. Roth IRAs, inherited IRAs, and employer plans (401(k)s, 403(b)s, governmental 457(b)s) are all excluded. The exclusion of employer plans is the hinge that makes the main workaround possible.
Two more points about the perimeter. The valuation is taken at the close of the calendar year, not the day you convert, so your December 31 balances drive the math even if you converted in April.1Office of the Law Revision Counsel. 26 U.S. Code 408 – Individual Retirement Accounts And aggregation is per taxpayer, not per household. If your IRA is clean and your spouse’s isn’t, your spouse’s balance has no effect on your conversion.
How the Calculation Works
Divide your total nondeductible (after-tax) basis across all non-Roth IRAs by the total year-end value of those accounts. That percentage is the tax-free share of any conversion. The rest is taxable at ordinary rates.
A Worked Example
Say you hold $100,000 across two traditional IRAs and a SEP IRA. Over the years, $10,000 of that came from nondeductible contributions. You convert $20,000 to a Roth.
Your nontaxable percentage is $10,000 ÷ $100,000, or 10%. So $2,000 of the conversion (10% of $20,000) is a tax-free return of basis. The other $18,000 hits your gross income for the year. Your remaining nondeductible basis drops from $10,000 to $8,000, and you report the whole transaction on Form 8606.3Internal Revenue Service. About Form 8606, Nondeductible IRAs
Why December 31 Matters
Because the denominator is your year-end balance, market movement after you convert can shift the answer. Gains before December 31 grow the denominator and shrink your nontaxable percentage; a downturn does the opposite. Large conversions done in the fourth quarter have less exposure to that drift, which is one reason many taxpayers convert late in the year.
Why This Wrecks a Backdoor Roth
The pro rata rule is the reason a backdoor Roth doesn’t work cleanly for everyone. The strategy assumes you contribute nondeductibly to a traditional IRA and then convert it, so 100% of the balance is basis and the conversion is essentially tax-free. That only holds if your traditional/SEP/SIMPLE IRAs contain nothing else.
Put a $7,000 nondeductible contribution alongside $93,000 in pre-tax IRA money and the nontaxable percentage falls to 7%. Convert the $7,000 and only $490 comes out tax-free; $6,510 is taxable. The pre-tax money in the other accounts contaminates the conversion. That’s exactly what the rule is designed to do. Without it, high earners could keep large pre-tax IRAs untouched, make nondeductible contributions annually, and convert only those clean dollars.
How to Reduce the Pro Rata Hit
The workaround isn’t a trick inside the formula. It’s changing what goes into the denominator before December 31.
Roll Pre-Tax IRA Money Into an Employer Plan
Because 401(k)s and similar employer plans are outside the IRA aggregation, moving your pre-tax IRA balance into your current employer’s plan removes it from the pro rata math entirely.4Internal Revenue Service. Rollovers of Retirement Plan and IRA Distributions What’s left in your IRA is basis, and a subsequent conversion is tax-free or close to it.
Take the earlier numbers. $90,000 pre-tax and $10,000 basis gives you a 10% nontaxable percentage. Roll the $90,000 into your 401(k) and you’re left with a $10,000 IRA that’s 100% basis. Converting that $10,000 costs nothing in tax.
Two conditions matter. The rollover has to be completed by December 31 of the conversion year, because that’s the aggregation date. And employer plans aren’t required to accept incoming IRA rollovers.4Internal Revenue Service. Rollovers of Retirement Plan and IRA Distributions Check your plan documents before you count on it. Self-employed filers can use a solo 401(k) for the same purpose if the plan document permits IRA rollovers in.
Draw Down IRA Balances With QCDs
If you’re 70½ or older, qualified charitable distributions send money directly from a traditional IRA to an eligible charity without generating taxable income. For 2026 the limit is $111,000 per person. QCDs shrink the pro rata denominator without a tax cost and also satisfy required minimum distributions once you reach RMD age. Over several years they can meaningfully move the ratio.
Tracking Basis on Form 8606
Form 8606 is how the IRS knows any of your IRA money was already taxed. File it for any year you make a nondeductible contribution and for any year you convert.3Internal Revenue Service. About Form 8606, Nondeductible IRAs The form carries your cumulative basis forward year over year. On Part I, Line 2 pulls in prior-year basis and Line 6 records the December 31 fair market value of your traditional, SEP, and SIMPLE IRAs. Part II runs the conversion math.5Internal Revenue Service. Instructions for Form 8606
Skip the form in years you made nondeductible contributions and the IRS has no record of basis. Your whole IRA balance defaults to pre-tax, and a conversion is fully taxable. There’s also a $50 penalty for each year you should have filed, unless you can show reasonable cause.6Internal Revenue Service. Instructions for Form 8606 The bigger cost is paying tax twice on money that was already taxed. You can file the form late to reconstruct prior years if you missed them.
Going the other direction (overstating your basis to reduce the taxable share) exposes you to an accuracy-related penalty of 20% of the resulting underpayment.7Internal Revenue Service. Accuracy-Related Penalty
Three Rules That Can Ambush a Conversion
Three limits sit outside the pro rata calculation itself but can change whether a conversion makes sense.
SIMPLE IRA two-year window. During your first two years of participation in a SIMPLE IRA, you can’t roll that money into a Roth or any non-SIMPLE account. Try it and the IRS treats the whole amount as a withdrawal, taxable as ordinary income with a 25% early distribution penalty (not the usual 10%) unless you’re 59½ or qualify for another exception.8Internal Revenue Service. SIMPLE IRA Withdrawal and Transfer Rules After two years, SIMPLE money converts like any other traditional balance and runs through the same pro rata formula.
Conversions are permanent. Recharacterization of a Roth conversion was eliminated by the Tax Cuts and Jobs Act. Any conversion made on or after January 1, 2018 cannot be undone.9Internal Revenue Service. Retirement Plans FAQs Regarding IRAs If the tax bill turns out higher than you modeled, there’s no reversing the transaction.
Five-year clock on each conversion. Every conversion starts its own five-year holding period. Withdraw converted principal before five years have passed and before you’re 59½, and the IRS assesses a 10% penalty on the converted amount. Regular Roth contributions can always come out tax- and penalty-free, so the five-year rule bites specifically on converted dollars. Convert at 52 and tap the funds at 55 and you clear the window; convert at 56 and withdraw at 58 and you don’t.
What the Taxable Share Actually Costs
The taxable portion of a conversion lands on your Form 1040 as ordinary income and lifts your adjusted gross income. That’s where the second-order costs show up.
Medicare Premium Surcharges
Medicare sets Part B and Part D premiums using your modified AGI from two years prior. A big conversion in 2026 can raise your premiums in 2028. For 2026, the income-related monthly adjustment amounts kick in above $109,000 single and $218,000 joint, running from $81.20 to $487.00 a month on top of the standard Part B premium, with parallel surcharges on Part D.10Centers for Medicare & Medicaid Services. 2026 Medicare Parts A and B Premiums and Deductibles Splitting a conversion across two calendar years can keep MAGI under a tier and save real money.
Net Investment Income Tax
The 3.8% NIIT applies to investment income once MAGI exceeds $200,000 single or $250,000 joint. The converted amount itself isn’t classified as net investment income, so the 3.8% doesn’t apply to the conversion directly. But the conversion raises MAGI and can pull your existing dividends, capital gains, or rental income above the threshold.
State Taxes and Other Ripples
Most states tax the taxable share of a conversion as ordinary income. Rates run from zero in the nine no-income-tax states up to 13.3% at the top. A higher AGI can also cut into income-based credits, increase the taxable portion of Social Security benefits, and raise the floor for deducting medical expenses. Before executing a large conversion, model the full return, not just the federal marginal bracket.