You cannot convert only your after-tax IRA dollars to a Roth. The IRS treats all your non-Roth IRAs as one combined account, so any conversion pulls out taxable and nontaxable money in the same proportion that exists across all those accounts. That proportion is tracked on Form 8606, using your account values as of December 31 of the conversion year.
TL;DR:
- Moving pre-tax IRA balances into an employer plan before December 31 can eliminate the pro-rata tax on Roth conversions, provided the plan accepts roll-ins.
- Your combined IRA balance on December 31 determines the taxable portion of any conversion, so year-end figures and recordkeeping are critical to avoid unexpected taxes.
- Making after-tax contributions or rollovers later in the year can alter the pro-rata percentage, increasing taxable amounts if done after a conversion.
- Inherited IRAs are excluded from your personal pro-rata calculation and require separate tracking to avoid miscalculations.
- Properly filing Form 8606 every year you make nondeductible contributions or conversions is essential to maintain accurate basis records and prevent double taxation.
Table of Contents
- What the pro-rata rule is and which accounts it counts
- How to calculate pro-rata taxes on a Roth conversion
- How pro-rata taxation changes the backdoor Roth math
- How to reduce or avoid the pro-rata tax hit
- Filing Form 8606: what you need to report
- Common mistakes that lead to bigger tax bills
- Modeling your scenario with Mali's calculators
- Spreading conversions across multiple years
- New contributions after a conversion can shift the math
- Regulatory changes to watch for 2026 and beyond
- Inherited IRAs follow different pro-rata rules
- A pragmatic closing note on planning around this rule
- Model your own numbers before you convert
- FAQ
- Sources
What the pro-rata rule is and which accounts it counts
The pro-rata rule comes from how the tax code treats individual retirement accounts for distribution and conversion purposes. Instead of letting you cherry-pick your nondeductible (after-tax) contributions when you convert to a Roth, the IRS requires you to look at every traditional, SEP, and SIMPLE IRA you own as a single pool. Whatever share of that pool is after-tax basis determines the tax-free share of any conversion, no matter which specific account the money comes from.
This aggregation only applies to certain account types:
- Counted together: traditional IRAs, SEP IRAs, and SIMPLE IRAs you own.
- Not counted: Roth IRAs, and qualified workplace plans like 401(k)s or 403(b)s, unless you roll that plan money into an IRA.
- Valuation date: your combined balance is measured as of December 31 of the year you convert, per Form 8606 instructions.
Two related rules matter here too. The once-per-year IRA-to-IRA rollover limit does not apply to Roth conversions or to trustee-to-trustee transfers, so you have more flexibility than many people assume when restructuring accounts before converting.
How to calculate pro-rata taxes on a Roth conversion
The formula the IRS uses on Form 8606 is straightforward once you have the right numbers in front of you.
- Add up your total basis. This is the sum of all nondeductible contributions you have ever made to traditional IRAs, minus any basis you have already recovered in past distributions or conversions.
- Find your year-end total. Add the December 31 value of every traditional, SEP, and SIMPLE IRA you own, including the amount you are converting, per Form 8606 instructions.
- Divide basis by the total. This gives you the nontaxable percentage of any conversion or distribution that year.
- Apply that percentage to your conversion amount. The result is your tax-free portion; the rest is taxable as ordinary income.
Here is a worked example. Say you have a significant basis from nondeductible contributions, and your combined traditional IRA balance on December 31 is substantially larger (including the conversion amount you just completed). Your nontaxable percentage is your total basis divided by your total IRA value, which determines the tax-free portion of any conversion.
Any unused basis carries forward. If you had converted less than your full basis amount, the remainder stays on Form 8606 to reduce taxable income on a future conversion or distribution, so accurate recordkeeping protects you for years, not just the current filing.
How pro-rata taxation changes the backdoor Roth math
A backdoor Roth conversion is simple in theory: you contribute to a traditional IRA with after-tax dollars because you earn too much to contribute directly to a Roth, then convert that balance to Roth. The pro-rata rule is where that plan either stays clean or gets expensive.
- Pure nondeductible contribution, no other IRAs: if that new contribution is your only traditional IRA money, the conversion is essentially tax-free, since your basis equals nearly 100% of the balance.
- Nondeductible contribution plus an old rollover IRA: if you also hold a $50,000 traditional IRA from a prior 401(k) rollover, your $7,000 contribution gets diluted across that much larger balance, and most of your conversion becomes taxable.
- Workplace plan balances: money sitting in a current employer's 401(k) or 403(b) is excluded from this calculation entirely, which is why many people with large old rollover IRAs look at moving that money into a workplace plan first.
The practical takeaway: a backdoor Roth is efficient when you have little or no other pre-tax IRA money. It creates a surprise tax bill when old rollover balances are sitting in the background, even if the new contribution itself was fully after-tax.
How to reduce or avoid the pro-rata tax hit
The most common fix is moving pre-tax IRA money out of the aggregation pool before you convert.
- Reverse rollover into an employer plan. If your current 401(k) or 403(b) accepts incoming rollovers, moving pre-tax IRA balances into it removes those dollars from the IRA aggregation entirely, per IRS rollover guidance.
- Use a trustee-to-trustee transfer, not a 60-day rollover. A direct transfer avoids mandatory withholding and does not count against the one-per-year IRA rollover limit.
- Check December 31 balances before you act. Since the formula uses year-end values, a transfer completed in January does nothing for the prior year's conversion.
- Confirm your plan accepts roll-ins first. Not every employer plan allows this, so get it in writing before you move money.
Pro Tip: Call your 401(k) administrator and ask for written confirmation that the plan accepts rollovers from traditional IRAs well before you plan to convert, not after.
A practical guide to IRA rollover mistakes echoes the same caution: direct transfers are safer than 60-day rollovers precisely because they sidestep withholding and timing traps.
Filing Form 8606: what you need to report
Whenever you make a nondeductible contribution or complete a Roth conversion, Form 8606 is the form that documents it.
- Report every nondeductible contribution in the year you make it, even if you convert the same year.
- Line 6 captures your total year-end IRA value across all traditional, SEP, and SIMPLE IRAs.
- Keep trustee statements showing December 31 balances and proof of nondeductible contributions for your own records.
- Filing matters long term: skipping Form 8606 can mean losing track of your basis, which risks paying tax twice on money the IRS has no record was already taxed.
Common mistakes that lead to bigger tax bills
A few recurring errors account for most unpleasant pro-rata surprises. Not filing Form 8606 in the contribution year is the most common, since it erases your basis record. Rolling pre-tax employer plan money into an IRA the same year you convert inflates your denominator right when you need it smallest. Assuming a recharacterization can undo a completed Roth conversion is another frequent misunderstanding, since recharacterization of conversions is no longer available. Finally, confusing inherited IRAs or Roth balances with the accounts that actually count toward your pro-rata calculation leads to miscalculated taxable amounts.
Modeling your scenario with Mali's calculators
Before you convert, it helps to see the actual numbers. Mali's Backdoor Roth Pro-Rata Rule Calculator lets you talk through your account balances out loud and see the taxable versus nontaxable split instantly, since your accounts are already linked and current. The Roth conversion calculator handles larger conversion scenarios. Mali's estimates are a planning aid, not a substitute for a tax professional at filing time.
Spreading conversions across multiple years
Some people convert a large traditional IRA balance over several years instead of all at once, often to manage the tax bracket impact of a big conversion. The pro-rata rule applies fresh each year, based on that year's December 31 total and that year's remaining basis.
This matters because your pro-rata percentage can shift from year to year. If you convert part of a mixed pre-tax and after-tax IRA in year one, your remaining basis carries forward on Form 8606, but your remaining year-end balance also shrinks. Depending on what else happens in the account, like additional contributions or partial rollovers, the ratio of basis to total balance in year two will not necessarily match year one.
A multi-year conversion strategy does not let you front-load the nontaxable portion into early years and leave the taxable portion for later. Each year's conversion draws proportionally from whatever basis and balance exist at that point, so there is no way to sequence conversions to intentionally front-load tax-free dollars. Tracking this accurately across years is exactly why keeping every Form 8606 on file matters: each year's carryforward basis depends on correctly reporting the prior year's figures.
New contributions after a conversion can shift the math
Timing is not just about what happens before you convert. If you make a new nondeductible contribution to a traditional IRA later in the same calendar year, after you have already converted, that contribution still factors into the December 31 total used in your pro-rata calculation for that tax year.
This can work in your favor or against you depending on sequencing. A contribution made after a conversion increases your basis for the year, which can increase the nontaxable percentage applied retroactively to that year's conversion, since the calculation is based on year-end totals, not account values on the day you converted. The same logic applies to employer plan rollovers completed later in the year: if you convert in March but roll a pre-tax 401(k) into a traditional IRA in November, that pre-tax money still inflates your denominator for the full year's calculation.
The practical implication is that you should not treat the pro-rata calculation as settled the moment you convert. Any contribution or rollover affecting your traditional IRA balances before December 31 of that same year can change your final taxable amount, which is one more reason to finalize all IRA moves for the year before filing.
Regulatory changes to watch for 2026 and beyond
The pro-rata rule itself, as a mechanism, has not changed. However, what shifts from year to year are the contribution limits and income phase-out ranges that determine who needs a backdoor Roth strategy in the first place. The IRS announced updated limits for 2026, including a higher 401(k) contribution limit and a higher IRA contribution limit.
Because income phase-out thresholds for direct Roth contributions also adjust annually, more or fewer people fall into backdoor Roth territory depending on where those thresholds land each year. Anyone modeling a 2026 conversion should check the current-year limits in Publication 590-B rather than relying on a prior year's figures, since using outdated thresholds can lead to contributing more than allowed or missing eligibility entirely.
There is no indication of a change to the underlying pro-rata aggregation formula itself. The mechanics on Form 8606, basis divided by year-end total, remain the operative calculation regardless of annual limit adjustments.
Inherited IRAs follow different pro-rata rules
Inherited IRAs are not lumped into your own pro-rata calculation. The IRS treats an IRA you inherit as a separate aggregation pool from the IRAs you contributed to yourself, which means an inherited traditional IRA's basis and value do not combine with your personal traditional, SEP, or SIMPLE IRAs when you calculate the taxable portion of your own Roth conversion.
That said, if you inherit a traditional IRA and later convert it to an inherited Roth IRA (where the plan allows this), the pro-rata rule still applies, but only within the inherited account's own basis and balance, separate from your personal IRAs. Beneficiaries managing both their own retirement accounts and an inherited IRA need to track two separate pro-rata calculations rather than one combined figure. Mali's inherited IRA RMD calculator can help separate these two tracks when you are modeling required distributions alongside a personal conversion strategy.
A pragmatic closing note on planning around this rule
Get your recordkeeping right before you get clever with strategy. Confirm your employer plan accepts roll-ins, model the conversion with real numbers, and file Form 8606 accurately every time.
— Todd
Model your own numbers before you convert
We built Mali's Backdoor Roth Pro-Rata Rule Calculator and Roth conversion calculator so you can talk through your actual account balances out loud instead of guessing at spreadsheet formulas. Because voice AI links to your real bank and IRA data, it can walk you through your specific pro-rata percentage in a conversation, the same way you would ask a knowledgeable friend to check the math.
No other personal finance app lets you talk to an AI about your real linked accounts and transactions the way this app does, which is exactly what makes scenario testing like this fast instead of tedious. Try the backdoor Roth calculator and see your own numbers before you talk to a tax professional about filing.
FAQ
What is the IRA pro-rata rule?
The pro-rata rule requires the IRS to treat all your traditional, SEP, and SIMPLE IRAs as one combined account when you convert to a Roth. Your taxable and nontaxable portions are based on the ratio of after-tax basis to your total year-end balance across those accounts, reported on Form 8606.
How can I avoid the IRA pro-rata rule?
The standard method is rolling your pre-tax traditional IRA balances into an employer 401(k) or 403(b) before December 31, since workplace plans are excluded from the pro-rata calculation, per IRS rollover guidance. This only works if your employer plan accepts incoming rollovers, so confirm that first.
What are the new IRA rules for 2026?
The IRS raised contribution limits for 2026, including higher 401(k) and IRA contribution caps. The underlying pro-rata calculation on Form 8606 has not changed, only the annual limits and phase-out ranges that determine backdoor Roth eligibility.
What are the exceptions to the pro-rata rule?
Qualified workplace retirement plans like 401(k)s and 403(b)s are excluded from the aggregation unless you roll that money into an IRA. Inherited IRAs are also tracked separately from your own personal IRAs, so they do not combine into your personal pro-rata calculation.
Do I need to file Form 8606 every year I make a nondeductible contribution?
Yes, Form 8606 should be filed any year you make a nondeductible contribution or complete a conversion, since it is the record the IRS uses to track your basis. Skipping it risks losing your basis record and paying tax twice on the same money, according to IRS guidance on Form 8606.