Backdoor Roth Pro-Rata Rule 2026: Taxable Conversion Math and Calculator

Fast answer

The pro-rata rule looks at your total nondeductible IRA basis and compares it to the combined value of your year-end traditional, SEP, and SIMPLE IRAs plus your distributions and conversions during the year. The result determines what percentage of a Roth conversion is nontaxable.

Use the pro-rata calculator to estimate the taxable and nontaxable portions of a 2026 Roth conversion.

Why the pro-rata rule matters

Many backdoor Roth explainers describe the clean version of the strategy: make a nondeductible Traditional IRA contribution and convert it soon after. That can work smoothly when you have no other pre-tax IRA balances. The trouble starts when you already have pre-tax money in other IRAs, because the IRS generally treats all of those IRA dollars as one pool for basis-tracking purposes.

That means a conversion that feels like it should be “mostly after-tax” can still trigger taxable income if there is a larger pre-tax IRA balance sitting elsewhere at year-end.

The core formula in plain English

Start with your total basis, which includes prior nondeductible IRA basis plus current nondeductible contributions. Then divide that basis by the denominator used in Form 8606-style pro-rata math: year-end value of all traditional, SEP, and SIMPLE IRAs plus your distributions and conversions during the year. That ratio tells you what share of the conversion is nontaxable.

Examples

Clean backdoor Roth case

If you have no year-end pre-tax IRA balance and convert a fresh nondeductible contribution, the conversion can be mostly or fully nontaxable, assuming there is little or no earnings difference.

Large pre-tax IRA balance case

If you have a $7,500 nondeductible contribution but also a $50,000 year-end pre-tax IRA balance, only a small fraction of the conversion may be treated as after-tax under the pro-rata formula. Most of the conversion can still be taxable.

Common mistakes

• Ignoring SEP and SIMPLE IRA balances when thinking about the denominator.
• Assuming the current nondeductible contribution can be isolated on its own.
• Forgetting to track prior-year basis.
• Missing the year-end timing effect of IRA balances.

Why year-end balance planning matters so much

The year-end IRA balance is often the hidden variable that changes the whole result. A taxpayer can make a clean nondeductible contribution and convert it quickly, but if there is still a large pre-tax IRA balance sitting elsewhere on December 31, the pro-rata denominator can still make much of the conversion taxable.

That is why many backdoor Roth discussions eventually turn into asset-location or rollover discussions. The backdoor Roth move itself may be simple, but the surrounding IRA balance picture determines whether the conversion stays efficient.

Examples that show the difference

Mostly clean conversion

A taxpayer with no meaningful year-end pre-tax IRA balance may see the conversion come out mostly nontaxable, especially if little time passed between contribution and conversion.

Messy pooled-balance conversion

A taxpayer with a large existing SEP or Traditional IRA balance can see most of the same-size conversion become taxable because the after-tax basis is small relative to the full IRA pool.

Partial basis carryforward

If not all basis is used during the year, some basis remains and continues to matter on later Form 8606 filings. That is why careful basis tracking is not optional here.

Who should read this carefully

This guide is most useful for taxpayers who are over the direct Roth IRA income limit and are considering a backdoor Roth strategy while still holding pre-tax IRA money. It also matters for anyone who already made a nondeductible IRA contribution and wants to understand the tax impact before completing or reporting a conversion.

Why SEP and SIMPLE IRA balances catch people off guard

A common misconception is that the pro-rata rule only cares about the specific Traditional IRA used for the backdoor Roth step. In reality, SEP IRA and SIMPLE IRA balances usually join the same pool for this purpose. That is why a taxpayer can follow the basic backdoor Roth checklist perfectly and still get a surprisingly taxable result.

This matters most for self-employed taxpayers and small-business owners, because those are the same people who often build larger pre-tax IRA balances through SEP or SIMPLE contributions. If that is your situation, the backdoor Roth question is really a balance-sheet question, not just a contribution question.

Common cleanup paths before December 31

The most discussed cleanup path is reducing or removing pre-tax IRA balances before year-end so the denominator is smaller. In some cases that means moving eligible pre-tax IRA money into an employer plan that accepts rollovers. In other cases it may mean deciding that a backdoor Roth is not the cleanest move this year.

The key point is not that everyone should do the same cleanup step. It is that the December 31 IRA balance can change the tax result dramatically. If you ignore that deadline and only focus on the conversion date, the pro-rata rule can undo the strategy you thought you were implementing.

FAQ

Does the pro-rata rule look only at the IRA I converted from?

No. It generally looks across your traditional, SEP, and SIMPLE IRAs as one pool for basis purposes. That is why money in a different account can still affect the taxable share of the conversion.

Can a quick contribution-and-conversion still be taxable?

Yes. Fast timing can reduce earnings between contribution and conversion, but it does not erase the pooled-balance rule. If you still have significant pre-tax IRA money at year-end, the conversion can remain partly taxable.

Where should I check if I am deciding between direct Roth and backdoor Roth?

Start with the Roth IRA income limits. If your income is above the direct Roth range, then compare that page with this pro-rata guide and the Traditional IRA deduction guide so you can see how the contribution and conversion pieces fit together.