Backdoor Roth Pro-Rata Calculator 2026

Estimate how much of a 2026 Roth conversion is taxable when you already have pre-tax traditional, SEP, or SIMPLE IRA money and the pro-rata rule applies.

Last updated: April 4, 2026

Calculator

This calculator follows the general Form 8606 pro-rata logic. It is meant for planning and does not replace your actual Form 8606 preparation.

What the pro-rata rule looks at

Your total nondeductible IRA basis
The amount converted to Roth IRA
Other IRA distributions during the year
The year-end value of all traditional, SEP, and SIMPLE IRAs

The IRS does not let you isolate only the after-tax dollars for a Roth conversion if you still hold pre-tax IRA money elsewhere. That is why the year-end balance matters so much.

How to use the result

The most useful output is often the taxable part of the conversion, not just the nontaxable percentage. That taxable amount is what tells you whether the backdoor Roth move is still clean or whether pre-tax IRA balances are making the conversion more expensive than expected.

The remaining basis figure matters too. If not all basis is used during the year, some after-tax basis can carry forward and continue affecting later conversions or distributions.

Examples

Clean conversion example

If the year-end IRA balance is zero and the only IRA money involved is a fresh nondeductible contribution, the conversion can be mostly nontaxable.

Large pre-tax balance example

If there is a large traditional, SEP, or SIMPLE IRA balance at year-end, only a fraction of the conversion may be treated as after-tax even though the current contribution itself was nondeductible.

Carryforward basis example

If the taxpayer does not use all basis during the year, some basis remains and must continue to be tracked on future Form 8606 filings.

Common mistakes

Ignoring SEP and SIMPLE IRA balances in the denominator.
Thinking only the converted IRA account matters.
Forgetting prior nondeductible basis from earlier years.
Assuming the entire conversion is tax-free because the new contribution was after-tax.

FAQ

Does this calculator handle every Form 8606 scenario?

No. It is designed for clear pro-rata planning, not every special reporting case.

Why does year-end value matter so much?

Because the IRS denominator includes year-end traditional, SEP, and SIMPLE IRA balances, which changes the after-tax percentage applied to the conversion.