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Backdoor Roth Pro-Rata Calculator

Enter pre-tax balance, after-tax basis and conversion amount to get the taxable portion.

Result

$6,494.85
$505.15
92.78%

How it is calculated

This tool applies the United States IRS pro-rata rule for Roth conversions. Enter the total pre-tax IRA balance (traditional, SEP and SIMPLE IRAs together), the after-tax basis you have in those IRAs, and the amount you are converting, all in US dollars.

taxable % = pre-tax ÷ (pre-tax + after-tax)

taxable amount = conversion × taxable %

tax-free amount = conversion − taxable amount

Every dollar you convert is treated as a blend of pre-tax and after-tax money in that same proportion; you cannot pick the after-tax dollars first. Rules for other countries differ, and the IRS looks at your balances at the end of the year, so later contributions or rollovers can change the percentage. The tool does not check that the conversion is no larger than your total balance, and it does not calculate the tax rate itself.

Worked example

You hold $90,000 in a pre-tax traditional IRA and just made a $7,000 non-deductible contribution, which you convert in full.

  1. Total IRA money: 90,000 + 7,000 = 97,000.
  2. Taxable share: 90,000 ÷ 97,000 = 92.78%.
  3. Taxable amount: 7,000 × 0.9278 = $6,494.85.
  4. Tax-free amount: 7,000 − 6,494.85 = $505.15.

Without the $90,000 pre-tax balance the taxable share would be 0%, and the whole $7,000 would be tax-free.

Try this example →

Common mistakes

  • Looking only at the account being converted. The rule counts all traditional, SEP and SIMPLE IRAs together, so a separate pre-tax IRA at another bank still raises the taxable percentage.
  • Assuming a 401(k) balance counts in the pre-tax figure. The pro-rata calculation covers IRAs only; a workplace plan balance is not included here.
  • Mixing up the after-tax field with the conversion amount. After-tax is the total basis you have built up in non-deductible contributions, not the amount you plan to move this time.
  • Thinking the taxable amount is the tax owed. The result is the portion added to your income; the tax is that amount times your marginal rate.
  • Entering an old balance. The figure should be the pre-tax balance that applies when the conversion is reported, since later contributions or rollovers into an IRA change the share.

Sources

About this calculator

A “backdoor Roth” — contributing after-tax dollars to a traditional IRA and then converting them to a Roth — seems simple, but the IRS's pro-rata rule complicates it if you hold any other traditional, SEP, or SIMPLE IRA balances. The rule requires the taxable portion of every conversion to be calculated across your entire IRA balance, not just the account you're converting: taxable % = pre-tax balance ÷ (pre-tax balance + after-tax basis).

Applying that percentage to your conversion amount tells you the taxable and tax-free portions — this is the calculation reported on IRS Form 8606. Financial planners and DIY investors use this before executing a backdoor Roth to avoid an unpleasant surprise tax bill: even a small pre-tax IRA balance elsewhere can make most of a “backdoor” conversion taxable.

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