Backdoor Roth Pro-Rata Calculator

How much of your backdoor Roth conversion is taxable?

If you hold pre-tax money in any traditional, SEP or SIMPLE IRA, the IRS will not let you convert only your after-tax dollars. This works the Form 8606 pro-rata formula for you, and shows what rolling that pre-tax money into a 401(k) before December 31 would save.

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1. Your Roth conversion

Amount converted to Roth in 2026 ($)

Nondeductible IRA contribution for 2026 ($)

Basis carried from prior Form 8606 ($)

Prior basis is line 14 of last year's Form 8606. Enter 0 if you have never made a nondeductible contribution.

2. Your IRAs on December 31

Total value of all traditional, SEP and SIMPLE IRAs on Dec 31, after the conversion ($)

Other IRA distributions in 2026, not conversions ($)

Add every account you own across every custodian. Roth IRAs and employer 401(k)/403(b) balances are not counted.

3. Optional

Your marginal federal tax rate (%)

Your age at the end of 2026

Enter your conversion and IRA balances, then click Calculate.

What this does not cover

Federal tax only. State treatment of conversions varies and is not modelled.
Simplified Form 8606. Line 4 (contributions made after year-end and counted for the prior year), inherited IRAs, and prior-year distributions of basis are not modelled.
Tax is estimated at one flat marginal rate. A large conversion can push part of your income into a higher bracket and affect credits, Medicare premiums and the taxable share of Social Security.
Roth income limits and the 5-year rule. This assumes the contribution and conversion were made correctly under the rules for your situation.
Point-in-time 2026 figures. Limits are re-indexed annually; confirm the current year's figures before relying on this in a later year.

Common questions

What is the pro-rata rule for a backdoor Roth IRA?

The IRS treats all of your traditional, SEP, and SIMPLE IRAs as one pot, measured on December 31. When you convert to Roth, the tax-free share of the conversion is your total after-tax basis divided by the total value of that pot plus the amount converted. You cannot pick only the after-tax dollars to convert.

How do I avoid the pro-rata rule?

Move your pre-tax traditional, SEP, and SIMPLE IRA money into an employer 401(k) or a Solo 401(k) that accepts roll-ins before December 31. Money in a 401(k) is not counted on Form 8606, so the ratio goes to 100% and the backdoor conversion is tax-free. Only pre-tax money can be rolled in; the plan must accept it.

Does the pro-rata rule count my 401(k) or my Roth IRA?

No. Only traditional, SEP, and SIMPLE IRAs are counted, on December 31 of the conversion year. Balances in an employer 401(k), 403(b), or any Roth account are not part of the calculation.

Which line of Form 8606 is the taxable amount?

Part I of Form 8606 divides your total basis by your year-end IRA value plus distributions plus the conversion. Multiplying the conversion by that ratio gives the nontaxable part; the rest is taxable and flows to your return.

Is this tax advice?

No. This tool applies the published Form 8606 formula and the 2026 IRA limits to the numbers you enter. It does not replace the form's instructions or a qualified tax professional.