Backdoor Roth Calculator: Headroom, Pro-Rata Tax and Form 8606

How it works

Headroom = (limit + catch-up, capped at compensation) − already contributed · Basis ratio = (this year’s non-deductible contribution + earlier basis) ÷ (December 31 pre-tax IRA balance + amount converted), max 1 · Tax-free part = converted × ratio · Taxable = converted − tax-free part · Tax ≈ taxable × (federal + state marginal rate) · Basis carried forward = total basis − tax-free part

A backdoor Roth is a non-deductible contribution to a traditional IRA that you then convert to a Roth IRA. It is the usual route for people whose income is too high to contribute to a Roth directly: for 2026 the direct Roth contribution phases out between $153,000 and $168,000 of modified AGI for single filers and between $242,000 and $252,000 for joint filers, while conversions have no income limit. Your headroom is the IRA limit ($7,500 in 2026, plus a $1,100 catch-up at 50 or older) minus what you have already put into any IRA for the year, and never more than your taxable compensation. The catch is the pro-rata rule. On Form 8606 the IRS adds up all your traditional, SEP and SIMPLE IRAs as of December 31 plus the amount converted, and only the share that is basis (after-tax money) converts tax-free; you cannot convert just the after-tax dollars. Earnings made before the conversion are also taxable. The calculator follows the Form 8606 lines, estimates tax at your marginal rate, shows the basis you carry into next year, compares the tax if you first roll pre-tax IRAs into a 401(k) (which is not counted), checks direct Roth eligibility with the phase-out formula, and projects tax-free growth if you repeat the backdoor each year. It assumes you contribute during the tax year, convert everything, and take no other IRA withdrawals.

Worked example

Single, under 50, $200,000 MAGI, nothing contributed yet and no pre-tax IRA money in 2026: the headroom is $7,500.00. MAGI is above $168,000, so a direct Roth is not allowed and the backdoor is the way in. The basis ratio is $7,500 ÷ (0 + $7,500) = 1, so the whole $7,500 converts tax-free and no basis is left over. Repeated each year for 20 years at 7%, the Roth would hold about $329,000 from $150,000 put in. With a $28,000 pre-tax IRA balance instead, the ratio is 7,500 ÷ 35,500 = 0.21127, so $5,915.49 of the conversion is taxable, about $1,419.72 at 24%, and $5,915.49 of basis carries forward.

Sources: IRS Notice 2025-67 2026 limits (Accounting Today summary) · National Tax Tools: Form 8606 backdoor Roth calculator · US Tax Tools: backdoor Roth calculator · US Tax Tools: Roth IRA pro-rata rule 2026 · Carry: Form 8606 for a backdoor Roth

Frequently asked questions