Backdoor Roth pro-rata calculator: know the bill before the back door

By Tom, CPA · Updated October 25, 2026

TL;DR: This calculator follows the Form 8606 pro-rata formula: your nondeductible basis divided by the year-end value of all your traditional, SEP and SIMPLE IRAs plus the amount you converted. The result is the nontaxable share of the conversion; the rest is taxable income.

Educational only, not tax, legal, or investment advice. Examples are illustrations, not predictions. Rules and limits change; verify current figures and talk to a qualified professional about your situation.

The backdoor Roth looks simple: contribute to a traditional IRA without a deduction, then convert it to a Roth. The catch is the pro-rata rule, which treats all your traditional IRA money as one pot. If that pot holds pre-tax dollars, part of every conversion is taxable, no matter which account you convert from. This calculator shows the bill before you walk through the back door.

CPA Warning Label: Year-end IRA balances drive the math, including SEP and SIMPLE IRAs and money rolled in later in the year. Employer plans like 401(k)s don't count. Form 8606 rules are detailed; check your actual return.

Inputs

• December 31 pre-tax IRA balance: the total value of all traditional, SEP and SIMPLE IRAs at year-end, after the conversion. Do not include Roth IRAs or employer plans.
• Nondeductible contribution: this year’s nondeductible traditional IRA contribution. The 2026 limit is $7,500, or $8,600 at age 50+.
• Prior basis: nondeductible contributions from earlier years not yet recovered (from last year’s Form 8606).
• Conversion amount: the total converted to Roth during the year.
• Marginal rate: your estimated federal bracket, used to estimate tax.

Defaults are examples: a clean backdoor with no other IRA money.

Formula

Simplified from Form 8606, Part I:

1. Total basis = prior basis + this year’s nondeductible contribution.
2. Total value = December 31 IRA balance + amount converted (plus any other distributions, which this tool doesn’t model).
3. Nontaxable percentage = total basis ÷ total value (capped at 100%).
4. Nontaxable conversion = conversion × nontaxable percentage. The rest is taxable.
5. Remaining basis carries forward to next year.

Example

Illustration: Lavish Larry contributes $7,500 nondeductible and converts $7,500, but forgot an old $92,500 rollover IRA. Total value = $92,500 + $7,500 = $100,000. Nontaxable share = $7,500 ÷ $100,000 = 7.5%. Only $562.50 of his conversion is tax-free; $6,937.50 is taxable, roughly $1,665 of tax at 24%. He keeps $6,937.50 of basis for later years.

Frugal Fran, with no other IRA money, converts $7,500 with 100% nontaxable. Tax: $0 (other than tax on any small earnings before converting).

What to do if taxable

• Roll pre-tax IRAs into your employer plan before December 31, if the plan accepts roll-ins. Plan balances aren’t in the formula.
• Convert everything in a low-income year, accepting the tax.
• Skip the backdoor this year and use other accounts.
• Track basis carefully: it isn’t lost, it just gets spread over future conversions and withdrawals.

Read the early access guide and the HSA as a retirement account, then run the contribution limits calculator or the Roth ladder planner.

How to use this calculator

1. Gather your numbers from statements and last year’s return; estimates are fine for a first pass.
2. Replace every example default with your own figure.
3. Change one input at a time to see which assumption moves the result most.
4. Save or screenshot the results with the date, since limits and rates change.
5. Bring the output to whoever prepares your return as a starting point, not a final answer.

Timing and reporting checklist

• Make the nondeductible contribution, then convert once it settles; small earnings in between are taxable.
• Before December 31, confirm no pre-tax money remains in any traditional, SEP or SIMPLE IRA, or that you accept the taxable share.
• File Form 8606 with your return for the contribution and the conversion. Missing it can make the IRS treat basis as zero.
• Expect a Form 1099-R from the custodian showing the conversion; it usually doesn’t know your basis, so the 8606 tells the story.
• Keep every year’s 8606 until all IRA money is withdrawn.

The habit that causes most surprise tax bills isn’t the conversion; it’s forgetting about one old IRA. List every account before you start.

Common questions about the inputs

Use the value on December 31 of the conversion year, not the date of the conversion. If you converted in January and rolled an old IRA into your 401(k) in November, the year-end balance is what counts, which is good news. If you opened a new rollover IRA in December, that balance counts too, even if it arrived after the conversion. When in doubt, pull every year-end statement before filing and add them up.

Limits

• Federal income tax only, at a flat marginal rate; a large conversion can cross brackets.
• Doesn’t model state tax, other IRA distributions in the same year, or inherited IRAs (which are separate).
• Earnings between contribution and conversion are taxable; convert promptly to keep them small.
• For education only; Form 8606 governs.