Backdoor Roth IRA Calculator
The contribution, the conversion, and every IRA you own
Your result will appear here
Fill in the fields on the left and this updates as you type.
Know what this estimate is based on
- Jurisdiction
- General mathematical model
- Scope and limitations
- Long-range scenario, not a guarantee. Small changes in returns, inflation, fees, taxes, and withdrawal timing can materially change the result.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Enter the nondeductible traditional IRA contribution you are making — $7,500, or $8,600 from age 50.
- 02
Enter the pre-tax balance across EVERY traditional, SEP and SIMPLE IRA you own on 31 December, not counting the new contribution. This is the field that decides everything.
- 03
Enter the amount you intend to convert to Roth, then your marginal federal rate and your state rate.
- 04
Add your modified AGI — the page reports what a direct Roth contribution would still allow, which is usually the reason you are here.
- 05
Read the tax, then the two figures beside it: the share of the conversion that is taxable, and what the pre-tax balance is costing you against a clean conversion.
Formula
Basis = prior Form 8606 basis + this year's nondeductible contribution. Total IRA value = pre-tax balance across all traditional, SEP and SIMPLE IRAs on 31 December + the new contribution + anything it earned. Nontaxable share = basis ÷ total value. Taxable amount = conversion × (1 − that share). Tax = taxable amount × (federal rate + state rate). Basis left behind = basis − the untaxed part of the conversion, carried forward on Form 8606 line 14.
Example
A $7,500 nondeductible contribution converted the same week, at $205,000 of MAGI where a direct Roth contribution is $0. With no other IRA money the pot is $7,500, all of it basis, and the conversion is tax-free — the manoeuvre working as intended. Add a $45,000 rollover IRA and the pot becomes $52,500 against the same $7,500 of basis: 14.3% of the conversion comes across untaxed and 85.7%, $6,429, is taxable. At 32% federal plus 5% state that is $2,379, and $6,429 of basis is stranded in the IRA to be recovered a slice at a time over the years.
Definitions
- Pro-rata rule
- The requirement that every distribution or conversion from a traditional IRA carry basis and pre-tax money in the same ratio as the whole of your traditional, SEP and SIMPLE IRAs.
- Form 8606
- The form that reports nondeductible contributions and carries the running basis figure forward. It exists nowhere else — an unfiled 8606 means the money is taxed twice.
- Reverse rollover
- Moving an IRA balance into a workplace 401(k) or 403(b). Plan balances sit outside the pro-rata denominator, so this is the standard fix.
- Conversion five-year clock
- Each converted amount must season five years, from 1 January of the conversion year, before it can be withdrawn before 59½ without the 10% penalty.
Good to know
A door that was left open on purpose
Two rules sit side by side in the code. You may not contribute to a Roth IRA once your income passes a band — $153,000 to $168,000 for a single filer in 2026, $242,000 to $252,000 married filing jointly. But you may contribute to a traditional IRA at any income, and since 2010 you may convert a traditional IRA to a Roth at any income too: the conversion income limit was repealed outright that year and never came back. So the manoeuvre is simply the two rules used in sequence — contribute non-deductibly, then convert. It is not a loophole in the sense of something hidden. The 2017 tax act's conference report explicitly noted that taxpayers do this, Congress declined to stop it, and no statute, regulation, ruling or case has ever challenged the sequence. There is no required waiting period, and the popular advice to "wait a year" mostly guarantees the account earns something that becomes taxable at conversion.
The rule that makes it go wrong
The step that fails is the conversion, and it fails because of an aggregation rule most people meet for the first time in the tax bill. For the purposes of Form 8606, every traditional, SEP and SIMPLE IRA you own is a single pot. When you convert, you cannot elect to convert only the freshly contributed after-tax dollars: the conversion comes out part basis and part pre-tax in the ratio the whole pot stands in. So the person for whom this works perfectly is the person who owns no other IRA money at all — pot $7,500, basis $7,500, tax $0. The person for whom it goes badly is the far more common one who rolled an old 401(k) into an IRA at some point. Put a $45,000 rollover IRA beside the same $7,500 and the pot becomes $52,500 with $7,500 of basis, so 85.7% of the conversion is taxable: $6,429 of income, and at 32% federal plus 5% state, $2,379 of tax on a move the internet told them was free. It gets worse as the balance grows, asymptotically: at $250,000 of pre-tax money 97% of the conversion is taxable and the manoeuvre is simply a conversion with extra steps.
The fix, and its deadline
The denominator counts IRAs. It does not count workplace plans — so moving the pre-tax balance into a 401(k), 403(b) or governmental 457(b) that accepts rollovers in takes it out of the calculation entirely and the conversion goes back to costing nothing. Most large plans accept incoming rollovers; someone with any self-employment income can open a solo 401(k) for the purpose. The detail that turns this from planning advice into a rescue is the measurement date: the pot is measured on 31 DECEMBER, not on the day you convert. So a conversion already made in March can still be cleaned up by a rollover completed in November, and the tax bill changes accordingly. Two things the fix does not reach: a spouse's IRAs, which are aggregated separately on their own Form 8606 and therefore never contaminate yours, and basis stranded from earlier years, which stays on line 14 and comes back a slice at a time for as long as the account exists.
Two ways to lose money on the plumbing
First, pay the tax from a bank account and never from the conversion. Withholding taken out of a conversion never arrives in the Roth — it is a distribution, and under 59½ it carries the 10% early-withdrawal penalty on top of the income tax you were trying to pay. A $2,379 bill settled by withholding costs a further $238 in penalty and permanently shrinks the Roth by the amount withheld. Second, each converted amount starts its own five-year clock, running from 1 January of the conversion year, before it can be withdrawn before 59½ without that same 10% penalty. That clock is separate from the five-year clock on Roth earnings, and separate again for every year you convert, so someone running the manoeuvre annually is running a queue of overlapping clocks. Neither of these matters at all if the money is genuinely long-term, which is the only sensible reason to be doing this — but the second is exactly why a backdoor Roth is a poor place to park money you might need in three years.
Frequently asked questions
What is the pro-rata rule?
Form 8606 treats every traditional, SEP and SIMPLE IRA you own as a single pot. Any conversion comes out part basis and part pre-tax in the ratio that pot stands in — you cannot elect to convert only the nondeductible money. So the taxable share is your pre-tax balance divided by the total, and it applies no matter which account the money physically left.
Why does one old rollover IRA ruin it?
Because it goes straight into the denominator. A $7,500 contribution converted with nothing else is 100% basis and costs nothing. Put a $45,000 rollover IRA beside it and the pot is $52,500, of which $7,500 is basis — so 85.7% of the conversion is taxable, and at 32% federal plus 5% state that is $2,379 on a move that was supposed to be free.
How do I fix it?
Move the pre-tax money somewhere the rule cannot see it. A 401(k), 403(b) or governmental 457(b) that accepts rollovers in will take it, and a workplace plan balance is not in the pro-rata denominator. The timing is the part people miss: the denominator is measured on 31 December, not on the day you convert, so a rollover completed by year end still rescues a conversion you have already made.
Is the backdoor Roth legal?
Yes. The income limit on Roth conversions was repealed in 2010, and there has never been a limit on contributing to a traditional IRA. The 2017 tax act's conference report explicitly acknowledged taxpayers doing exactly this, and no statute, regulation or ruling has challenged the sequence. There is no required waiting period between the contribution and the conversion.
Should I pay the tax out of the conversion?
No. Anything withheld from the conversion never reaches the Roth, and under 59½ it is a distribution that carries the 10% early-withdrawal penalty on top of the income tax. Pay from a bank account.
Does my spouse's rollover IRA count?
No. Each person's IRAs are aggregated on their own Form 8606 — there is no household pot. On a joint return the spouse with no pre-tax IRA can run a clean backdoor Roth while the other cannot. Enter one person's numbers at a time.