Backdoor Roth IRA Calculator: How to Use It

What this calculator does

The Backdoor Roth IRA Calculator applies the pro-rata rule to your specific IRA situation. Enter your total pre-tax IRA balances, your non-deductible contribution amount, and the calculator shows what percentage of your conversion is taxable, the tax owed at your marginal rate, and the after-tax cost of doing the conversion versus skipping it.

The calculator also shows the long-term benefit of getting money into Roth: no future required minimum distributions, tax-free growth, and tax-free withdrawals in retirement. The break-even shows when the tax cost of the conversion is recovered in future tax savings.

What each input means

Total pre-tax IRA balance

The combined December 31 balance of all your traditional IRAs, SEP IRAs, and SIMPLE IRAs (not 401(k)s or TSP). This is the denominator in the pro-rata calculation. If you have $200,000 in a traditional IRA rolled over from a 401(k), that entire balance counts even if you're only contributing $7,000 this year.

The December 31 balance is what matters for the pro-rata calculation in that tax year. If you plan to roll pre-tax IRAs into your 401(k) before year-end to clear them out, that matters for the current year's calculation.

Non-deductible contribution amount

The amount you're contributing to a traditional IRA without taking a deduction. The 2026 contribution limit is $7,000 ($8,000 if you're 50 or older). If your income exceeds the Roth IRA phase-out ($150,000 for single filers, $236,000 for married filing jointly in 2026), you can't contribute directly to Roth. The backdoor approach lets you get money in anyway.

Existing IRA basis (prior non-deductible contributions)

The total of all non-deductible contributions you've made to traditional IRAs in prior years that haven't yet been converted or withdrawn. This tracks your cumulative after-tax money already in the IRA system. Find it on line 14 of your most recent Form 8606. If you've never filed Form 8606, this is likely zero.

How the pro-rata rule works

The taxable percentage of a conversion equals your total pre-tax IRA balance divided by (total pre-tax balance plus total after-tax basis). If you have $190,000 pre-tax and $10,000 of basis (non-deductible contributions), 95% of any conversion is taxable and 5% is tax-free.

You can't choose to convert only the after-tax money. The IRS aggregates all your IRAs. The only way to get a clean backdoor Roth with no pro-rata tax is to have zero pre-tax IRA balances before December 31 of the year you convert.

The 401(k) rollover solution

If you have a large pre-tax IRA balance, the most common solution is to roll it into your current employer's 401(k) or 403(b) before doing the backdoor Roth. Many plans accept incoming rollovers. Once the pre-tax IRA is emptied into the 401(k), the pro-rata calculation becomes zero pre-tax IRA balance, and the backdoor Roth conversion is fully tax-free. Confirm your plan accepts rollovers before counting on this strategy.

Form 8606 is not optional

Every year you make a non-deductible IRA contribution, file Form 8606 with your tax return. It tracks your cumulative basis. Without it, the IRS assumes the full conversion is taxable income. If you've been doing backdoor Roth for years without filing 8606, file amended returns for prior years to establish your basis record. The statute of limitations for 8606 is unusual; there's no penalty for late filing if no tax was owed.