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 December 31 IRA value, your after-tax basis and the amount you converted, and the calculator follows Form 8606 lines 5 through 18 to show what share of the conversion is taxable and how much basis carries forward.

Money in a Roth IRA has no required minimum distributions during your life, and qualified withdrawals are tax-free. The calculator doesn't project future growth or a break-even; it reports the tax result for the year you convert.

What each input means

December 31 IRA balance (Form 8606 line 6)

The combined value on December 31 of the conversion year of all your traditional IRAs, SEP IRAs, and SIMPLE IRAs (not 401(k)s or TSP), after the conversion has left. If your only IRA was the one you converted, enter $0. The calculator adds the conversion back in (line 9 = lines 6 + 7 + 8), so don't add it yourself. 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 $7,500 this year.

The December 31 value is what counts for that tax year. A pre-tax IRA rolled into your 401(k) any time before December 31 is out of the calculation, even if you converted earlier in the year.

After-tax basis (Form 8606 line 5)

This year's non-deductible contribution plus any basis from earlier years (line 14 of your most recent Form 8606). If you've never filed Form 8606, your prior basis is likely zero. The 2026 contribution limit is $7,500 ($8,600 if you're 50 or older). The Roth IRA direct contribution phases out between $153,000 and $168,000 of modified AGI for single and head of household filers, between $242,000 and $252,000 for married filing jointly, and between $0 and $10,000 for married filing separately in 2026 (IRS Notice 2025-67). Above those upper limits you can't contribute directly to Roth at all. The backdoor approach lets you get money in regardless of income.

Conversion amount (Form 8606 line 8)

The net amount you converted from traditional IRAs to Roth IRAs during the year. If the contribution earned a little before you converted, enter the full amount converted; the earnings are taxable.

How the pro-rata rule works

Form 8606 line 10 divides your basis (line 5) by line 9, which is your December 31 IRA value plus this year's conversions and other distributions. Say you hold $192,500 of pre-tax money, contribute $7,500 non-deductible and convert that $7,500. On December 31 the IRA is worth $192,500, so line 9 is $200,000 and 3.75% of the conversion ($281) is tax-free. The other $7,219 is taxable, and $7,219 of basis carries forward on line 14.

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 on 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, you have no record of basis and the conversion can be taxed as if it were all pre-tax. Not filing a required 8606 for a non-deductible contribution carries a $50 penalty unless you can show reasonable cause. If you skipped it in earlier years, a tax preparer can help you file the missing forms to establish your basis.