Backdoor Roth IRA Calculator | Estimate Your Pro-Rata Tax Before Converting

You earn too much for a Roth IRA. That’s a good problem, and it has a fix.

Here’s the annoying part about Roth IRAs: the people who could benefit most from decades of tax-free growth are the ones locked out. For 2025, if you’re single and your modified adjusted gross income lands between $150,000 and $165,000, your ability to contribute directly starts shrinking. Above $165,000, the door shuts completely.

The backdoor Roth IRA is the legal workaround. Nothing shady, no loophole the IRS is trying to close. It’s two ordinary, allowed steps done in sequence: you contribute to a traditional IRA (which has no income limit), then you convert that money to a Roth IRA (which also has no income limit). Congress knows about it. Thousands of high earners do it every year.

The catch, and it’s a real one, is the pro-rata rule. That’s what the calculator above is for. Enter your numbers in the calculator above before you convert anything, because the tax bill on a backdoor Roth can range from basically zero to painfully large depending on what else sits in your IRA accounts.

How does a backdoor Roth IRA work?

Step one is the contribution. For 2025 you can put up to $7,000 into a traditional IRA ($8,000 if you’re 50 or older). Because your income is too high, you almost certainly can’t deduct this contribution, so it goes in as after-tax money. That’s fine. That’s the plan.

Step two is the conversion. You move that $7,000 from the traditional IRA into your Roth IRA. A conversion is technically a taxable event, but you’re converting dollars that were already taxed, so there’s nothing new to tax. As long as the pro-rata rule doesn’t get involved.

Step three is the paperwork. You report the non-deductible contribution and the conversion on Form 8606 with your tax return. Don’t skip this. Form 8606 is how you prove to the IRS which dollars were already taxed. Without it, you can end up paying tax twice on the same money, and that’s a miserable discovery to make years later.

The pro-rata rule, in plain English

This is the part that trips people up, so let’s slow down.

The IRS won’t let you point at your accounts and say “convert just the after-tax $7,000, please.” Instead, it treats all your traditional, rollover, SEP, and SIMPLE IRAs as one big combined account. (Your 401(k) doesn’t count. Your spouse’s IRAs don’t count. Inherited IRAs don’t count.) When you convert, the IRS insists your conversion contain the same mix of pre-tax and after-tax money as that combined pool.

The math is simple division: your after-tax basis divided by your total IRA balance across all those accounts. That percentage of your conversion is tax-free. The rest is taxed as ordinary income.

A real backdoor Roth example with numbers

Meet Priya. She’s 38, single, earns $185,000, and has no existing IRA balances. She contributes $7,000 (non-deductible) to a traditional IRA on Monday and converts $7,000 to her Roth on Tuesday.

Her after-tax share: $7,000 divided by $7,000 = 100%. Taxable amount of the conversion: about $0. (If the money earned $12 of interest overnight, that $12 is taxable. Convert quickly and this stays tiny.) Clean, simple, done.

Now meet David. Same income, same $7,000 contribution. But David has a $93,000 rollover IRA from an old 401(k) sitting around. His combined IRA pool is $100,000, and only $7,000 of it is after-tax.

His after-tax share: $7,000 divided by $100,000 = 7%. So of his $7,000 conversion, only about $490 escapes tax. The other $6,510 gets taxed as ordinary income. At a 24% marginal rate, that’s roughly $1,560 of federal tax to move $7,000 into a Roth. Same strategy, wildly different bill.

David’s fix, and the standard advice here: roll that pre-tax IRA money into his current employer’s 401(k) before December 31 of the conversion year. Since 401(k) balances are excluded from the pro-rata calculation, his pool drops back to just the $7,000 and his conversion becomes clean again. Check with HR first. Not every plan accepts incoming rollovers.

Why the December 31 rule matters

The IRS measures your IRA balances as of December 31 of the year you convert. Move money around in January all you want. What counts is where things stand on New Year’s Eve. Plan any 401(k) rollover early in the year so you’re not scrambling in December.

What the calculator assumes

The calculator above follows the same logic as IRS Form 8606. It takes your total pre-tax IRA balance, your after-tax basis, and your planned conversion amount, then applies the pro-rata fraction. A few things to know about the estimate:

• IRA balances are measured at year-end, per IRS rules.
• 401(k), 403(b), Roth IRA, and inherited IRA balances are excluded (correct, the IRS excludes them too).
• Any growth between contribution and conversion is taxable. Converting within days keeps this near zero.
• Tax is estimated at the marginal rate you enter. Your actual bill depends on your full return.

It doesn’t model state taxes, the 5-year rules, or RMD interactions. For the RMD side of the picture, the RMD calculator shows the withdrawals the IRS will eventually force on whatever traditional balance you leave unconverted. Those matter at the edges, and they’re covered in the FAQs below.

Mistakes that cost people real money

Forgetting Form 8606. Even good tax software sometimes needs a nudge. Confirm the form is actually in your filed return.

Converting late in the year with a messy IRA. If you can’t clear pre-tax balances by December 31, consider waiting until January so the conversion lands in a cleaner tax year.

Ignoring the 5-year rules. Converted dollars have their own 5-year clock before you can withdraw them penalty-free, separate from the 5-year rule on your first Roth contribution. Every conversion starts its own clock.

Treating it as a one-time trick. You can do a backdoor Roth every year you have eligible compensation. It compounds. Literally.

Who this is for (and who should skip it)

This strategy fits high earners above the Roth IRA income limits who have little or no pre-tax IRA money, or who can move pre-tax money into a 401(k) first. If your income sits below the phase-out, just contribute to a Roth directly and skip the extra steps.

If you’re carrying a large pre-tax IRA you can’t move into a 401(k), run the numbers honestly. Sometimes the conversion tax still makes sense measured against decades of tax-free growth. Sometimes it doesn’t. The calculator gives you the taxable amount. The judgment call is yours, or your CPA’s. And if the real alternative is investing that money in a taxable brokerage account instead, the capital gains tax calculator shows what the IRS takes when you eventually sell, which is the number a conversion’s tax bill has to beat.

One more thing. The mega backdoor Roth is a different animal: after-tax contributions inside a 401(k), converted to Roth, with a far higher ceiling. If your employer’s plan allows it, it’s worth a separate look. But the mechanics and limits are completely different from everything described here.

This calculator gives an educational estimate, not tax advice. Roth conversions are irreversible, so talk to a qualified tax professional before you convert.