For high earners, the front door to a Roth IRA is locked. Once your income passes the IRS limit, direct Roth contributions are off the table. The backdoor Roth exists for exactly this situation. You contribute to a Traditional IRA without taking a tax deduction, then convert that money to a Roth IRA.

That two-step move sounds clean, but one IRS rule decides how much of your conversion is actually tax-free. It is called the pro-rata rule, and it says you cannot point at just the after-tax dollars and convert only those. Every dollar you convert carries pre-tax and after-tax money in the same ratio as your entire IRA balance.

The Backdoor Roth Calculator runs that ratio for you. Enter your nondeductible contribution, your existing pre-tax IRA balance, and the amount you plan to convert, and it shows the taxable part and the nontaxable part side by side, with the exact percentage the IRS will apply.

What the pro-rata rule actually says

The pro-rata rule is the IRS answer to a simple temptation. Imagine you have thirty thousand dollars of pre-tax IRA money and you add seven thousand dollars of after-tax money. It would be convenient to convert only the seven thousand and call it tax-free. The rule blocks that move. It treats every dollar in your Traditional, SEP, and SIMPLE IRAs as a blend of pre-tax and after-tax money, in proportion to the totals. When you convert any amount, the taxable slice of that conversion equals your pre-tax share of the whole pot. The rule does not care which account the money sits in or which contribution you intended to convert. It only cares about the totals across every IRA you own on December 31 of the conversion year.

Why the IRS treats all your IRAs as one pot

The aggregation part of the rule surprises almost everyone. The IRS does not look at the single IRA you converted from. It adds up the balances of every Traditional, SEP, and SIMPLE IRA in your name, even if they sit at different brokerages. Your Roth IRAs are excluded, and your employer plan such as a 401(k) is excluded, but every flavor of individual retirement account that holds pre-tax money counts. This is why opening a fresh empty IRA for the backdoor contribution does not help. The new account holds your seven thousand dollars of after-tax money, but the aggregation rule immediately mixes it with the old rollover IRA across the street. One pot, one ratio, no exceptions.

Your nondeductible contribution this year

This is the after-tax money you add as the first half of the backdoor move. You contribute to a Traditional IRA and deliberately do not claim a deduction, which creates basis, the already-taxed portion of your IRA. For most people this is the annual IRA limit, seven thousand dollars, or eight thousand if you are fifty or older. You report it on Form 8606 so the IRS knows that slice of your IRA has already been taxed. In the calculator, this is the first input, and it forms the nontaxable side of the ratio. Without this contribution there is no backdoor, and without the Form 8606 filing the IRS has no record of your basis.

Your existing pre-tax IRA balance

This is the number that usually causes the trouble. It includes every pre-tax dollar in your Traditional, SEP, and SIMPLE IRAs: old 401(k) rollovers, deductible contributions from years when your income was lower, and all the investment growth those dollars produced. Growth is pre-tax even if the original contribution was after-tax, which catches people off guard. If you rolled a fifty thousand dollar 401(k) into an IRA five years ago, that entire balance, plus whatever it has earned, sits on the pre-tax side of the ratio. The larger this balance, the larger the taxable slice of any conversion. In the calculator, this is the second input, and it is the single biggest driver of your result.

The amount you plan to convert

This is the dollar figure you intend to move from Traditional to Roth. Most backdoor Roth savers convert the full contribution, often within days of making it, so the conversion amount equals the nondeductible contribution. But the calculator lets you enter any amount up to your combined balance, because partial conversions follow the same rule. Whether you convert seven thousand or seventy thousand, the taxable percentage is identical. Only the dollar amounts change. One guardrail applies: the conversion cannot exceed your combined IRA balance, because you cannot convert money that does not exist.

Your combined IRA balance

The calculator adds your nondeductible contribution to your existing pre-tax balance to get the denominator of the pro-rata fraction. This combined balance is the total pot the IRS pretends all your IRAs form. With a seven thousand dollar contribution and a thirty thousand dollar pre-tax balance, the combined balance is thirty seven thousand dollars. Every percentage in the result flows from this total. If the combined balance is wrong, the taxable share is wrong, so it pays to use the December 31 balances and to include every Traditional, SEP, and SIMPLE IRA you own.

Your pre-tax share of the total

Divide the pre-tax balance by the combined balance and you get the master percentage. In the example above, thirty thousand divided by thirty seven thousand is about 81.08 percent. That single number decides everything. It means 81.08 percent of every dollar converted is taxable ordinary income, and the remaining 18.92 percent converts tax-free. The calculator displays this share and then applies it to your conversion amount. Notice what the share ignores: it does not matter how long each dollar has been in the account, which brokerage holds it, or what you intended. The ratio is purely mechanical.

Worked Example: The clean first-year backdoor

Maria has no existing IRA balance at all. She contributes seven thousand dollars as a nondeductible Traditional IRA contribution and converts seven thousand dollars to Roth a week later. Her combined balance is seven thousand dollars. Her pre-tax share is zero divided by seven thousand, which is zero percent. Her taxable conversion is seven thousand times zero percent, which is zero dollars. Her nontaxable conversion is the full seven thousand dollars. This is the textbook backdoor Roth, the result everyone hopes for, and it only happens when the pre-tax balance is truly zero on December 31.

Worked Example: The rollover-heavy saver

James has a thirty thousand dollar rollover IRA from an old job. He adds a seven thousand dollar nondeductible contribution and converts seven thousand dollars. His combined balance is thirty seven thousand dollars. His pre-tax share is thirty thousand divided by thirty seven thousand, which is 81.08 percent. His taxable conversion is seven thousand times 81.08 percent, which is 5,675.68 dollars. His nontaxable conversion is seven thousand minus 5,675.68, which is 1,324.32 dollars. James expected a tax-free conversion and instead owes tax on more than eighty percent of it. This is the classic pro-rata surprise, and it is exactly what the calculator is built to reveal before you convert.

Worked Example: The partial conversion

Priya has a thirteen thousand dollar pre-tax IRA balance and makes a seven thousand dollar nondeductible contribution, giving a combined balance of twenty thousand dollars. She decides to convert ten thousand dollars, not the full twenty thousand. Her pre-tax share is thirteen thousand divided by twenty thousand, which is 65 percent. Her taxable conversion is ten thousand times 65 percent, which is 6,500 dollars. Her nontaxable conversion is ten thousand minus 6,500, which is 3,500 dollars. Converting less did not change the percentage. The same 65 percent would apply if she converted the full twenty thousand, which would make thirteen thousand dollars taxable.

Worked Example: Earnings before conversion

David contributes seven thousand dollars and waits three months before converting. The money grows to seven thousand two hundred dollars. He has no other IRA balance, so he assumes the conversion is tax-free. It is not. The two hundred dollars of growth is pre-tax money, so his combined balance is seven thousand two hundred dollars and his pre-tax share is two hundred divided by seven thousand two hundred, which is 2.78 percent. Converting the full seven thousand two hundred makes two hundred dollars taxable and seven thousand dollars nontaxable. The lesson is to convert quickly, before earnings create a pre-tax balance out of nothing.

The pro-rata formula

The formula is:

Taxable conversion = Conversion amount x (Pre-tax IRA balance / (Nondeductible contribution + Pre-tax IRA balance))

The nontaxable part is simply the conversion amount minus the taxable part. The fraction in parentheses is your pre-tax share of the total, and it is the only number that matters. If you remember one formula from this page, remember that every conversion is split by that single ratio, no matter how many accounts you use or how carefully you label your transfers.

How to shrink your taxable share

The taxable share has only two ingredients, so shrinking it means changing one of them. You can raise the after-tax side by converting in a year when your basis is high, but the practical lever is lowering the pre-tax side before December 31. Every dollar of pre-tax balance you remove from your IRAs before year end reduces the numerator of the fraction. Some people convert their entire IRA balance, pre-tax and all, accepting one large tax bill to clear the decks. Others move the pre-tax money somewhere the aggregation rule cannot reach. That second option has a name, and it deserves its own section.

The reverse rollover escape hatch

A reverse rollover moves your pre-tax IRA balance into an employer plan such as a 401(k). Employer plans are invisible to the pro-rata rule, so the money stops counting toward your pre-tax share. After the rollover, your IRAs hold only after-tax basis, your pre-tax share drops to zero, and the next conversion is fully tax-free. The timing rule is strict. The money must be out of your IRAs by December 31 of the conversion year. A rollover that settles on January 3 does not help the prior year. Also confirm your 401(k) accepts incoming rollovers and will take only the pre-tax portion, since after-tax basis cannot go into most employer plans.

What changes when your pre-tax balance is zero

Zero is the magic number. With no pre-tax balance in any Traditional, SEP, or SIMPLE IRA on December 31, the pre-tax share is zero percent and the entire conversion is nontaxable. This is why experienced backdoor Roth savers guard a zero pre-tax balance so carefully. They roll old 401(k) money into their current employer plan instead of an IRA, they convert promptly to avoid earnings, and they check every IRA they own before year end. If your balance is already zero, the calculator will confirm what you hope: a fully tax-free conversion, every year, with no pro-rata drag.

Mistakes that inflate the taxable share

The most expensive mistake is converting first and rolling over later. If the pre-tax balance is still in your IRA on December 31, the conversion is already prorated, and a January rollover cannot undo it. The second mistake is forgetting an old SEP or SIMPLE IRA from freelance work, which quietly counts in the aggregation. The third is letting the contribution sit for months, so earnings add pre-tax dollars you never planned for. The fourth is using an estimated balance instead of the actual December 31 figure, which makes the Form 8606 math wrong. Each of these mistakes raises the taxable share, and each is avoidable with a year-end checklist.

Frequently Asked Questions

1. Does the pro-rata rule apply if I have no pre-tax IRA money?

No. With a zero pre-tax balance across all your Traditional, SEP, and SIMPLE IRAs, your pre-tax share is zero percent, so the entire conversion is nontaxable. This is the ideal backdoor Roth setup.

2. Do my Roth IRA balances count in the pro-rata calculation?

No. Roth IRAs are completely excluded from the aggregation. Only Traditional, SEP, and SIMPLE IRA balances enter the ratio.

3. Does my 401(k) balance affect the pro-rata rule?

No. Employer plans are invisible to the rule. Money inside a 401(k), 403(b), or similar plan does not count toward your pre-tax IRA balance.

4. Can I convert only my after-tax contribution and leave the pre-tax money alone?

No. The rule forbids cherry-picking. Any conversion you make is split by your overall pre-tax share, regardless of which account or contribution you meant to convert.

5. Do earnings on my contribution count as pre-tax money?

Yes. Any growth that happens before you convert is pre-tax, even if the original contribution was after-tax. Converting quickly keeps this number near zero.

6. Which date determines my IRA balances for the rule?

December 31 of the year you do the conversion. Balances on that single date decide the ratio, so year-end moves matter more than mid-year ones.

7. Can I dodge the rule by using a separate IRA account for the contribution?

No. The IRS aggregates every Traditional, SEP, and SIMPLE IRA you own, at every brokerage. A separate account does not create a separate ratio.

8. What happens if my pre-tax balance dwarfs my contribution?

Your taxable share approaches one hundred percent. With a one hundred thousand dollar pre-tax balance and a seven thousand dollar contribution, about 93.5 percent of the conversion is taxable.

9. Does the rule apply to partial conversions too?

Yes. Converting ten thousand out of a twenty thousand dollar balance uses the exact same taxable percentage as converting the full twenty thousand.

10. How do I report the taxable and nontaxable split on my taxes?

You file Form 8606 with your return. It walks through the basis, the conversion, and the December 31 balance to compute the taxable amount.

11. What if I already converted without knowing about the rule?

The conversion is still prorated, so report it correctly on Form 8606 and pay the tax due. For future years, consider a reverse rollover to clear the pre-tax balance first.

12. Does the pro-rata rule apply when converting a 401(k) directly to Roth?

That conversion follows its own rules, not IRA aggregation. The pro-rata rule described here applies specifically to Traditional, SEP, and SIMPLE IRA conversions.

13. Can I do more than one backdoor conversion in a single year?

Yes. Each conversion is prorated against the same December 31 balances, so doing two conversions of three thousand five hundred dollars gives the same total result as one conversion of seven thousand.

14. Does my spouse pre-tax IRA balance affect my conversion?

No. IRAs are individual. Your spouse balance is never aggregated with yours, so a spouse with a large rollover IRA does not change your taxable share.

15. Is the nontaxable part of the conversion really tax-free forever?

Yes. That money was already taxed when you earned it, so converting it to Roth creates no new tax. All future growth inside the Roth is tax-free as well.