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How the Pro Rata Rule Affects Your Backdoor Roth Conversion

By Felix Thorpe · Retirement Accounts Writer

The pro rata rule forces you to count all your pre-tax IRA money when calculating taxes on a backdoor Roth conversion

When you convert money from a traditional IRA to a Roth IRA, the IRS taxes you on the pre-tax portion of that conversion. The pro rata rule means you cannot cherry-pick only the after-tax money to convert while leaving pre-tax money behind. Instead, the IRS treats all your IRAs as a single pool for tax purposes, and you owe tax on a percentage of the conversion equal to the percentage of pre-tax money in that entire pool.

This rule exists because the IRS wants to prevent people from stashing pre-tax money in traditional IRAs, converting only the after-tax portion to a Roth, and avoiding tax on the pre-tax side. The calculation itself is straightforward arithmetic, but the consequences are large enough that many people discover this rule only after they have already moved money around.

Key Takeaways

  • The pro rata rule requires you to add up all your traditional, SEP, and SIMPLE IRAs as of December 31 of the conversion year, then calculate what percentage is pre-tax money.
  • You owe federal income tax on that same percentage of the amount you convert, even if you are converting only after-tax contributions.
  • The calculation uses the fair market value of each account on December 31, not the contribution amounts, so investment gains and losses matter.
  • Employer plans like 401(k)s and 403(b)s are not included in the pro rata calculation, which is why rolling pre-tax IRA money into your employer plan before converting can reduce or eliminate the tax hit.
  • You report the conversion and the pro rata calculation on Form 8606, which you file with your tax return for the year of the conversion.

How to identify your pre-tax and after-tax IRA balances

Start by listing every traditional IRA, SEP IRA, and SIMPLE IRA you own as of December 31 of the year you are converting. Do not include Roth IRAs—those are already after-tax and do not factor into the pro rata calculation. If you have multiple accounts at different institutions, you still add them all together.

For each account, separate the balance into two categories: pre-tax money and after-tax money. Pre-tax money includes all contributions you deducted on your tax return, all employer contributions (in a SEP or SIMPLE), and all investment gains on that pre-tax money. After-tax money is the portion of your contributions that you did not deduct—usually non-deductible traditional IRA contributions you made in years when your income was too high for a deduction.

If you have made non-deductible contributions in the past, you should have received a Form 5498 from your IRA custodian each year showing the contribution amount. Your tax return for that year should also show the non-deductible contribution on Form 8606. If you cannot find this record, contact your IRA custodian and ask for a statement showing the basis (after-tax contributions) in each account.

The pro rata calculation step by step

Use December 31 fair market values for all accounts, not the values on the day you convert. This date is fixed by the IRS—it does not matter if you convert on January 15; you still use December 31 of that same year.

Here is the formula:

  1. Add up the pre-tax balance across all your IRAs as of December 31.
  2. Add up the after-tax balance across all your IRAs as of December 31.
  3. Add those two numbers to get your total IRA balance.
  4. Divide pre-tax balance by total balance. This is your pro rata percentage.
  5. Multiply the amount you are converting by your pro rata percentage. This is the taxable portion.

Example: On December 31, you have a traditional IRA with $60,000 in pre-tax money (old 401(k) rollover) and $10,000 in after-tax money (non-deductible contributions). You also have a SEP IRA with $30,000 in pre-tax money. Total pre-tax: $90,000. Total after-tax: $10,000. Total: $100,000. Pro rata percentage: $90,000 ÷ $100,000 = 90 percent. You convert $10,000 of after-tax money. Taxable amount: $10,000 × 90 percent = $9,000. You owe income tax on $9,000 of that conversion.

Why the conversion date does not change the calculation

Many people assume that if they convert on January 2, they should use January 2 values. The IRS rule is different: you always use December 31 of the conversion year, period. This is true even if the market moves significantly between December 31 and your conversion date, or if you add new money to an IRA in early January.

The reason is administrative—the IRS wants a single, fixed date that applies to everyone, so there is no ambiguity about which values to use. If you convert in February and the market has dropped, you cannot use February values to lower your pro rata percentage. If the market has risen, you cannot use higher values to increase it.

How rolling pre-tax money to your employer plan changes the calculation

If you have access to a 401(k), 403(b), or similar employer plan, you can move pre-tax IRA money into that plan before you convert. Employer plans are not included in the pro rata calculation—only IRAs count. This is one of the few legal ways to reduce or eliminate the pro rata tax hit.

The timing matters: you must complete the rollover before you do the conversion. If you convert first and then roll money to your employer plan, the pro rata calculation still uses the pre-tax money that was in your IRA on December 31. Rolling money out after the conversion does not retroactively change the tax you owe.

Example using the same numbers: Before converting, you roll the $60,000 pre-tax 401(k) rollover from your traditional IRA into your employer 401(k). Now your IRA has only $30,000 pre-tax (the SEP) and $10,000 after-tax. Total: $40,000. Pro rata percentage: $30,000 ÷ $40,000 = 75 percent. You convert $10,000 of after-tax money. Taxable amount: $10,000 × 75 percent = $7,500. You owe tax on $7,500 instead of $9,000—a savings of $1,500 in taxable income.

Reporting the pro rata calculation on Form 8606

You report the conversion and calculate the pro rata tax on Form 8606, which you file with your federal tax return for the year of the conversion. The form has separate sections for traditional IRA conversions and non-deductible contributions. You will need the December 31 balances and the amount converted.

Part II of Form 8606 is where you enter your pro rata calculation. You list your total basis (after-tax contributions) in all IRAs, your total IRA balance, and the amount converted. The form then calculates how much of the conversion is taxable. If you make a mistake on this form, the IRS will recalculate it and send you a notice—but you are responsible for paying any additional tax owed, plus interest.

If you do not file Form 8606 in the year of the conversion, you can still file it later, but the IRS may assess penalties. If you convert after-tax money and do not report it, the IRS may treat the entire conversion as taxable, which is worse than the pro rata calculation.

Common mistakes that increase your tax bill

The most common mistake is forgetting about a small SEP IRA or SIMPLE IRA from a previous job. Even if you have not contributed to it in years, it still counts toward the pro rata calculation. Many people discover this when they convert and receive a larger tax bill than expected.

Another mistake is using the wrong date for account values. Some people use the value on the day they convert, or the value on the day they open the Roth account. The IRS rule is December 31 only. If the market has moved significantly, this can change the pro rata percentage by several percentage points.

A third mistake is converting in December and assuming you can roll pre-tax money to your employer plan in January to reduce the tax. The rollover must happen before the conversion. If you convert in December and roll in January, the pro rata calculation still uses the December 31 balance that included the pre-tax money.

Frequently Asked Questions

Do I have to include my spouse's IRAs in the pro rata calculation?

No. The pro rata rule applies to each person separately. Your spouse's IRAs are not included in your calculation, and your IRAs are not included in theirs. If you are both doing backdoor Roths in the same year, each of you calculates your own pro rata percentage based on your own accounts.

What if I have a Roth IRA already—does that count?

No. Roth IRAs are excluded from the pro rata calculation. Only traditional, SEP, and SIMPLE IRAs count. This is one reason some people keep their Roth IRAs separate from their traditional IRAs—to keep the pro rata calculation simpler.

Can I split my conversion across two years to lower the pro rata tax?

No. The pro rata calculation is based on your total IRA balance on December 31 of the conversion year. Converting $5,000 in December and $5,000 in January does not change the calculation—both conversions use the December 31 balance from their respective years. If your pro rata percentage is high, splitting conversions across years does not help.

What happens if my IRA loses money between December 31 and when I convert?

You still use the December 31 value for the pro rata calculation, even if the account is worth less on the conversion date. The IRS does not allow you to use a lower value to reduce your pro rata percentage. However, if the account value on December 31 was already lower due to market losses, that lower value is what you use.

If I convert and owe tax, do I have to pay it by April 15?

The tax is due when you file your return for that year, which is normally April 15 of the following year. You can pay it with your return, or you can make estimated tax payments during the year if you expect a large bill. If you do not pay by April 15, the IRS charges interest and penalties on the unpaid amount.