If you are considering a Roth IRA conversion, one tax rule catches many people by surprise: the IRA pro-rata rule. At first glance, a backdoor Roth seems simple, you make a nondeductible contribution to a traditional IRA, then convert it to a Roth IRA, and assume that because you already paid tax on the contribution, the conversion is tax-free. The IRS, however, looks at all of your traditional, SEP, and SIMPLE IRAs together, and that is what creates the pro-rata issue.
What is the IRA pro-rata rule?
The IRA pro-rata rule means you cannot pick and choose only the after-tax money in your IRA to convert tax-free. The IRS looks at the total balance of all your non-Roth IRAs, determines what percentage is after-tax versus pre-tax, and applies that percentage to calculate the taxable and nontaxable parts of your conversion.
In plain terms, if you already have pre-tax money in any traditional, SEP, or SIMPLE IRA, your backdoor Roth conversion may be partly taxable, even if the amount you just contributed was nondeductible. The rule lives in IRC Section 408(d)(2) and is enforced through the calculation on Form 8606.
Why does the pro-rata rule matter for a backdoor Roth?
A backdoor Roth is partly taxable whenever you hold pre-tax IRA balances, which defeats much of the strategy's appeal. The backdoor Roth is commonly used by taxpayers whose income is too high to contribute directly to a Roth IRA, and the usual process is to make a nondeductible contribution to a traditional IRA, then convert that amount to a Roth.
For 2026, the general IRA contribution limit is $7,500, or $8,500 if you are age 50 or older (the $1,000 catch-up). The problem is that the IRS does not let you isolate only the new after-tax contribution. The conversion is taxed based on the ratio of after-tax funds to your total IRA balances, so existing pre-tax balances pull the taxable percentage up.
How is the taxable portion calculated?
The taxable portion equals the pre-tax percentage of your combined non-Roth IRA balance applied to the converted amount. Consider a taxpayer who:
- Contributes $7,500 of after-tax money to a traditional IRA
- Already has $92,500 of pre-tax money in other IRAs
- Converts $7,500 to a Roth IRA
The total IRA balance is $100,000, and only 7.5% of it is after-tax. That means only 7.5% of the conversion is tax-free, while the remaining 92.5% is taxable. Many people expect the entire $7,500 to be tax-free, but the pro-rata rule produces a very different result, which is exactly why this calculation belongs on Form 8606.
Which accounts count toward the pro-rata rule?
Traditional IRAs, SEP IRAs, and SIMPLE IRAs are aggregated; Roth IRAs are not. These three account types are combined when calculating the taxable portion of a conversion whenever you have basis in your IRAs.
Crucially, balances held in an employer plan such as a 401(k) or 403(b) are not counted in the aggregation. That single distinction is what makes the cleanup strategy below possible.
Why does year-end planning matter for a backdoor Roth?
The calculation uses your December 31 IRA balance, not the balance on the conversion date. One of the biggest mistakes taxpayers make is focusing only on the date of the conversion, when in reality Form 8606 looks at the total value of your traditional, SEP, and SIMPLE IRAs at year-end.
That means timing is everything. Even if your plan looks clean when you make the contribution, a year-end balance in another IRA, including a SEP funded later in the year, can still make the conversion partly taxable. Coordinating the move with the rest of your return is core to proactive tax planning.
Why is Form 8606 so important?
Form 8606 is required for any nondeductible traditional IRA contribution or Roth conversion, and it is what prevents you from being taxed twice on the same dollars. The form tracks your after-tax basis year over year and calculates the taxable portion of each conversion.
If Form 8606 is missed or prepared incorrectly, it can create problems for years, including lost basis and over-taxation on future distributions. Common mistakes include assuming each IRA is tested separately, forgetting that SEP and SIMPLE IRAs count, completing a backdoor Roth without reviewing existing balances, and failing to file the form at all.
Can the pro-rata problem be reduced?
Yes, in many cases. The most common fix is rolling pre-tax traditional, SEP, or SIMPLE IRA balances into an eligible employer 401(k) before December 31, because employer plans are not part of the IRA aggregation used for this calculation. With the pre-tax balance moved out, only the after-tax contribution remains, and the conversion can be substantially or fully tax-free.
This should be reviewed carefully before any move, because the right strategy depends on your full tax picture, your plan's rollover rules, and your other individual tax considerations. Before completing a backdoor Roth, confirm your current IRA balances, any prior nondeductible contributions, and whether Form 8606 has been filed correctly in past years.
Have questions about the backdoor Roth pro-rata rule? Contact TS CPA for a free consultation. We respond within the same day.