The Pro-Rata Rule: Why Your IRA Balance Can Wreck a Backdoor Roth

The pro-rata rule (Section 408(d)(2) of the Internal Revenue Code) says that when you convert or withdraw from any traditional individual retirement account (IRA), the IRS treats every traditional, simplified employee pension (SEP), and Savings Incentive Match Plan for Employees (SIMPLE) IRA you own as one combined pool: the taxable share of your conversion equals the pre-tax share of the whole pool. Hold pre-tax IRA money anywhere and a backdoor Roth conversion is mostly taxable, no matter which account the converted dollars came from.

How the Math Works

A separate IRA opened just for nondeductible contributions doesn't matter, nor do different custodians; everything is aggregated, and the taxable portion of your conversion is pre-tax dollars divided by total IRA dollars.

If you have $90,000 of pre-tax money in a rollover IRA, make a $7,500 nondeductible contribution (the 2026 limit under age 50), and convert that $7,500, your total IRA balance is $97,500 and only about 7.7% of the pool is after-tax. So 7.7% of your conversion (about $577) is tax-free, and the other $6,923 is ordinary income.

The IRS uses Form 8606 to track your basis (your nondeductible contributions) and calculate the taxable share of any distribution or conversion.

The Rules and the Watchouts

• The aggregation test uses your IRA balances on December 31 of the conversion year, whatever the balance was on the day you converted.
• Roth IRAs are excluded entirely. Only Traditional, SEP, and SIMPLE IRAs count.
• 401(k) and other employer plan balances are also excluded. That exclusion makes the main fix possible.

The biggest watchout is the rollover IRA from a previous employer's 401(k), often holding $200,000, $500,000, or more in pre-tax dollars. That balance alone can make backdoor Roth contributions nearly worthless from a tax standpoint for years.

Waiting months between contribution and conversion does not help, either. The IRS looks at December 31 balances, whatever order the transactions happened in during the year.

When This Applies vs. When It Doesn't

Suppose you're a physician earning $450,000, above the 2026 Roth phase-out (which ends at $168,000 for single filers, $252,000 for married filing jointly). You make a $7,500 nondeductible contribution, but you have a $280,000 rollover IRA from a former hospital employer. Your pool is $287,500, your basis is about 2.6% of it, and the conversion produces roughly $196 tax-free and $7,304 of taxable income.

With no pre-tax IRA balance, the same $7,500 conversion is 100% tax-free (assuming no growth between contribution and conversion). That's the clean backdoor Roth.

The fix is rolling the pre-tax IRA balance into a current employer's 401(k) before December 31, which removes it from the calculation entirely. Not every plan accepts incoming rollovers, and the rollover has to settle by year-end, so check with your plan administrator in the third quarter. Moving IRA money into a workplace plan has tradeoffs of its own (the plan's investment menu, costs, and access rules), so weigh those before you start.

How This Connects to Roth Conversion Strategy Broadly

How much to convert, which years fit your bracket, and how conversions interact with Income-Related Monthly Adjustment Amount (IRMAA) surcharges or Affordable Care Act (ACA) premiums require a wider lens than this one rule. The parent page, Roth Conversion: A Practical Guide for High Earners and Pre-Retirees, covers that fuller picture, including bracket arbitrage and the 5-year rule watchouts.

Frequently Asked Questions

Can I just open a new IRA at a different bank to avoid the rule?

No. The IRS aggregates all your Traditional, SEP, and SIMPLE IRAs regardless of custodian; multiple accounts count as one pool.

What happens if I never filed Form 8606 for my nondeductible contributions?

You may have a basis you haven't claimed, meaning you've been treating money as taxable that wasn't. You can file late or amended 8606 forms to establish it; work with a CPA first.

Can my spouse's IRAs affect my pro-rata calculation?

No. The aggregation rule applies per individual, so your spouse's IRA balances are separate from yours for this purpose.

Is there a limit to how much I can roll into a 401(k) to clear out my IRA balance?

There's no IRS cap on rollover amounts, but your employer plan must accept the rollover and may have its own timing and eligibility rules. Confirm with your plan administrator before counting on this strategy.

What to Do Next

The year-end ratio. The ratio of after-tax basis to your total pre-tax IRA balance, measured on December 31, across every traditional, SEP and SIMPLE IRA in your name. Your spouse's accounts are separate. Yours are all one pool.

Common mistakes. The rule feels like it should apply to the account you touched, and it does not. It also uses the year-end balance, which trips people who try to clean up late. A rollover into a workplace plan initiated in mid-December can settle in January, which means it was not done, which means the conversion you made in March is taxed as though nothing changed. The problem usually surfaces at tax time.

Worth a conversation if you hold more than one IRA, you have a SEP or SIMPLE from consulting or self-employed years, or you are planning a rollover specifically to clear the decks. That rollover needs a margin of months before December 31. Book a call.

The information provided is for educational purposes only and does not constitute investment, legal, or tax advice. Tax law changes frequently, so verify current rules before acting. Consult with qualified professionals for guidance specific to your situation.