The Pro-Rata Rule and Roth Conversions

If you've ever made non-deductible contributions to a traditional IRA — contributions that didn't give you a tax deduction — you can't convert just those dollars to Roth and skip the tax. The IRS treats all your traditional IRA money as a single pool and taxes conversions proportionally. This is the pro-rata rule, and getting it wrong can cost tens of thousands of dollars in unexpected taxes.

What the pro-rata rule is

Under IRC § 408(d)(2),¹ when you take a distribution or convert from a traditional IRA, the IRS calculates the taxable fraction based on the ratio of your total pre-tax balance to your total IRA value — across all your traditional, SEP, and SIMPLE IRAs combined. You cannot designate a specific IRA or specific contributions to be the ones you're converting.

This matters because many retirees have accumulated non-deductible IRA contributions over the years — particularly those who were phased out of deductible IRA contributions due to income but still contributed to traditional IRAs. Those after-tax contributions create what the IRS calls "IRA basis," tracked on Form 8606.²

The pro-rata formula

Step by step:

1. Add up all IRA balances. Sum the year-end values of all your traditional, SEP, and SIMPLE IRAs. Include any amounts you rolled out to another IRA during the year. Do NOT include Roth IRAs or inherited IRAs.
2. Find your total after-tax basis. This is the cumulative total of non-deductible contributions you've made over the years, minus any after-tax amounts you've already withdrawn or converted. Your Form 8606 history shows this number.
3. Calculate the non-taxable fraction.

Concrete example: $1.5M traditional IRA with $75K of basis

Say you've contributed $75,000 of non-deductible (after-tax) contributions to IRAs over 20 years. Your total IRA balance today is $1,500,000. You want to convert $150,000 to Roth.

Only $7,500 of the $150,000 conversion is tax-free — even though you contributed $75,000 of after-tax money. The other 95% is fully taxable at your current bracket. Converting $150K at the 22% bracket: you owe taxes on $142,500 = roughly $31,350.

If you had mistakenly assumed you could convert just the $75K of after-tax basis tax-free, you would owe $0. The actual bill is much larger. This surprise hits people who haven't tracked their Form 8606 history or don't realize how the pooling works.

Which IRA accounts are pooled?

The pro-rata calculation pools these account types:¹

• Traditional IRAs — deductible and non-deductible alike
• SEP IRAs
• SIMPLE IRAs (after the 2-year holding period)

These are not included in the pro-rata pool:

• Roth IRAs (already post-tax)
• Inherited IRAs (treated as separate accounts)
• 401(k), 403(b), 457(b), TSP, or other employer-plan balances

That last point — employer plans are excluded — is the foundation of the most common workaround.

The 401(k) rollover workaround

If your current employer's 401(k) plan accepts incoming rollovers from IRAs (most large plans do), you can roll your pre-tax IRA balance into the 401(k). This removes the pre-tax money from the pro-rata calculation, leaving only your after-tax basis in the IRA — which you can then convert to Roth nearly tax-free.

Step-by-step:

1. Confirm your 401(k) plan accepts IRA rollovers. Check the plan document or ask HR.
2. Roll your pre-tax IRA balance (not the after-tax basis) into the 401(k). The plan typically only accepts pre-tax money.
3. The remaining IRA balance is now almost entirely after-tax basis.
4. Convert that remaining IRA balance to Roth. Nearly 100% is tax-free.

Example continued: Before the rollover, you had $1,500,000 IRA ($75K basis, $1,425K pre-tax). Roll $1,425,000 to your 401(k). Your IRA now holds approximately $75,000 — all basis. Convert the IRA to Roth: the conversion is essentially tax-free. You've moved $1,425,000 into the 401(k) where it will be subject to RMDs and future conversions, but you've cleanly extracted the $75K basis.

When the workaround doesn't apply

Several situations make the 401(k) rollover workaround unavailable or less useful:

• Retired with no current employer plan. Most people reading this page have left the workforce. You can't roll to a 401(k) you no longer have access to. Solo 401(k)s are an option if you have self-employment income — even consulting or side work qualifies.
• The 401(k) plan doesn't accept incoming rollovers. This is increasingly rare, but some plans restrict it.
• Very small basis. If your after-tax basis is $10,000 against a $2M IRA, the pro-rata rule only makes 0.5% of conversions tax-free — barely worth the complexity of engineering around it. Just convert and accept 99.5% is taxable.
• All IRA money is pre-tax. If you have no after-tax basis (never made non-deductible contributions), the pro-rata rule doesn't change anything — every dollar of conversion is already taxable.

Backdoor Roth and the pro-rata rule

The backdoor Roth strategy — contributing to a non-deductible IRA and immediately converting — is designed to get high earners (above Roth IRA income limits) into a Roth account. But it only works cleanly if you have no other pre-tax IRA balances.

If you have a $1.5M traditional IRA and contribute $7,000 to a non-deductible IRA, converting that $7,000 is not tax-free. The pro-rata rule sees: $7,000 basis / ($1,507,000 total IRA) = 0.46% non-taxable. You owe tax on 99.54% of the conversion — nearly all of it.

For retirees with large traditional IRA balances, the backdoor Roth is generally pointless until the pre-tax IRA is cleared out through conversions or the 401(k) rollover technique above.

Form 8606: track your basis or lose it

Every year you make a non-deductible IRA contribution, you must file Form 8606 to report and track your basis.² If you skip years, you can reconstruct from prior tax returns, but it's burdensome. Without proper basis tracking:

• The IRS assumes 100% of every withdrawal and conversion is taxable.
• You pay tax again on money you already paid tax on — double taxation.
• Reconstructing years of missed 8606s is possible but requires amended returns or a formal basis calculation under Rev. Proc. 87-57.

If you've been making non-deductible contributions for 15 years without filing Form 8606, the first step before any conversion strategy is reconstructing your basis history. A fee-only advisor who specializes in IRA distribution planning can do this work systematically.

State tax and the pro-rata rule

Most states follow federal pro-rata treatment, but a few diverge. California follows federal rules closely. Some states have their own IRA basis tracking separate from federal Form 8606 — if you've moved states, both the federal and state pro-rata calculations may apply independently. This adds complexity for retirees who made contributions in high-tax states and are now retiring to a no-income-tax state.

Summary: when pro-rata matters and what to do

Sources

1. IRC § 408(d)(2) — IRA Distribution Rules and Pro-Rata Aggregation. The statutory basis for treating all traditional/SEP/SIMPLE IRAs as a single pool for pro-rata purposes.
2. IRS Form 8606 — Nondeductible IRAs. Required annually whenever you make non-deductible IRA contributions or take distributions with after-tax basis.
3. IRS Publication 590-B — Distributions from Individual Retirement Arrangements. Covers basis tracking, pro-rata calculation, and Form 8606 requirements in detail.
4. Kitces — Pro-Rata Rule and IRA Aggregation for Roth Conversions. Plain-English explanation of how the IRS aggregates IRA accounts for the pro-rata calculation.
5. IRS — Rollovers of Retirement Plan and IRA Distributions. Governs the 401(k) rollover workaround: which account types can receive incoming IRA rollovers.

Pro-rata rule mechanics are based on IRC § 408(d)(2) and IRS Publication 590-B. The 401(k) rollover workaround has no time-sensitive thresholds — its availability depends on your employer plan documents. Verify with a specialist before executing.