The Roth IRA pro-rata rule — codified at IRC §408(d)(2) — is the single most misunderstood mechanic in the Roth conversion process. It applies whenever a taxpayer has any after-tax basis in a Traditional IRA and executes a distribution or conversion: the pre-tax and after-tax dollars come out proportionally, not in the order the taxpayer chooses.
The rule defeats the naïve backdoor-Roth assumption that a non-deductible contribution can be converted tax-free as long as the conversion happens before any earnings accrue. It doesn’t matter how clean the contribution looks in isolation — the IRS aggregates every Traditional, SEP, and SIMPLE IRA the taxpayer owns and computes a single pro-rata fraction across the lot.
This guide walks through the formula, the aggregation rules, the Form 8606 mechanics, and the workarounds. For step-by-step backdoor Roth process see the Backdoor Roth IRA Guide and the Backdoor Roth Calculator.
1. The pro-rata formula
For any distribution or conversion in a tax year, the taxable portion is:
Taxable portion = Distribution amount × (Pre-tax balance ÷ Total IRA balance)
The non-taxable portion (your basis recovery) is the complement:
Non-taxable portion = Distribution amount × (After-tax basis ÷ Total IRA balance)
The fractions are computed using:
- Pre-tax balance: Total balance across all Traditional, SEP, and SIMPLE IRAs on 31 December of the conversion/distribution year.
- After-tax basis: Cumulative non-deductible contributions reported on Form 8606 Part I, accumulated across years.
- Total IRA balance: Pre-tax + after-tax, again on 31 December.
The IRS uses the year-end balance, not the conversion-date balance. This is the trap: a taxpayer who converts in March and waits to see the result has the formula recomputed against the December balance — which can swing significantly with market movements or additional contributions during the year.
2. Aggregation — which accounts count
IRC §408(d)(2) requires aggregation across all of the taxpayer’s:
- Traditional IRAs (personal accounts)
- SEP-IRAs (employer-funded but treated as Traditional)
- SIMPLE IRAs (employer-funded but treated as Traditional)
Aggregation is per taxpayer, not per account. A working spouse with a $500,000 Traditional IRA and a non-working spouse with $0 IRA balance who makes a $7,000 non-deductible contribution and immediately converts to Roth — the working spouse’s $500,000 has no impact on the non-working spouse’s pro-rata fraction. Each spouse is computed separately.
What does not aggregate
- Roth IRAs. The pro-rata rule does not apply to Roth distributions. Roth distributions follow the ordering rules of §408A(d)(4): contributions → conversions → earnings. See the Roth Conversion 5-Year Rules for the Roth ordering mechanic.
- 401(k), 403(b), 457(b). Employer plan balances do not count toward IRA aggregation. This is the foundation of the workaround in section 5.
- Inherited IRAs. Pre-tax balances inherited from a deceased account owner are not aggregated with the recipient’s own IRAs for pro-rata purposes — they are tracked as a separate inherited account.
- Spouse’s IRAs. As above, each spouse computes their own pro-rata fraction.
3. The backdoor Roth pro-rata trap
The backdoor Roth strategy works only if the taxpayer has no pre-tax balance in any Traditional/SEP/SIMPLE IRA at year-end. Otherwise:
Worked example: failed backdoor Roth
Maria is single, age 35, with a $94,000 Traditional IRA balance (from a 401(k) rollover years ago) — all pre-tax. She makes a $7,000 non-deductible Traditional IRA contribution in March 2026, then immediately converts the $7,000 to Roth.
- Pre-tax balance on 31 Dec 2026 (post-conversion): $94,000 (unchanged — she only converted the new $7,000)
- After-tax basis from non-deductible contribution: $7,000
- Total IRA balance: $94,000 + $7,000 = $101,000 (before conversion); after conversion of $7,000 = $94,000 left
- The $7,000 conversion is tested against the year-end aggregate basis ratio:
- Total IRA balance at year-end (post-conversion): $94,000
- But the formula uses balance + amount distributed: $94,000 + $7,000 = $101,000
- Basis ratio: $7,000 / $101,000 = 6.93%
- Taxable portion of conversion: $7,000 × (1 − 6.93%) = $6,515 taxable as ordinary income
- Tax-free portion (basis recovery): $7,000 × 6.93% = $485
Maria thought she was doing a clean $7,000 tax-free backdoor Roth. She actually paid tax on $6,515 of it. The remaining basis ($485 not yet recovered) carries forward on Form 8606 to future years.
How long does the basis follow her?
The $485 unrecovered basis sits in Maria’s Traditional IRA pool and reduces the taxable fraction of every future distribution or conversion proportionally. Eventually it is recovered, but only at the same pro-rata rate — across a $94,000 pre-tax balance, the $485 will be recovered very slowly.
4. Form 8606 — tracking basis correctly
Form 8606 Part I is the federal recordkeeping mechanism for after-tax basis. Every year in which any of the following occurs requires filing:
- A non-deductible contribution is made to a Traditional IRA.
- A distribution is taken from a Traditional/SEP/SIMPLE IRA that holds after-tax basis.
- A conversion is made from a Traditional/SEP/SIMPLE IRA to a Roth IRA.
The 8606 fields that matter
- Line 1: Non-deductible Traditional IRA contributions for the year.
- Line 2: Total basis as of the end of the prior year (from prior 8606 line 14).
- Line 3: Line 1 + Line 2 = cumulative basis going into the conversion math.
- Line 6: Total Traditional, SEP, SIMPLE IRA value on 31 December (year-end).
- Line 7: Distributions (non-conversion withdrawals) made during the year.
- Line 8: Conversions made during the year.
- Line 10: Basis ratio = Line 3 ÷ (Line 6 + Line 7 + Line 8).
- Line 11: Basis recovered = Line 8 × Line 10.
- Line 14: Year-end basis = Line 3 − basis recovered in Line 11 − Line 7’s basis portion. Carries forward to next year’s Line 2.
If Form 8606 is not filed in a year where a non-deductible contribution was made, the IRS treats the contribution as a deductible contribution (which it isn’t allowed to be if the taxpayer is over the AGI deduction limit) — leading to a notice or audit. Worse, in any subsequent year, the IRS treats the entire IRA balance as pre-tax (no basis), so any conversion is fully taxable. Reconstructing missed 8606s requires amending each affected year’s return.
5. The 401(k) rollover workaround — the main fix
The fundamental defeat of the pro-rata trap is to remove all pre-tax Traditional IRA balance before doing the backdoor Roth conversion. Two main paths:
5.1 Roll Traditional IRA into a current employer 401(k)
If the current employer’s 401(k) plan accepts inbound rollovers from IRAs (most do), the entire Traditional IRA pre-tax balance can be rolled into the 401(k) tax-free. After the rollover, the Traditional IRA has only the after-tax basis (or is empty if the basis was already converted). A subsequent backdoor Roth is then clean.
Critical sequencing:
- The IRA rollover into the 401(k) must complete before 31 December of the conversion year. The pro-rata formula uses year-end balance.
- The 401(k) must accept pre-tax IRA rollovers. Verify with the plan administrator; not all plans permit this.
- Only pre-tax balance can be rolled into a 401(k). The 401(k) does not accept after-tax basis from an IRA. The basis stays in the IRA.
5.2 Convert the entire pre-tax IRA to Roth (the “rip-the-band-aid” approach)
If the pre-tax IRA balance is moderate ($20K-$50K) and the taxpayer has the cash to pay the conversion tax, converting the entire pre-tax balance to Roth in one year removes the pro-rata complication forever. This is best done in a low-income year (early retirement, sabbatical, business loss carryforward) to minimize the tax cost.
After the full conversion, the Traditional IRA is empty. Future backdoor Roth contributions can flow through cleanly.
5.3 Spouse-only basis isolation
If the working spouse has a large pre-tax Traditional IRA and the non-working spouse has none, the non-working spouse can do an unencumbered backdoor Roth without involving the working spouse’s balance. Each spouse computes their own pro-rata fraction.
This is the simplest workaround when one spouse has a clean slate.
6. SEP-IRA and SIMPLE IRA — the silent traps
The pro-rata rule applies equally to SEP-IRA and SIMPLE IRA balances. A self-employed taxpayer who contributes to both a SEP-IRA (for employer-side contribution) and a non-deductible Traditional IRA (for personal backdoor Roth) cannot escape pro-rata. The SEP-IRA balance is in the same aggregation bucket.
The workaround is the same: roll the SEP-IRA into a Solo 401(k) (which can accept pre-tax IRA rollovers if the plan document permits). The SEP-IRA vs Solo 401(k) Calculator covers this tradeoff in detail.
SIMPLE IRA contributions cannot be rolled out of the SIMPLE for the first 2 years from the first contribution. Plan accordingly — the backdoor Roth strategy is locked for 2 years if using SIMPLE.
7. Mega backdoor Roth — different mechanic
The mega backdoor Roth uses after-tax contributions to a 401(k), then converts them in-plan to a Roth 401(k) or rolls out to a Roth IRA. The pro-rata rule does not apply to this strategy because:
- 401(k) after-tax balance is tracked separately from pre-tax 401(k) balance.
- The in-plan Roth conversion happens within the 401(k), not crossing into the IRA aggregation bucket.
See the Mega Backdoor Roth: After-Tax 401(k) Conversion Guide for the full mechanic.
8. Common mistakes
- Forgetting older non-deductible contributions. Basis from contributions made 10 years ago still counts and must be tracked on Form 8606 forever. Reconstructing missed basis from old 1099-Rs and statement archives is painful but worthwhile — recovered basis is permanent tax savings.
- Rolling into a Roth from a SEP-IRA expecting pro-rata immunity. SEP balances aggregate. A backdoor Roth attempted alongside a SEP-IRA contribution is not clean.
- Filing MFS to escape pro-rata for one spouse. Spousal IRAs are already separately aggregated; MFS filing has no effect on this. Filing MFS does, however, lock both spouses into the lower MFS standard deduction and IRMAA tier — a net cost.
- Re-pre-tax-ing a backdoor Roth. Once non-deductible contribution is made and basis tracked, you cannot retroactively convert it to a deductible contribution. The basis is fixed.
- Doing the conversion in the same year as a 401(k) → IRA rollover. The rollover lands in the Traditional IRA at year-end; pro-rata then takes a slice of the backdoor contribution. Time the rollover for January of the year after the backdoor Roth, or sequence the backdoor conversion before any rollover.