Key Takeaways
- The pro-rata rule taxes conversions in proportion to your pre-tax share of ALL IRA balances.
- You cannot isolate after-tax basis — the IRS averages it across every IRA you own.
- Rolling pre-tax IRAs into a 401k before December 31 is the standard fix.
What the Rule Does
When you convert any amount from a Traditional IRA to a Roth, the IRS determines the taxable portion using a ratio: your total after-tax basis (reported on Form 8606) divided by your total Traditional IRA balances at year-end. The rule exists to stop people from "cherry-picking" — converting only after-tax dollars while leaving pre-tax dollars behind. The consequence is that a $7,500 non-deductible contribution is not converted tax-free if you also hold pre-tax money anywhere in the IRA universe: rollover IRAs, SEP-IRAs, SIMPLE IRAs, and inactive accounts all count. The averaging is done once a year, on December 31 — a balance on that date drags into next year's math too.
Worked Example
Meet Sam. Sam has a $60,000 rollover IRA from an old 401k and makes a $7,500 non-deductible contribution, then converts $7,500 to a Roth, expecting a tax-free conversion. Total IRA balance: $67,500. After-tax basis: $7,500. Pro-rata share: $7,500 ÷ $67,500 = 11.1%. So only 11.1% of the $7,500 conversion — about $833 — is tax-free; the other $6,667 is taxable ordinary income. And Sam still has $6,667 of basis "stuck" in the Traditional IRA, complicating next year. If Sam had instead rolled the $60,000 into a 401k first, the conversion would have been 100% tax-free. That one step is worth roughly $1,600 in the 24% bracket — every single year.
How to Fix It
- Inventory every IRA: rollover IRAs, SEP-IRAs, SIMPLE IRAs, inactive accounts — all count.
- Roll pre-tax balances into a workplace plan (401k, 403b, 457) before December 31 if your plan accepts incoming rollovers.
- Convert any remaining after-tax basis separately once the pre-tax money is gone.
- File Form 8606 every year — the basis only helps if it is documented.
- If a 401k is unavailable, consider whether the conversion is still worth its tax cost — sometimes it is not.
The SEP and SIMPLE Traps
SEP-IRAs are a favorite small-business retirement vehicle and a pro-rata time bomb: a $50,000 SEP balance makes every backdoor conversion 87% taxable. SIMPLE IRAs have an additional trap — money rolled out of a SIMPLE IRA within two years of the first contribution carries a 25% penalty. If you are self-employed and use the backdoor Roth, consider a solo 401k instead of a SEP: it does not count toward the pro-rata calculation because it is a qualified plan, not an IRA.
Why December 31 Matters
The pro-rata calculation is performed using your IRA balances on December 31 of the conversion year — a date that can undo careful planning. Two traps follow. First, if you convert in January and then roll a 401k into an IRA in July, the year-end balance includes the rollover, and the conversion's taxability is recalculated against it — the IRS averages across the whole year's balances. Second, the December 31 balance also determines next year's ratio: converting in December but leaving basis behind means next year's conversions inherit a messy ratio. The professional play is to do the conversion and any cleanup rollovers in the same calendar year, and to get pre-tax IRA balances to zero by December 31 if you plan a backdoor conversion for that year. A one-day difference — December 31 versus January 1 — can change the taxability of an entire strategy.
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Use the Calculator →Why December 31 Is the Key Date
The pro-rata rule looks at your IRA balances on December 31 of the conversion year — not the day you convert. This creates both a trap and an opportunity. The trap: convert in June with no pre-tax IRA money, then roll a $50,000 401k into a Traditional IRA in November — the December 31 balance is now $50,000, and a portion of your conversion becomes taxable. The opportunity: if you have pre-tax IRA money, you can roll it into an employer 401k before December 31 to zero out the pre-tax balance, making the conversion fully non-taxable. Example: you have $40,000 in a Traditional IRA (all pre-tax) and contribute $7,500 non-deductible. Your basis is $7,500 of $47,500 — about 15.8%. A $7,500 conversion is 15.8% tax-free and 84.2% taxable. The rule applies across all your IRAs combined — Traditional, SEP, and SIMPLE — so consolidate and plan the year-end balance deliberately.