The Pro-Rata Rule and Why It Matters for Backdoor Roths

If you have any pre-tax money in any Traditional, SEP, or SIMPLE IRA, the pro-rata rule under IRC §408(d)(2) follows your basis around like a shadow. Every conversion is partly tax-free and partly taxable, in the same proportion that your basis bears to your total pre-tax IRA value. For backdoor Roth users, this is the rule that ruins everything if you don't plan around it.

The basic mechanic

When you make a non-deductible Traditional IRA contribution, the contribution becomes "basis" — money on which you already paid tax. Your Form 8606 tracks the running total. Any later distribution or conversion is split between basis (tax-free) and pre-tax money (taxable) in the ratio of basis to total IRA value as of December 31 of the conversion year.

The "all IRAs" aggregation rule under §408(d)(2) means every Traditional, SEP, and SIMPLE IRA you own is treated as one big IRA for the calculation. A 401(k) balance is not aggregated. A Roth IRA is not aggregated. A spouse's IRA is not aggregated.

Worked example: the trap

You have a $100,000 Rollover IRA from a prior 401(k) — all pre-tax money. You want to do a $7,000 backdoor Roth. You contribute $7,000 non-deductible to a new Traditional IRA, then immediately convert that $7,000 to your Roth.

The pro-rata calculation:

• Total Traditional IRA basis: $7,000
• Total Traditional IRA value (year-end, before conversion): $107,000
• Basis percentage: 7,000 / 107,000 = 6.54%
• Tax-free portion of the conversion: $7,000 × 6.54% = $458
• Taxable portion: $7,000 − $458 = $6,542

You expected a tax-free backdoor Roth and instead owe ordinary-income tax on $6,542. Worse: your remaining $100,000 Rollover IRA now carries $6,542 of basis that you must track on Form 8606 for the rest of your life.

Three legitimate workarounds

1. Roll the pre-tax IRA into a 401(k). A 401(k) balance is not in the pro-rata aggregation. If your current employer's 401(k) accepts incoming rollovers (most do), move the $100,000 Rollover IRA in. The pro-rata denominator drops to $7,000 and the backdoor Roth is clean. The conversion must happen by December 31 of the year the rollover completes — the year-end balance is what counts.
2. Convert everything. Pay the tax on the full $100,000 in a single year (or split across two), and from that point on your Traditional IRA balance is zero. Subsequent backdoor Roths are clean. Useful in a low-income year (sabbatical, early retirement, business loss).
3. Use SEP/SIMPLE strategically. The pro-rata aggregation includes SEP and SIMPLE IRAs. If you have a SEP from self-employment, consider switching to a Solo 401(k), then rolling the SEP balance into the Solo 401(k). The Solo 401(k) is also exempt from pro-rata.

Spouses are independent

The pro-rata rule applies per IRA owner. If your spouse has no pre-tax IRA balance, she can do a clean backdoor Roth regardless of yours. This is occasionally a planning point — funding the spouse's backdoor first, deferring yours until you can solve the pro-rata problem.

Form 8606 — the most important form you've never heard of

Every year you have basis in a Traditional IRA, you must file Form 8606 with your tax return. Skipping a year is fixable via amendment, but losing the basis is a permanent and expensive mistake. Custodians do not track basis for you. If you have done backdoor Roths for five years and never filed an 8606, file amended returns for each year before the next conversion.

Common mistakes

• Doing the backdoor Roth without checking the IRA balance. The 401(k)-rollover trick must happen before December 31 of the conversion year. Plan in October, not December 30.
• Ignoring the inherited IRA. An inherited IRA is not aggregated with your own IRAs for pro-rata purposes — it has its own basis tracking. Useful to know.
• Believing the "immediately convert" trick avoids the rule. The pro-rata calculation uses December 31 balances, not transaction-date balances. Converting in January and rolling the pre-tax IRA into a 401(k) in March still triggers the rule if any pre-tax balance exists at year-end.
• Failing to file Form 8606. Without the basis record the IRS will tax the basis again at conversion. The mistake compounds annually.

Sources

• Internal Revenue Code §408(d)(2), pro-rata rule and IRA aggregation (Cornell LII): law.cornell.edu/uscode/text/26/408
• IRS Form 8606, Nondeductible IRAs: irs.gov/forms-pubs/about-form-8606
• IRS Publication 590-A, Contributions to IRAs: irs.gov/forms-pubs/about-publication-590-a
• IRS Publication 590-B, Distributions from IRAs: irs.gov/forms-pubs/about-publication-590-b
• IRS Notice 2014-54, allocation of after-tax amounts (not directly pro-rata but related): irs.gov/pub/irs-drop/n-14-54.pdf

If you have a Rollover IRA and want to backdoor Roth, RetirementCheck101 flags the pro-rata trap and shows the 401(k)-rollover fix. Explore the free educational tool.