What is the pro-rata rule and why does it matter?

The pro-rata rule (IRC § 408(d)(2)) treats all of your traditional IRA money as a single pool for tax purposes, regardless of which account it came from or when it was contributed. If 5% of your total IRA pool is after-tax (basis) and 95% is pre-tax, then every dollar you convert to Roth is 5% tax-free and 95% taxable — even if you convert directly from an account holding only after-tax contributions. You cannot selectively convert only the after-tax portion. This is what makes the backdoor Roth costly when you have a large rollover IRA.

How do I calculate the Form 8606 denominator?

The denominator is: year-end value of ALL traditional/SEP/SIMPLE IRAs (after conversion) PLUS the conversion amount. This is the total value of your IRA pool as if you had not yet converted. The numerator is your total basis (current year + prior years). Non-taxable fraction = numerator ÷ denominator. If your $7,000 non-deductible contribution is the only IRA money and you convert it all, denominator = $0 + $7,000 = $7,000, fraction = 100%, fully tax-free.

What is a "reverse rollover" and does it help?

A reverse rollover moves pre-tax IRA money into an employer 401(k) plan — the opposite of the typical rollover direction. Once pre-tax money is in the 401(k), it is no longer included in the pro-rata IRA denominator. If your employer plan accepts rollovers from IRAs (most do), this strategy clears the contamination. After the rollover, your only remaining IRA balance is the after-tax (basis) amount, and the backdoor conversion is fully tax-free. Check your 401(k) plan documents — most large employer plans accept rollovers.

Can I do a backdoor Roth if I have a SEP-IRA from self-employment?

Yes, but the SEP-IRA balance counts in the pro-rata denominator. If your SEP-IRA has $200,000 pre-tax and you contribute $7,000 non-deductible to a traditional IRA and convert, about 96.6% is taxable. The reverse rollover to a Solo 401(k) is a common workaround for self-employed individuals — move the SEP-IRA balance to a Solo 401(k), then convert only the after-tax IRA to Roth tax-free.

What happens to my remaining basis after a partial conversion?

Unused basis carries forward on Form 8606. If you have $7,000 of basis and convert only $3,500, roughly $175 (5%) applies to the conversion and $6,825 of basis remains for future years. The basis tracks you for life — it applies to any future Roth conversions or non-qualified traditional IRA distributions. When you eventually fully deplete your traditional IRA through conversions or withdrawals, your remaining basis reduces the final taxable amount.

Does a backdoor Roth affect my RMDs?

Roth IRAs are not subject to Required Minimum Distributions during the owner's lifetime. Each dollar converted from a traditional IRA to Roth reduces your future RMD base, which can reduce SS taxation and IRMAA surcharges in your 70s. The backdoor Roth is effectively an early Roth conversion strategy — the same math applies whether you fund it via backdoor or direct contribution.

Retirement & Investing

Backdoor Roth Pro-Rata Rule Calculator — Form 8606 Tax Cost

See exactly how much of your backdoor Roth conversion is taxable if you have pre-existing IRA money — and whether a reverse rollover to your 401(k) is worth it.

By The FinCalc TeamUpdated May 29, 2026

High earners above the Roth IRA income limits ($161,000 single / $240,000 MFJ in 2026) can still fund a Roth via the "backdoor" strategy: contribute to a non-deductible traditional IRA, then immediately convert to Roth. The catch — if you have any pre-tax IRA money, the IRS pro-rata rule forces you to convert a proportional mix of pre-tax and after-tax dollars. This calculator shows exactly how much of your conversion is taxable and what it costs you.

How the Backdoor Roth Pro-Rata Rule Works

The Backdoor Roth Strategy

When income exceeds Roth IRA contribution limits, the backdoor Roth provides an alternative:

Contribute to a traditional IRA without taking a deduction (non-deductible contribution — this creates "basis")
Convert the traditional IRA to Roth immediately
Pay tax only on any earnings that accumulated between contribution and conversion

If done quickly and you have no other pre-tax IRA money, this is fully tax-free. The IRS acknowledges this strategy is legal.

When the Pro-Rata Rule Bites

The problem arises when you have existing pre-tax IRA money — whether from prior deductible contributions, a 401(k) rollover, or a SEP-IRA. The IRS treats all your traditional IRA money as a single pool:

| Scenario | Non-taxable fraction | Tax cost on $7,000 conversion (22% bracket) | |----------|---------------------|---------------------------------------------| | No pre-tax IRA money | 100% | $0 | | $50,000 rollover IRA | 12.3% | $1,344 | | $200,000 rollover IRA | 3.4% | $1,474 |

The Fix: Reverse Rollover

Move pre-tax IRA money into your employer 401(k) before December 31. Once it's out of your IRA, the denominator collapses and the backdoor conversion becomes tax-free:

Before: $200,000 pre-tax IRA + $7,000 non-deductible → 3.4% tax-free
After rollover: $0 pre-tax IRA + $7,000 non-deductible → 100% tax-free

Most 401(k) plans accept rollovers from traditional IRAs. Solo 401(k) plans work for self-employed individuals with SEP-IRAs.

Who Should Use This Calculator

If you earn above the Roth IRA income limits and want to contribute to Roth, this calculator is your planning tool. Run it with your actual year-end IRA balance to see exactly what the contamination costs — and whether a reverse rollover is worth pursuing.

If you have rollover IRAs from prior employers, model the pro-rata cost before assuming the backdoor Roth is tax-free. A $300,000 rollover IRA means over 97% of your backdoor conversion is taxable — the tax cost may make the strategy counterproductive.

If you are self-employed with a SEP-IRA, this calculator shows why moving SEP assets to a Solo 401(k) before attempting a backdoor Roth matters. The Solo 401(k) is ineligible for the pro-rata rule, so the SEP balance disappears from the denominator.

If you are tracking accumulated Form 8606 basis, enter your prior basis to see the correct remaining carry-forward amount after this year's conversion.

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Social Security Taxation Calculator — Roth conversions reduce future combined income and SS taxation; model the long-term benefit
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Understanding the Inputs

Non-Deductible IRA Contribution: The amount you contribute to a traditional IRA this year without taking a tax deduction (because your income exceeds the deductibility phase-out). For 2026, the IRA contribution limit is $7,000 ($8,000 if age 50+). This contribution becomes your "basis" — after-tax dollars that are not taxed again on conversion. Enter this year's contribution only; prior years' accumulated basis goes in the next field.
Prior Non-Deductible Basis (Form 8606): The total after-tax IRA basis you have accumulated in prior years, reported on the most recent Form 8606 you filed, Line 14. If this is your first backdoor Roth, this is $0. If you have made non-deductible contributions for several years without converting, enter the cumulative total. This basis is tracked across all traditional IRAs — it is not account-specific.
Traditional IRA Balance at December 31: The combined fair market value of ALL traditional IRAs, SEP-IRAs, and SIMPLE IRAs at December 31 of the conversion year, AFTER the conversion. Do not subtract the amount you converted — that money has already moved to your Roth account. This is the single most important input for the pro-rata rule. Crucially, this includes any rollover IRAs from old 401(k)s. A $100,000 rollover IRA will contaminate the pro-rata calculation even if it is held at a completely different custodian.
Roth Conversion Amount: The dollar amount you are converting from traditional IRA to Roth this year. For a standard backdoor Roth, this equals your current-year contribution ($7,000). If you are doing a partial conversion of a larger pre-tax balance, enter the actual conversion amount. A larger conversion removes more pre-tax money from the IRA denominator, which slightly improves the non-taxable fraction for future years.
Your Marginal Federal Tax Rate: The federal income tax rate that applies to your next dollar of income. Used to estimate the federal tax cost of the taxable portion of your conversion. The taxable piece is treated as ordinary income — no special capital gains rate applies. State income tax on the conversion is not included in this estimate.

Frequently Asked Questions

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The FinCalc Team

Personal Finance Experts

The FinCalc team is a group of personal finance writers, analysts, and engineers dedicated to building accurate, transparent financial calculators. Every formula is verified against industry standards and explained in plain language.

Last reviewed and updated: May 29, 2026