Tax Cost of a Partial IRA-to-Roth Conversion

When you convert part of a traditional IRA to a Roth, the IRS treats the conversion as a taxable event on a pro-rata basis—meaning you cannot cherry-pick only the pre-tax portion and leave the after-tax dollars behind. The tax on a partial traditional IRA to Roth conversion depends on how much of your total IRA assets are pre-tax versus after-tax, and ordinary income tax applies to the pre-tax share.

How the Pro-Rata Rule Works

The pro-rata rule is a single, blunt calculation that applies whenever you convert any traditional IRA to Roth. It ignores which account you physically take the money from. Instead, it looks at your total traditional IRA balance across all IRAs (including SEP-IRAs and SIMPLE IRAs) and calculates what fraction is pre-tax.

Here’s the formula:

Pro-rata % = (Total pre-tax IRA balance) / (Total IRA balance, all types)

Then you multiply the amount you convert by this percentage to find the taxable portion.

Example 1: Mostly pre-tax, one account

You have one traditional IRA with:

• $60,000 in pre-tax contributions
• $10,000 in after-tax contributions (nondeductible)
• Total: $70,000

You convert $20,000 to Roth.

• Pro-rata % = $60,000 / $70,000 = 0.857 or 85.7%
• Taxable conversion = $20,000 × 0.857 = $17,140
• After-tax portion = $20,000 × 0.143 = $2,860 (no tax)

You report $17,140 as ordinary income.

Why the Rule Exists and What It Prevents

Congress enacted the pro-rata rule to prevent “conversion arbitrage”—the ability to convert only after-tax dollars while leaving pre-tax balances to grow tax-deferred, or to strategically time conversions around basis. Without it, someone with $100,000 in pre-tax IRAs and $10,000 in after-tax IRAs could convert $10,000 and claim zero tax, then later roll the pre-tax balance into a 401k-plan to eliminate pro-rata complications. The rule treats all traditional IRAs as a single pool for tax purposes.

The rule applies to all traditional, SEP, and SIMPLE IRAs you own, whether or not they’re at the same institution. If you have one IRA with $50,000 pre-tax at Vanguard and another with $30,000 after-tax at Fidelity, a conversion from either account uses the blended ratio.

Worked Example: Multi-Account Scenario

You own three IRAs:

You decide to convert $30,000 from IRA C (your after-tax bucket) to Roth, hoping to avoid tax.

• Pro-rata % = $120,000 / $140,000 = 0.857 or 85.7%
• Taxable conversion = $30,000 × 0.857 = $25,710
• Tax-free conversion = $30,000 × 0.143 = $4,290

Even though IRA C is entirely after-tax, the pro-rata rule forces 85.7% of your $30,000 conversion into taxable ordinary income because of what you hold elsewhere. You owe marginal-tax-rate-investor on $25,710.

Strategies to Reduce the Tax Hit

One legitimate approach is to roll all your pre-tax and after-tax balances into an employer plan—a 401(k), 403(b), or 457—before converting. Most plans accept direct rollovers. Once the pre-tax dollars leave the IRA ecosystem, the pro-rata calculation excludes them, and you can convert only the after-tax IRA dollars with little to no tax.

Caveat: Not all plans accept IRA rollovers, and some exclude after-tax dollars. Check your plan documents first. Also, you must complete the rollover before the conversion; doing it after has no effect.

Another approach is to spread conversions over multiple years, converting smaller amounts in lower-income years when you’re in a lower marginal-tax-rate-investor. This doesn’t reduce the overall pro-rata percentage, but it may let you pay tax at a lower rate.

You cannot reduce the pro-rata percentage itself—it’s determined by your total IRA balances, not the account you convert from or the amount you convert.

Timing and the Year of Conversion

The pro-rata rule applies based on your IRA balances on December 31 of the year of conversion. If you convert $30,000 in January, the pro-rata calculation uses December 31 balances from that same year. Market volatility, distributions, and additional contributions all affect the December 31 number, so if you’re planning a conversion, be aware that year-end portfolio moves change your tax bill.

Converting late in the year lets you see December 31 balances with little uncertainty; converting early in the year means you’re locking in a calculation that may shift if you make contributions, receive distributions, or market values change by year-end.

Reporting and Record-Keeping

You report the conversion and its tax on Form 8606, filed with your tax return for that year. Form 8606 asks for:

• Total amount converted
• Non-taxable (after-tax) portion
• Taxable portion

Keep records of your basis in all traditional, SEP, and SIMPLE IRAs. The IRS shares no mechanism to track basis automatically, so you must maintain statements showing after-tax contributions and earnings. When you file Form 8606, you’re certifying the after-tax amounts; incorrect reporting can trigger audit and penalties.

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