How a Roth Conversion Is Taxed

A Roth conversion converts assets from a traditional IRA or 401(k) into a Roth IRA, and the value converted becomes taxable ordinary income in the year of conversion. But when you have both pre-tax and after-tax balances, the pro-rata rule ensures you cannot cherry-pick only the after-tax portion—you must convert all account types proportionally.

The basic conversion income rule

The simplest scenario: you have a traditional IRA with $100,000 of pre-tax contributions and gains. You convert the entire balance to a Roth. You owe federal ordinary income tax on the full $100,000, as if you had withdrawn it. The IRS treats a conversion as a distribution followed immediately by a contribution to the Roth.

If you have pre-tax deferrals (from 401(k) rollovers), employer matching, or accumulated gains, all of it becomes taxable income in the year you convert. There is no step-up in basis, no deferral—just immediate taxation at your marginal rate.

This is the cost of accessing a Roth IRA: you pay the tax upfront, but then the account grows tax-free forever. Many conversions are strategic, done in years when income is unusually low or when a major life transition (retirement, sabbatical, job change) temporarily reduces adjusted gross income.

The pro-rata rule: the trap for mixed accounts

Here is where most conversion surprises occur. Suppose you have multiple IRAs:

• A traditional IRA with $80,000 pre-tax balance
• A Roth IRA (already opened) with $20,000
• A SEP-IRA with $50,000 pre-tax

You want to convert just $10,000 of after-tax contributions you made to the traditional IRA, assuming you will only pay tax on gains.

The IRS says: no. You must look at all of your IRAs combined. Your total pre-tax balance is $130,000 ($80,000 + $50,000). Your after-tax balance is $10,000. If you convert $10,000, the IRS calculates the ratio:

After-tax portion = $10,000 / $140,000 total = 7.14% after-tax.

Only 7.14% of your conversion is after-tax (non-taxable). The remaining 92.86% is pre-tax (fully taxable). So converting $10,000 means $9,286 is taxable income and $714 is not—not the clean $10,000 non-taxable result you hoped for.

The pro-rata rule applies across all IRAs you own (traditional, SEP, SIMPLE). It does not apply to 401(k) plans, which are separate entities. This distinction is critical: a $1 million pre-tax 401(k) is not counted in the pro-rata calculation. Only your IRAs count.

Why the pro-rata rule exists

The IRS created the pro-rata rule to prevent sophisticated savers from converting only the “cheap” (after-tax) portions of their IRA balance and leaving the pre-tax gains untouched. The rule enforces proportionality: if your IRA is 80% pre-tax, then 80% of any conversion is pre-tax. You cannot selectively pluck out just the after-tax slices.

This rule has made the “backdoor Roth” strategy more complex for high-income earners with lingering pre-tax IRA balances. If you want to do a backdoor Roth conversion (contributing after-tax dollars and immediately converting to Roth), but you already have a traditional IRA with pre-tax dollars, your conversion becomes partially taxable under the pro-rata rule.

Calculating the pro-rata ratio

The calculation is straightforward but requires careful data collection:

1. List all IRAs (traditional, SEP, SIMPLE).
2. Sum the pre-tax balances as of December 31 of the prior year.
3. Sum the after-tax contributions (basis) as of the same date.
4. Divide after-tax by (pre-tax + after-tax) to get the after-tax percentage.
5. Multiply your conversion amount by that percentage to find the non-taxable portion.

Example: If your IRAs total $150,000 and $30,000 is after-tax, then 20% of any conversion is non-taxable. A $50,000 conversion means $40,000 is taxable and $10,000 is not.

The IRS looks at account values as of December 31 of the year before the conversion. If your IRA grows significantly between year-end and the conversion date, the proportions shift slightly, but the rule uses the prior year-end snapshot.

After-tax contributions are never double-taxed

One common fear: if you have $10,000 in after-tax contributions and you withdraw them, do you owe tax again? No. You have already paid tax on the contribution itself. Your cost basis is $10,000. The pro-rata rule recognizes this basis and does not tax that portion of the conversion. You owe tax only on the gains and pre-tax balance.

If you have $50,000 after-tax contributions and $20,000 in gains (total $70,000 after-tax value), a conversion of that amount is taxable only on the $20,000 gain, not the $50,000 contribution. The contribution is basis and is not taxed.

Timing and the year of income recognition

A conversion is complete when the assets reach your Roth account. If you initiate a conversion on December 15 and it settles on January 5 of the next year, the income is recognized in January’s tax year. The settlement date, not the request date, determines the year.

This timing flexibility lets savvy investors coordinate conversions with market downturns or income-deferral strategies. Converting after a market drop means less income is recognized, and you are locking in a lower dollar amount in the tax-free account.

The five-year rule for early withdrawals

In a Roth IRA, contributions (including converted amounts treated as contributions) can be withdrawn at any time without penalty. But the earnings on a conversion are subject to a five-year rule: if you withdraw earnings from a converted amount before age 59½ and before five years have passed since the conversion, you owe both income tax and a 10% penalty on the earnings.

A pro-rata converted balance means part of your Roth balance is earnings subject to the five-year rule, and part is basis (non-taxable). This distinction matters if you need to access your Roth account before retirement.

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