The backdoor Roth and the pro-rata trap.
High earners over the Roth IRA income limit can still fund a Roth — through a back door. But scattered pre-tax IRA balances can turn the backdoor into a partial tax event that nobody warns you about until it's on your return.
The 'backdoor Roth' is a two-step move: contribute non-deductible dollars to a traditional IRA, then convert that IRA balance to a Roth. For someone with no existing pre-tax IRA balance, the conversion is tax-free (you've already paid tax on the contribution). For someone with a substantial pre-tax IRA balance, it's not — and the IRS has rules that prevent you from picking which dollars convert. That's the pro-rata trap.
Who needs the backdoor
High earners. The direct Roth IRA contribution phases out and disappears once your modified adjusted gross income crosses roughly $161,000 single / $240,000 married filing jointly (2025 limits, adjust for current year). Above those numbers, you cannot contribute directly to a Roth IRA. The backdoor is the workaround that the tax code allows but doesn't advertise.
The pro-rata rule, in plain English
When you convert any traditional IRA dollars to Roth, the IRS treats every dollar in every traditional IRA you own as one big pool. The taxable portion of your conversion is the percentage of that pool that is pre-tax dollars. If you have $100,000 of pre-tax money in a rollover IRA and you contribute $7,000 non-deductible and try to convert just that $7,000, the IRS says: 'no, this conversion is 93.5% pre-tax and 6.5% after-tax — so 93.5% of $7,000 is taxable.' Most of the conversion gets taxed even though that's not what you intended.
Why this catches people
Many high earners have an old 401(k) rollover sitting in a traditional IRA from a previous job. Or a SEP-IRA. Or a SIMPLE IRA. Every one of those balances counts in the aggregation. The Roth conversion calculator at the custodian doesn't know about your other IRAs at other custodians. You only discover the problem when your CPA prepares Form 8606 and the bill is much higher than expected.
The workarounds that actually work
The cleanest fix: roll all pre-tax IRA balances into your current employer's 401(k) (if the plan accepts incoming rollovers — most do). Now your IRA aggregate is zero except for the non-deductible contribution, and the backdoor conversion is tax-free. Time it carefully: the IRA aggregate is measured on December 31 of the year of the conversion. Do the 401(k) rollover before year-end, do the backdoor conversion before year-end, and you're clean.
If you're self-employed
A solo 401(k) often accepts incoming IRA rollovers and gives self-employed clients the same escape from the pro-rata trap. SEP-IRAs and SIMPLE IRAs themselves cannot be rolled to a Roth without the pro-rata rule applying — but they can usually be rolled to a 401(k) first.
Form 8606 — track or lose
The non-deductible contribution is tracked on Form 8606 each year. Skip the form and the IRS doesn't know which dollars are after-tax. When you eventually convert or withdraw, you'll be taxed on dollars you've already paid tax on. Keep the 8606 forms forever; they're the only record of your basis.
Mega backdoor Roth — adjacent but different
A separate strategy uses after-tax contributions to a 401(k) (above and beyond the normal $23,000 deferral limit, up to the $69,000 annual addition limit) that are then converted to Roth in-plan or rolled to a Roth IRA. Different mechanism, different rules. If your 401(k) plan supports after-tax contributions and in-service conversions, this is one of the highest-leverage retirement-savings opportunities in the code.
The backdoor Roth is technically legal, IRS-acknowledged, and routinely used. It's also one of the easiest strategies to execute incorrectly. Sequence matters.