The Roth IRA income limit for 2025 is $161,000 for single filers and $240,000 for joint filers. If you earn above those thresholds, you can't contribute directly. But you can use the backdoor Roth conversion -- a two-step process that's perfectly legal and explicitly addressed in IRS guidance. The catch is a tax rule called pro-rata that makes the strategy worthless (or worse) for many people who think it's working for them.
How the backdoor works
Step one: contribute to a traditional IRA. At high incomes, this contribution is not tax-deductible (you're above the phase-out range if you have a workplace retirement plan). But non-deductible contributions to a traditional IRA are permitted regardless of income. You contribute after-tax dollars and track the basis using Form 8606.
Step two: convert the traditional IRA to a Roth IRA. The conversion is a taxable event -- you owe ordinary income tax on any pre-tax amounts converted. Since you just contributed after-tax money, there should be nothing taxable. The $7,000 (or $8,000 if 50+) you just put in converts to Roth with zero tax owed.
That's the theory. It works exactly as described when you have no other traditional IRA balances.
The pro-rata rule
The IRS does not let you choose which IRA dollars to convert. All your traditional IRA balances -- at every institution, in every account -- are aggregated and treated as one pool. When you convert, the taxable portion is determined by the ratio of pre-tax money to total IRA money across that entire pool.
Example: you have $90,000 in a traditional rollover IRA from a prior employer (pre-tax) and you contribute $7,000 after-tax for the backdoor. Your total IRA balance is $97,000. Of that, $7,000 is after-tax basis. The ratio of after-tax money is $7,000 / $97,000 = 7.2%. When you convert the $7,000, only 7.2% of it ($504) is tax-free. The remaining $6,496 is taxable at ordinary income rates.
You haven't successfully done a backdoor Roth. You've just done a partially taxable conversion that didn't accomplish what you intended. The $90,000 rollover IRA is the problem.
How to fix the pro-rata problem
The cleanest solution is to roll the pre-tax traditional IRA into your current employer's 401(k) plan. Most 401(k) plans accept incoming rollovers from traditional IRAs. Once the pre-tax balance is inside the 401(k), it's no longer in the pro-rata calculation. Your IRA pool is now only your after-tax basis, and the conversion is tax-free.
This works only if your employer's plan accepts rollovers. Many do -- check with your HR department or plan administrator. Plans are not required to accept rollovers, and some don't.
If your plan doesn't accept rollovers, or if you're self-employed without a plan, you have a few options: establish a Solo 401(k) (self-employed people can do this and then roll the IRA balance in), accept the partial taxation and do the conversion anyway if the long-term Roth tax-free growth justifies it, or skip the backdoor until your situation changes.
The timing trick
The pro-rata calculation uses your IRA balance as of December 31 of the conversion year. If you contribute in January and convert immediately, your IRA balance on December 31 should show only the converted amount. The rollover IRA needs to be moved to the 401(k) before December 31 of the same year, not before the conversion.
This means you can contribute after-tax dollars in January, convert in February, and still clean up the rollover IRA by moving it to the 401(k) before December 31. The pro-rata calculation will be clean as of the year-end date that matters.
Form 8606
Every non-deductible contribution and every conversion must be tracked on Form 8606. If you skip this form for years of non-deductible contributions, you lose track of your basis and may end up paying tax twice -- once when you contributed, and again when you convert as if it were all pre-tax money. Keep every 8606 forever. The IRS has no obligation to remember your basis if you can't document it.
Mega backdoor Roth
If your 401(k) plan allows after-tax (not Roth) contributions above the standard $23,500 elective deferral limit, and allows in-service withdrawals or in-plan Roth rollovers, you can contribute up to the $70,000 total annual addition limit and then convert the after-tax portion to Roth. This is the mega backdoor Roth -- it requires plan support that most plans don't offer. Check your plan documents or ask your HR team specifically whether after-tax 401(k) contributions plus in-service withdrawals are allowed.