What is the pro-rata rule and does it affect me?

If you have ever researched the backdoor Roth IRA, you have probably run into the term pro-rata rule. It sounds technical, but the underlying idea is simple: the IRS will not let you pick and choose which dollars in your IRA are taxable when you do a conversion. Every conversion pulls proportionally from the entire pool.

For most people doing a clean backdoor Roth with no other IRA balances, the rule is irrelevant. But for anyone who has rolled an old 401(k) into a Traditional IRA, or who has a SEP IRA from self-employment income, the pro-rata rule can turn what looks like a tax-free move into a significant and entirely unexpected tax bill.

Does it affect you?

The pro-rata rule only matters if two things are true at the same time: you are doing a Roth conversion, and you have pre-tax money in any Traditional IRA, SEP IRA, or SIMPLE IRA as of December 31 of the conversion year.

One thing that trips people up: it does not matter how many separate IRA accounts you have or which brokerage holds them. The IRS aggregates every Traditional IRA, SEP IRA, and SIMPLE IRA you own into one combined balance for the calculation. Opening a new account at a different institution does not create a separate pro-rata pool. Your 401(k) at work, however, is excluded entirely.

The calculation with real numbers

Say you earn $185,000 as a single filer, which puts you above the 2026 Roth IRA income limit of $168,000. You want to do a backdoor Roth. You contribute $7,500 to a Traditional IRA with after-tax dollars in January. You also have a $92,500 rollover IRA from a job you left three years ago.

At a 32% marginal tax rate, that $6,937 costs you over $2,200 in federal taxes on a conversion you assumed would be free. The conversion itself did not go wrong. The problem was the pre-existing rollover IRA that you forgot was included in the calculation.

One more thing the rule ignores

The IRS does not care when you made the contribution or when you did the conversion within the same year. If you contributed in January and converted in February, but you still had that rollover IRA sitting there on December 31, the pro-rata calculation uses the December 31 balance. Timing the contribution and conversion close together does not help if the underlying IRA balance problem is not resolved before year end.

How to fix it

The cleanest solution is to roll your pre-tax IRA balances into your current employer's 401(k) plan before December 31 of the year you want to do a clean conversion. Most 401(k) plans accept incoming rollovers, though you need to confirm with your plan administrator. Once that pre-tax money is inside the 401(k), it disappears from the pro-rata calculation entirely. Your IRA balance drops to just your new after-tax contribution, and the conversion is clean.

If your 401(k) does not accept rollovers, your options narrow. You could convert the entire pre-tax IRA in a single year and pay the taxes upfront, which makes sense if you expect your tax rate to be higher in the future. Or you can skip the backdoor Roth for now and invest in a taxable brokerage account instead, which is a perfectly sound alternative.

SIMPLE IRA timing rule

One additional wrinkle: if you have a SIMPLE IRA, you cannot roll it into a 401(k) until two years after your first contribution to that account. Contributions made in 2024 become eligible to roll out in 2026. If you are inside that two-year window, you will need to wait or plan around it.

Form 8606 and why it matters

Every year you make a non-deductible Traditional IRA contribution, you must file Form 8606 with your tax return. This form tracks your after-tax basis across years. Without it, the IRS has no record that any of your IRA money was already taxed, and the entire conversion could be treated as ordinary income. If you have been making non-deductible contributions for years without filing Form 8606, a tax professional can help you reconstruct the history and file amended returns to establish your basis before you convert.

The quick version

• The pro-rata rule affects you only if you have pre-tax IRA balances and are doing a Roth conversion
• The IRS aggregates all Traditional, SEP, and SIMPLE IRAs into one pool regardless of where they are held
• 401(k) balances are excluded from the calculation
• The taxable portion equals your pre-tax balance divided by your total IRA balance, applied to every dollar you convert
• The fix: roll pre-tax IRA money into your 401(k) before December 31 of the conversion year
• SIMPLE IRAs require a two-year holding period before they can be rolled into a 401(k)
• Always file Form 8606 when you make a non-deductible IRA contribution