The Backdoor Roth IRA Pro-Rata Rule: Why Your Conversion Got Taxed.

A software engineer whose RSUs pushed her household income past the Roth limit does exactly what every personal finance blog tells high earners to do. She puts $7,500 into a nondeductible traditional IRA in January, converts it to a Roth a week later, and expects a tax-free backdoor Roth. In February her tax software tells her almost the entire conversion is taxable. The culprit is the backdoor Roth IRA pro-rata rule, and the reason it bit her is the $50,000 rollover IRA still sitting at her old 401(k) provider from two jobs ago. Here is how the rule works in 2026, why it taxed a conversion she thought was free, and the single move that fixes it, but only if it happens before December 31.

Start with why the backdoor Roth exists at all. Roth IRA contributions phase out at higher incomes. For 2026, under Notice 2025-67, the ability to contribute directly to a Roth phases out between $242,000 and $252,000 of modified AGI for married couples filing jointly, and between $153,000 and $168,000 for single filers. Above the top of the range you cannot contribute directly at all. But there is no income limit on converting a traditional IRA to a Roth, and anyone can make a nondeductible traditional IRA contribution. So the backdoor is two steps: contribute up to $7,500 (or $8,600 if you are 50 or older, including the $1,100 catch-up) to a traditional IRA without taking a deduction, then convert that balance to a Roth. When you have no other IRA money, the contribution creates basis, the conversion uses it up, and almost nothing is taxable. The problem starts when you do have other IRA money.

Under IRC §408(d)(2), you cannot choose which dollars come out of your IRAs. The statute treats every traditional, SEP, and SIMPLE IRA you own as one combined account and taxes any distribution or conversion proportionally. The nontaxable share of your conversion equals your after-tax basis divided by the total value of all of your non-Roth IRAs. You report the calculation on Form 8606: your basis goes on line 5, the year-end value of all traditional, SEP, and SIMPLE IRAs goes on line 6, the converted amount goes on line 8, and the ratio on line 10 decides how much of the conversion escapes tax. You cannot cherry-pick the $7,500 of fresh after-tax money and leave the pre-tax balance behind. The IRS makes you blend them.

Two details make this worse than people expect. First, the balance that counts is the value on December 31 of the conversion year, not the value on the day you convert. So a pre-tax IRA you ignore all year still sits in the denominator. Second, Roth IRAs are excluded from the calculation, and so are employer plans. Money inside a 401(k) or 403(b) is not an IRA and does not count toward the pro-rata denominator. That exclusion is exactly what the fix relies on.

Both paths convert the same $7,500. In Path A the pre-tax rollover IRA dilutes the basis, so 87% of the conversion is taxable even though the engineer paid for the contribution with already-taxed dollars. She does not lose that basis, it carries forward on Form 8606 and reduces tax on a future distribution, but she has handed the IRS roughly $2,380 now to convert money she will eventually have to convert anyway. Path B removes the pre-tax money from the IRA system entirely before the calendar turns, so the denominator is just the $7,500 she contributed and the conversion is fully tax-free. The strategy is identical. The only difference is where the $50,000 sits on December 31.

The reverse rollover is the cleanest fix. Move every pre-tax traditional, SEP, and SIMPLE IRA dollar into your current employer's 401(k) or 403(b), if the plan accepts incoming rollovers, and the pro-rata denominator drops to zero pre-tax money. Three things have to be true for it to work. The plan has to accept the rollover, and many do but not all, so check before you contribute. You can only roll pre-tax money into the plan, not after-tax basis, so the $7,500 nondeductible contribution stays in the IRA to be converted. And the timing is unforgiving: because Form 8606 uses the December 31 balance, the rollover has to land in the plan by year-end, not by the filing deadline. If you are self-employed and have no employer plan to receive the money, a solo 401(k) you sponsor can serve the same purpose, though most solo 401(k) documents will not accept a rollover of SEP or SIMPLE balances, so confirm the plan language first.

Most of the backdoor Roth mistakes I see are variations on the same theme: a pre-tax IRA balance nobody accounted for, or a basis nobody tracked. Three are worth calling out.

• ◆Forgetting that SEP and SIMPLE IRAs count. A consultant who funds a SEP IRA every year and then tries a backdoor Roth is blending after-tax basis into a large pre-tax SEP balance under the same §408(d)(2) aggregation. The SEP has to be emptied into a 401(k) too, or the conversion is mostly taxable.
• ◆Assuming a spouse's IRA taints yours. The pro-rata rule is applied per individual, not per household. Your spouse's $300,000 traditional IRA has no effect on your conversion, and each of you files a separate Form 8606. A married couple can each run a backdoor Roth independently.
• ◆Losing the Form 8606 trail. You must file Form 8606 for every year you make a nondeductible contribution, even years you do not convert, because that is the only place your basis is recorded. Miss the form and the IRS has no record of your after-tax dollars, and you can end up paying tax twice on the same money. The fix for past years is to file the missing 8606s.

There is also a worry that does not deserve the attention it gets. People ask whether converting the day after contributing is a problem under the step transaction doctrine. In practice it is not. There is no statutory waiting period between the contribution and the conversion, and converting promptly actually helps, because it minimizes the pre-conversion earnings that would otherwise be taxable. The real risk is never the speed of the conversion. It is the pre-tax balance sitting in the denominator.

Utah taxpayers feel the Path A mistake twice. The state starts from federal taxable income and taxes it at the 4.5% individual rate H.B. 106 set for tax years beginning in 2025, so the $6,522 the federal pro-rata rule made taxable also lands on the Utah return. The same bracket-management logic that drives smart inherited IRA distributions applies here: a conversion taxed at a marginal rate you control beats a conversion taxed at whatever rate the pro-rata rule leaves you with. And like the income-in-respect-of-a-decedent problem in a net unrealized appreciation election on company stock, the cost is entirely about which account the money sits in when the year closes, not about the dollars themselves.