This one is worth walking through slowly, because the mistake is common and the cost is not the immediate tax bill — it is the years of compounding lost while you are sorting it out. If you are about to do a backdoor Roth and you cannot yet explain the words "pro-rata rule" to a friend, read this first.
Setup: who the Roth is for
A Roth IRA is an individual retirement account funded with after-tax money. The contributions you put in have already been taxed. Inside the account, investments grow tax-free, and qualified withdrawals in retirement are tax-free. It is, by far, the most generous retirement wrapper available to US individual investors. The IRS lays out the rules and limits on its retirement plans pages.
There is a catch. You can only contribute directly if your modified adjusted gross income is below a certain threshold — in 2026, the phase-out for single filers runs from $146,000 to $161,000. Above that, direct contributions are not allowed.
The backdoor, briefly
The "backdoor Roth" is the legal workaround. The two steps:
- Contribute up to the annual maximum (see the IRS IRA contribution limits; $7,000 in 2026) to a traditional IRA. There is no income limit on traditional IRA contributions, only on the deduction.
- Convert that traditional IRA balance to a Roth IRA. The conversion is a taxable event — you pay income tax on the amount converted minus your basis — but if you converted the same year you contributed and held nothing else in pre-tax IRAs, the tax should be approximately zero.
That last clause is the trap.
Where it goes wrong
Consider a common scenario. An investor rolled an old employer 401(k) into a rollover IRA, leaving a $38,400 pre-tax balance. Then they make a $6,000 non-deductible contribution to a new traditional IRA in January and convert it to a Roth in March, assuming the only thing the IRS cares about is that $6,000.
The IRS cares about the pro-rata rule. When you have multiple traditional IRA balances — including SEP-IRAs, SIMPLE-IRAs, and rollover IRAs from old 401(k)s — and you convert any of them, the IRS treats your entire pre-tax IRA balance as one pool. The portion of the conversion treated as "after-tax basis" is proportional to the share of your total balance that was after-tax.
In this example: $6,000 of after-tax basis ÷ $44,400 total traditional IRA balance = 13.5% basis. The other 86.5% of the $6,000 conversion is treated as taxable income. At a 32% marginal rate, that is roughly $1,660 of federal tax on a conversion the investor assumed was tax-free.
The direct dollar cost is $1,660. The opportunity cost is larger.
How to clean it up
There are two paths. Option one is to keep doing the backdoor while owing pro-rata tax every year — manageable, but suboptimal forever. Option two is to roll the pre-tax rollover IRA into an active employer 401(k) — which, unlike IRAs, does not count toward the pro-rata calculation — and clear the path for tax-free backdoors going forward.
Option two is usually the better fix, when available. Many (not all) employer plans accept rollovers in. The mechanics: open a 401(k) rollover ticket with the plan, fill out the receiving form, and request the IRA balance be transferred via direct rollover. It typically takes a few weeks and a phone call. From the next year, backdoor conversions are clean.
The $1,660 of federal tax is unrecoverable. The bigger cost is the compounding lost while sorting the structure out. If the prior year's surprise leads an investor to skip the backdoor entirely for two years, the headline cost is roughly $4,200 — back-of-envelope: two years of $6,000 contributions not made, multiplied by six years of compounding at about 9% annualized, against a baseline where they were made.
The expensive part of a Roth mistake is not the tax bill. It is the years you sit out the contribution while you figure out how to fix it.
Four rules to follow
- Check your pre-tax IRA balance before doing a backdoor. If it is more than zero, the conversion is not tax-free. Plan for the pro-rata math or roll the balance into a 401(k) first.
- Do the contribution and conversion in the same calendar year. Holding the contribution in the traditional IRA for several months has no benefit and creates tax complexity if it gains value before conversion.
- File Form 8606 every year you contribute non-deductible. Even if it feels like paperwork for $6,000. Years from now, when you start withdrawing, the 8606s are the proof of your basis.
- If you mess it up, do not freeze. Pay the tax, fix the structure, get back on schedule. Sitting out the contribution is the worse outcome.
None of this is complicated once it is laid out. The good news is that the steps are clear when you read them in the right order — which is exactly what trips people up the first time.
