Backdoor Roth With $50,000 Left in a Traditional IRA: $6,522 of a $7,500 Conversion Is Taxable in 2026

A “backdoor” Roth IRA is a nondeductible contribution to a traditional IRA, converted to a Roth IRA soon after. It works as advertised, with almost no tax, only if your traditional IRAs hold nothing else. If a rollover IRA from an old 401(k) holds $50,000, the conversion is mostly taxable, and the test is made on one date: December 31 of the year you convert. This guide puts numbers on that for a $7,500 contribution (the 2026 limit for someone under 50), using the engine behind the backdoor Roth pro-rata calculator: what each balance costs, what happens to the part that was not taxed, and what moving the pre-tax money into an employer plan changes.

The test is on December 31, and every traditional, SEP and SIMPLE IRA counts

The rule is in 26 U.S.C. 408(d)(2): for working out the tax on an IRA distribution, all your individual retirement plans are one contract, all of a year's distributions are one distribution, and the value is taken at the close of the year. A Roth conversion is a distribution (408A(d)(3)), so it is taxed on the same share as everything else you take out. On Form 8606 that is line 10: your basis (lines 1 to 5) divided by the December 31 value of all your traditional, SEP and SIMPLE IRAs (line 6) plus what came out during the year (lines 7 and 8). Whatever share of the conversion is not basis is added to income (line 18). Converting the same day you contribute does not help, because line 6 is the December 31 value, not the value on the conversion date. A 401(k), 403(b), Roth IRA or inherited IRA is not on line 6 (an inherited IRA has its own Form 8606 basis).

When the contribution is the only basis and it is what you convert, the taxable share is the balance left divided by the balance left plus the contribution. In the first row below the IRAs are empty on December 31, so nothing is taxable. In the fifth, $50,000 is left:

Assumptions: a single filer with $100,000 of other income, no other deductions beyond the 2026 standard deduction, the 2026 brackets (Revenue Procedure 2025-32), so every extra dollar is taxed at 22%; no state tax. The curve is steep at the start: a balance of just $7,500 already makes half the conversion taxable, $10,000 makes 57.1% taxable, and from $100,000 on the strategy stops doing what it is usually described as doing (tax-free by design): 93% is taxable at $100,000 and 98.5% at $500,000. A balance of $67,500 makes 90% taxable.

The part that was not taxed is not lost

The fear is paying tax twice on the $7,500 you already paid tax on. That does not happen if Form 8606 is filed: of the $7,500 converted, the $978 that is not taxed uses up part of your basis, and the rest of the basis, $6,522, stays in your traditional IRAs and is carried to line 2 of next year's form (line 14 of this year's). Next year, that basis is in the numerator again. The table runs the same $7,500 contribution and conversion for eight years with $50,000 of pre-tax money in the IRAs and no investment growth:

The taxable part falls from $6,522 to $2,452, and the tax-free share passes half in year 5 (50.3%). Over the eight years $60,000 goes into the Roth IRA, of which $33,655 was pre-tax money, taxed when converted, and the pre-tax balance falls from $50,000 to $16,345. Read that way, the pro-rata rule does not charge extra tax on the contribution; it makes each conversion carry a slice of your pre-tax money into the Roth IRA, and you pay the tax on that slice now instead of when you would have taken it out of the traditional IRA. Whether paying sooner is a good trade depends on the bracket you would otherwise pay later, which no tool here can know, and in this table every dollar is taxed at 22%.

Moving the pre-tax money into an employer plan

Pre-tax money in a 401(k) or 403(b) is not on line 6. Publication 590-A says a traditional IRA can be rolled into a qualified plan, that the part you can roll over is the part that would otherwise be taxable, and that a special rule treats the move as only taxable amounts as long as what you leave in your IRAs is at least your basis (the limit is in 408(d)(3)(A)(ii)); plans may, but do not have to, accept such rollovers. If the whole $50,000 goes to the plan before December 31, line 6 is zero, the $7,500 conversion is all basis and the $1,435 of tax is gone. With $22,500 of basis already in the IRAs and $60,000 on December 31, only the pre-tax part, $37,500, can go. Unmoved, the same $7,500 conversion is $4,167 taxable; with $37,500 moved, line 6 is $22,500, the basis fills line 5 ($30,000 of $30,000) and none of it is taxable.

• It has to be finished by December 31. The ratio is tested on that date, so do not count on a rollover completed in January helping the previous year. That makes the date a deadline for the plan paperwork, not for the conversion; ask the plan and your IRA custodian how long a direct rollover takes.
• The plan must accept it. Nothing in the tax rules makes a plan take IRA money. Ask the plan whether it accepts rollovers from IRAs, whether the pre-tax money has to be sent directly, and what the plan's investments and fees are, because the money is then in the plan's hands.
• Only the pre-tax part can go. The basis stays behind in the IRA, and the calculator holds your move to the pre-tax amount.
• The money is still pre-tax. It is taxed when it comes out of the plan, as other pre-tax plan money is, so this removes the conversion tax from this year; it does not remove the tax on the pre-tax money.

Where it goes wrong

• Forgetting Form 8606. Publication 590-A says that without the form your contributions are treated as deductible and the distributions are taxed, that a $50 penalty applies for not filing a required form (unless there is reasonable cause), and a $100 penalty for each overstatement of nondeductible contributions. The form is also how the $6,522 of leftover basis gets carried to next year.
• Counting only the IRA you are converting from. Line 6 is every traditional, SEP and SIMPLE IRA you own, at every provider. A forgotten rollover IRA changes the answer.
• Using today's balance. The line asks for December 31. A balance that grows or shrinks through the year changes the ratio; the calculator takes whatever value you give it for that date.
• Mixing up two spouses. Each spouse files their own Form 8606 for their own IRAs; balances are not combined. Publication 590-B's worksheet example (Rose Green: $300 of basis, a $2,000 contribution, $20,000 left and $5,000 converted) gives a ratio of 0.092, so $460 is never taxed and $4,540 is taxable, and the calculator reproduces it.

What this guide leaves out

One illustrative filer, one balance, no investment growth, the 2026 brackets for every year, no state tax, and no change in the other income you have. Not modelled: whether you may contribute and in what amount (see the IRA limit calculator), whether the strategy suits you, the five-year clock on a conversion before you withdraw it early (see the Roth IRA withdrawal calculator), disaster distributions and repayments, inherited IRAs, and what extra income does to credits or Medicare premiums. The form lines follow the 2025 Form 8606; the 2026 form is not out. Estimates, not tax advice or financial advice: check the amounts on your own return, with your IRA custodian or plan, or with a tax professional.