A backdoor Roth conversion is generally not taxable if you have no pre-tax money in any traditional, SEP, or SIMPLE IRA on December 31 of the year you convert. That is the whole answer in one line. The catch is that the IRS decides how much of your conversion is taxable by looking at every non-Roth IRA you own as a single pool, not just the account you funded for the backdoor. If that pool contains pre-tax dollars, a proportional share of your conversion becomes ordinary income, no matter which dollars you thought you were moving.
Why the Pro-Rata Rule Decides the Tax
The IRS does not let you cherry-pick which dollars you are converting. It treats every traditional, SEP, and SIMPLE IRA you own as one combined pool and applies a ratio to figure out how much of any conversion is tax-free.1Internal Revenue Service. Publication 590-B – Distributions From Individual Retirement Arrangements (IRAs)
The formula: divide your total after-tax basis by the total value of all your non-Roth IRAs as of December 31 of the conversion year. That percentage is the tax-free share of your conversion. Everything else is taxable at ordinary income rates.
Roth IRAs sit outside this calculation. So do employer plans like 401(k)s and 403(b)s. Your spouse’s IRAs also don’t count; the math runs per person. But a SIMPLE IRA from a side business or a SEP IRA from freelance income absolutely counts.2Internal Revenue Service. Retirement Plans FAQs Regarding SIMPLE IRA Plans
Here is what that looks like in practice. Say you contribute $7,500 non-deductibly and convert it, but you also have a $92,500 traditional IRA rollover from an old 401(k), all pre-tax. Your total IRA value on December 31 is $100,000, and your basis is $7,500. The tax-free percentage is 7.5%. Of the $7,500 you converted, only $562 comes across tax-free. The remaining $6,938 is taxable ordinary income. At a 35% marginal rate, that is roughly $2,428 in unexpected tax, which largely defeats the point of the strategy.
Pay close attention to the December 31 timing. Even if you complete the conversion in February, the IRS uses your total IRA balance at year-end. Roll a 401(k) into a traditional IRA in November and that balance joins the denominator, raising the taxable portion of a conversion you finished months earlier.
When the Conversion Really Does Cost Zero
With no other traditional, SEP, or SIMPLE IRA balances, the math is clean. You contributed $7,500 of after-tax money. You convert $7,500. Basis equals the conversion, so the taxable portion is zero. After-tax dollars have moved into a Roth wrapper without any additional tax bill.
One small wrinkle: any growth between contribution and conversion is pre-tax and becomes taxable when you convert. If the account earns $12 in interest before you convert, you owe tax on that $12 at your marginal rate. At a 35% bracket, that is about $4. Trivial, but a reason most people convert within a day or two of contributing rather than letting the money sit.
Clearing Out Pre-Tax IRA Balances
If pre-tax money in a traditional, SEP, or SIMPLE IRA is standing between you and a tax-free backdoor Roth, you have two realistic ways to move it out of the pro-rata denominator by December 31.
The most common is a reverse rollover: move the pre-tax traditional IRA money into your current employer’s 401(k), 403(b), or similar workplace plan. Employer plans are ignored in the pro-rata calculation, so the dollars leave the equation. The rollover itself is not a taxable event because the money stays pre-tax. Not every employer plan accepts incoming rollovers, so confirm with your plan administrator before counting on it.
If no employer plan will take the money, the other option is to convert all your pre-tax IRA balances to a Roth in one shot. You would owe income tax on the entire pre-tax amount in the year of conversion, which can be a large bill. Once done, though, every future backdoor conversion is clean. Whether that upfront tax hit is worth it depends on the size of the pre-tax balance, your current bracket, and how many years of tax-free growth you expect ahead.
Why Form 8606 Determines Whether You Actually Owe Tax
Every backdoor Roth conversion requires IRS Form 8606, which tracks your non-deductible contributions and calculates the taxable portion of your conversion.3Internal Revenue Service. IRS Form 8606 – Nondeductible IRAs Part I records the non-deductible contribution and any carryover basis. Part II runs the pro-rata math using your December 31 IRA balance and the amount converted. Whatever taxable number Part II produces flows to your Form 1040 as ordinary IRA income.
For a clean backdoor Roth, Part II will show zero or a tiny amount for stray earnings. File it anyway. Skipping it carries a $50 penalty per missed form, with a reasonable-cause exception available.4Office of the Law Revision Counsel. 26 U.S. Code 6693 – Failure to Provide Reports on Certain Tax-Favored Accounts or Annuities The bigger risk is not the $50. Without Form 8606 on file, the IRS has no record that your contribution was non-deductible. If you are audited or need to prove basis years later, the missing form can mean paying tax on money you already paid tax on. Each spouse files their own Form 8606 if both execute conversions.
Missed it in a prior year? The cleanest fix is to file an amended return (Form 1040-X) for that year with the missing Form 8606 attached. You can also mail a standalone Form 8606 with a cover letter, though there is some risk the IRS will not link it to the original return.
State Income Tax
Federal rules govern the mechanics above, but your state may add its own layer. Nine states have no income tax at all, so no state tax hits the conversion. A handful of states with income taxes specifically exempt retirement account conversions or IRA rollover distributions. Most income-tax states, though, follow the federal treatment: if the conversion is taxable federally, it is taxable at the state level too. If you live in a high-tax state and are considering a large conversion to clear out pre-tax balances, running the numbers with state tax included often changes the answer on whether to do it all in one year or spread it across several.
Contribution Deadline vs. Conversion Deadline
One timing point trips up people trying to figure out what year a conversion falls into. A non-deductible traditional IRA contribution for 2026 can be made any time from January 1, 2026, through the tax filing deadline in April 2027. A Roth conversion, by contrast, is dated to the calendar year it happens in.1Internal Revenue Service. Publication 590-B – Distributions From Individual Retirement Arrangements (IRAs) A conversion completed on January 3, 2027, is a 2027 conversion reported on your 2027 return, even if it uses a contribution you designated for 2026. That means the pro-rata test looks at your December 31, 2027 balances, not your 2026 balances, and any tax on the conversion lands on your 2027 return.