A backdoor Roth conversion is generally not taxable, provided you have no pre-tax money sitting in any traditional, SEP, or SIMPLE IRA on December 31 of the year you convert. That single condition decides almost everything. If your only IRA balance is the non-deductible contribution you just made, the conversion moves after-tax dollars into a Roth wrapper and nothing gets added to your income. If you hold other pre-tax IRA money, the IRS forces you to treat a proportional share of the conversion as taxable, no matter which dollars you thought you were converting.
How the Two Steps Actually Work
The strategy exists because the IRS caps who can contribute directly to a Roth IRA but places no income limit on contributing to a traditional IRA or on converting traditional IRA funds to a Roth. For 2026, direct Roth contributions phase out between $153,000 and $168,000 for single filers and between $242,000 and $252,000 for joint filers, with no direct contribution allowed above those ceilings.1Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs High earners who want Roth exposure contribute to a traditional IRA without claiming a deduction, then convert that balance.
The 2026 contribution ceiling across all your IRAs is $7,500, or $8,600 if you’re 50 or older.2Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026; IRA Limit Increases to $7,500 Making the contribution on a non-deductible basis creates what the IRS calls basis: money you’ve already paid tax on. The conversion then transfers that basis into the Roth.
When the Conversion Creates Zero Tax
With no other traditional, SEP, or SIMPLE IRA balances and a quick conversion, the arithmetic is clean. You contributed $7,500 of after-tax money. You’re converting $7,500. Your basis equals the conversion amount, so the taxable portion is zero.
Speed matters here. If the account earns interest or market gains between the contribution and the conversion, that growth is pre-tax money and will be taxable when you convert. Most people convert within a day or two to keep the earnings sliver negligible. Even a small amount of growth doesn’t ruin the strategy: $12 of interest at a 35% marginal bracket costs about $4 in tax. The core contribution still moves across tax-free.
The Pro-Rata Rule, Where Most People Get Caught
The IRS does not let you choose which dollars you’re converting. For the tax calculation, it treats every non-Roth IRA you own as one combined pool.3Internal Revenue Service. Publication 590-B – Distributions From Individual Retirement Arrangements (IRAs) Traditional IRAs, SEP IRAs, and SIMPLE IRAs all count.4Internal Revenue Service. Retirement Plans FAQs Regarding SIMPLE IRA Plans Roth IRAs and employer plans like 401(k)s and 403(b)s do not. Your spouse’s IRAs don’t count either; the calculation is per person.
The formula divides your total after-tax basis by the total value of all your non-Roth IRAs as of December 31 of the conversion year. That ratio is the tax-free percentage of what you convert.
A realistic example shows how badly this can bite. Suppose you contribute $7,500 non-deductibly and convert it, but you also hold a $92,500 traditional IRA from an old 401(k) rollover, 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 is tax-free. The remaining $6,938 is taxable as ordinary income. At a 35% marginal rate, that’s about $2,428 in tax on a conversion you expected to cost nothing.
Notice the December 31 timing. Even if you converted 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 gets added to the denominator, increasing the taxable portion of a conversion you finished months earlier.
Clearing Out Pre-Tax Balances First
If you want the backdoor Roth to stay tax-free, you need zero pre-tax money in any traditional, SEP, or SIMPLE IRA by December 31 of the conversion year.
The most common fix is a reverse rollover: move your pre-tax traditional IRA money into your current employer’s 401(k), 403(b), or similar workplace plan. Employer plans sit outside the pro-rata calculation, so this removes those dollars from the formula. The rollover itself isn’t a taxable event because the money stays pre-tax. Not every employer plan accepts incoming rollovers, so confirm with your plan administrator before relying on it.
If no workplace plan is available to receive the money, the alternative is converting all your pre-tax IRA balances to a Roth in one year and paying the full income tax up front. That can be a steep bill, but once the slate is clean, every future backdoor Roth conversion stays tax-free. Whether the upfront cost 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.
Reporting on Form 8606
Every backdoor Roth conversion requires Form 8606, which tracks your non-deductible contributions and calculates the taxable portion of any conversion.5Internal Revenue Service. IRS Form 8606 – Nondeductible IRAs Each spouse files their own if both execute conversions.
Part I records your non-deductible contribution for the year and any prior-year basis you haven’t yet converted. Part II walks through the conversion math: total IRA value on December 31, amount converted, and the taxable portion after the pro-rata calculation. Whatever Part II produces flows to your Form 1040 as ordinary IRA income.
For a clean backdoor Roth with no other IRA balances, the taxable number on Part II is zero or a trivial figure for minor earnings. File the form anyway. The direct penalty for skipping it is $50, with a reasonable-cause exception available.6Office 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 that without Form 8606 on file, the IRS has no record your contribution was non-deductible. If you’re audited later or need to prove basis years down the road, the missing form can mean paying tax on money you already paid tax on.
If you missed Form 8606 in a prior year, the cleanest fix is an amended return (Form 1040-X) for that year with the missing 8606 attached. You can also mail a standalone Form 8606 with a cover letter, though it’s less certain the IRS will link it to the original return.
Deadlines That Decide Which Year the Tax Lands In
Contributions and conversions follow different calendars, and confusing them can push tax into the wrong year or cost you a year of Roth growth.
You can make your 2026 non-deductible traditional IRA contribution any time from January 1, 2026, through the tax filing deadline in April 2027. Conversions, by contrast, must be completed by December 31 to count for that tax year.3Internal Revenue Service. Publication 590-B – Distributions From Individual Retirement Arrangements (IRAs) A conversion done on January 3, 2027, is a 2027 conversion, taxed on your 2027 return, regardless of what tax year the underlying contribution belonged to.
The planning move is to contribute in early January and convert the next day. The money enters the Roth almost a full year earlier than if you waited until April of the following year to contribute, and the tax year of both events is the same, which keeps the Form 8606 reporting straightforward.
State Income Tax
Federal rules govern the mechanics above, but your state may add another layer. Nine states have no income tax, so a conversion creates no state-level tax there. A few other states specifically exempt retirement conversions or IRA rollover distributions. Most states with an income tax follow the federal treatment: whatever is taxable federally is taxable at the state level too. In a high-tax state, factoring in state income tax matters a lot when deciding whether to convert a large pre-tax balance in one year or spread it over several.