How to Report Backdoor Roth on Your Tax Return: Form 8606 and 1040

To report a backdoor Roth on your tax return, file IRS Form 8606 alongside your Form 1040. Part I documents the nondeductible contribution to your traditional IRA, Part II reports the conversion to a Roth IRA, and the results flow to Form 1040 lines 4a and 4b. Done correctly, line 4b (the taxable amount) is zero or nearly zero, because you already paid tax on the dollars before contributing. Skip the form or fill it in wrong and the IRS has no record that the contribution was after-tax, which sets you up to be taxed on the same money twice.

Documents to Have in Front of You

Your custodian will send two forms that drive the reporting. Form 1099-R reports the distribution from the traditional IRA (which is how the IRS sees the conversion). Box 1 shows the gross distribution, and Box 7 carries a distribution code: 2 if you were under 59½ at the time of conversion, 7 if you were 59½ or older.¹ Either code is normal for a backdoor Roth, and the IRA/SEP/SIMPLE checkbox in Box 7 should be marked.

Form 5498 confirms how much you contributed to the traditional IRA for the tax year, with the amount shown in Box 1.² Custodians have until the end of May to send it, so it often arrives after you’ve already filed. You don’t need to wait for it; keep it for your records when it shows up.

If you’ve made nondeductible contributions in past years, pull last year’s Form 8606. Line 14 carries your accumulated after-tax basis forward into the current year. Losing track of that running number is one of the most common backdoor Roth mistakes, and reconstructing it years later is painful.

Filling Out Form 8606 Part I

Part I establishes that your traditional IRA contribution was made with after-tax money. Without it, the IRS treats the contribution as deductible, and the conversion becomes fully taxable.³

Line 1: Your nondeductible contribution for the tax year.
Line 2: Your total basis from all prior years, meaning the running total of past nondeductible contributions not yet distributed or converted. Enter zero if this is your first backdoor Roth.⁴
Line 3: Lines 1 and 2 added together, which is your total cost basis for the year.
Line 6: The total value of all your traditional, SEP, and SIMPLE IRAs as of December 31, plus any outstanding rollovers. This number drives the pro-rata calculation.⁵
Line 8: The amount you converted to a Roth IRA during the year.

In the textbook case, you contributed the annual limit, had no prior basis, held no other traditional IRA balances on December 31, and converted the full amount before year-end. The form’s built-in math then produces a non-taxable portion equal to the full conversion, and nothing gets added to your income.

The Pro-Rata Rule Can Create a Tax Bill

The pro-rata rule is where most backdoor Roth returns go sideways. The IRS won’t let you pick which dollars inside your traditional IRAs get converted. Every conversion is treated as a proportional mix of pre-tax and after-tax money across all your traditional, SEP, and SIMPLE IRA accounts combined.⁵

Say you have $45,000 of pre-tax money in a rollover IRA from a former employer, and you make a $5,000 nondeductible contribution to a separate traditional IRA. Your total IRA pool is $50,000, and only 10 percent of it is after-tax basis. Convert $5,000 to a Roth and the IRS treats 90 percent of that conversion as taxable income. You’d owe tax on $4,500. Different brokerages, different account numbers, none of it matters. The IRS looks at the combined pool.

The cleanest fix is to have zero pre-tax balances in traditional, SEP, or SIMPLE IRAs on December 31 of the conversion year. If you have pre-tax money sitting in old rollover IRAs or from deductible contributions, consider rolling those balances into a current employer’s 401(k) before you do the backdoor Roth. Most 401(k) plans accept incoming rollovers, and once the money is inside the 401(k) it drops out of the pro-rata calculation. One rollover can turn a partially taxable conversion into a fully tax-free one.

Filling Out Form 8606 Part II

Part II takes the basis you established in Part I and applies it to the actual conversion. Complete it if you moved any amount from a traditional IRA to a Roth IRA during the year.³

Line 16: The total amount you converted from your traditional IRA to a Roth IRA. If you completed line 8 in Part I, this number carries forward.
Line 17: The non-taxable portion of the conversion, based on the Part I basis calculation.
Line 18: Line 16 minus line 17. This is the taxable amount that gets reported as income on Form 1040.

For a clean backdoor Roth with no pre-tax IRA balances and no earnings between contribution and conversion, line 18 should be zero. If a few dollars of interest or gains built up before you converted, line 18 will show that small taxable amount. Converting quickly after contributing keeps this number as low as possible.

Check line 14 before you finish. It carries any remaining basis forward to next year. If you converted everything, line 14 should be zero. If you contributed but converted only part, or you had leftover basis from prior years that stayed behind, line 14 shows what you’ll bring into next year’s Form 8606.

Moving the Numbers to Form 1040

The Form 8606 results feed directly into your Form 1040. Line 4a shows the total IRA distribution, which should match the gross distribution on Form 1099-R and line 16 of Form 8606. Line 4b shows the taxable portion, taken from line 18 of Form 8606.⁶

If the whole conversion was after-tax basis with no earnings, line 4b is zero. Line 4a still reflects the total distribution so the IRS can see the transaction happened; line 4b tells them no additional tax is owed. Most tax software handles this transfer automatically once you enter the 1099-R and answer the prompts about nondeductible contributions.

When any portion is taxable, that amount adds to your adjusted gross income. A larger-than-expected taxable figure from a botched pro-rata calculation can push you into a higher bracket or shrink your eligibility for income-based tax credits for the year.

Contribution and Conversion Deadlines

The two steps of a backdoor Roth follow different deadline rules, and mixing them up delays reporting by a full year.

The nondeductible traditional IRA contribution for a given tax year can be made any time from January 1 of that year through the tax filing deadline the following April, generally April 15. A filing extension does not push this deadline back.²

The conversion is different. There’s no separate calendar deadline, but the calendar year you actually convert determines the tax year the conversion belongs to. Contribute in December and convert in January and you’ll have a contribution on this year’s Form 8606 and a conversion on next year’s. That’s why most people convert as soon as the contribution settles, often within days: it keeps both steps on the same return and minimizes taxable growth in between.

Penalties for Skipping or Misfiling Form 8606

Failing to file Form 8606 when required carries a $50 penalty per failure, unless you can show reasonable cause.⁷ The bigger cost is evidentiary. Without a filed 8606 on record, the IRS has no documentation that your contribution was nondeductible, and proving your basis when you later take distributions gets significantly harder.

Overstate your basis in a way that leads to a substantial understatement of tax and the IRS can impose an accuracy-related penalty of 20 percent of the underpayment.⁸ Overstate the basis by 150 percent or more of the correct figure and the IRS can treat it as a substantial valuation misstatement. At 200 percent or more, the penalty doubles to 40 percent.

File Form 8606 every year you make a nondeductible contribution, even if you don’t convert that year. It builds the paper trail that protects you from double taxation later. If you missed filing it in a prior year, you can submit a standalone Form 8606 for that year to establish the basis retroactively.

The Conversion Can’t Be Undone

Before 2018, a Roth conversion could be reversed through recharacterization if it produced an unexpected tax bill. That option is gone. The Tax Cuts and Jobs Act permanently eliminated recharacterization of Roth conversions completed after December 31, 2017.⁹

So the pro-rata math has to be right before you convert, not after. If you convert while large pre-tax IRA balances are still sitting there, you’ll owe income tax on the pre-tax portion and there’s no way to walk it back. Run the numbers on Form 8606 (or let your tax software run them) before you tell the brokerage to move the money.

Keep copies of every Form 8606 you file for as long as you hold the Roth IRA. Those forms are your cumulative proof of basis. If the IRS ever questions whether a later distribution is taxable, the stack of 8606s is the documentation that ends the conversation.

1
Internal Revenue Service. Instructions for Forms 1099-R and 5498 (2025)
2
Internal Revenue Service. Form 5498 – IRA Contribution Information
3
Internal Revenue Service. Instructions for Form 8606 (2025)
4
Internal Revenue Service. Form 8606
5
Internal Revenue Service. 2025 Instructions for Form 8606
6
Internal Revenue Service. 2025 Instruction 1040
7
Office of the Law Revision Counsel. 26 U.S. Code 6693 – Failure to Provide Reports on Certain Tax-Favored Accounts or Annuities
8
Office of the Law Revision Counsel. 26 U.S.C. 6662 – Imposition of Accuracy-Related Penalty on Underpayments
9
Internal Revenue Service. Retirement Plans FAQs Regarding IRAs