How to Fill Out Form 8606 for a Backdoor Roth IRA

Form 8606 is the IRS document that keeps a backdoor Roth conversion from being taxed twice, and filling it out for a backdoor Roth IRA comes down to two parts: Part I establishes that your traditional IRA contribution was made with after-tax dollars, and Part II reports the conversion to a Roth. Done correctly with no other pre-tax IRA money in the picture, line 18 shows $0 taxable and the conversion costs you nothing in income tax. The 2026 IRA contribution limit is $7,500, or $8,600 if you’re 50 or older.¹

Check the Pro-Rata Rule Before You Start

The pro-rata rule is the reason most backdoor Roth attempts go sideways, and it lives inside line 6 of the form. The IRS treats every traditional, SEP, and SIMPLE IRA you own as a single pool when it calculates how much of your conversion is taxable. You cannot cherry-pick which dollars to convert.

Say you have $92,500 of pre-tax money in a rollover IRA from an old employer and you contribute $7,500 of after-tax money to a separate traditional IRA. The IRS sees one $100,000 pool. Only 7.5% is after-tax, so only 7.5% of any conversion is tax-free. Convert $7,500 and roughly $6,940 of it becomes taxable.² That defeats the entire strategy.

What counts toward the pro-rata calculation: traditional IRAs, rollover IRAs, SEP IRAs, and SIMPLE IRAs. What does not count: 401(k)s, 403(b)s, 457(b)s, Roth IRAs, inherited IRAs, and your spouse’s IRAs.

If you have pre-tax IRA money and your current employer’s 401(k) accepts incoming rollovers, moving that balance into the 401(k) before December 31 removes it from the pro-rata calculation. The December 31 balance is what matters, so a rollover in October zeroes out a balance you had in March.³ If your plan won’t accept the rollover, you’re stuck with the pro-rata math, and a backdoor Roth may not make sense until you resolve those balances.

Documents You Need

Three numbers drive the form:

Your non-deductible contribution for the tax year. For most 2026 backdoor Roth contributors, $7,500, or $8,600 if 50 or older.
Your December 31 balance across all traditional, SEP, and SIMPLE IRAs. Your custodian reports this on Form 5498, but that form doesn’t arrive until late May or early June, well after the April filing deadline. Use your year-end account statement instead.
Your total Roth conversion amount for the year, reported on Form 1099-R (arriving by early February). Box 7 will show distribution code 2 if you’re under 59½ or code 7 if you’re 59½ or older.⁴

If you’ve done a backdoor Roth in prior years, pull your last Form 8606. Line 14 from that return carries forward as this year’s starting basis. Download the current year’s version from IRS.gov to be sure you’re on the right edition.⁵

Part I: Establishing Your Non-Deductible Basis

Part I proves how much after-tax money is inside your traditional IRAs. Every line builds toward a single ratio: what fraction of your total IRA balance is money you already paid tax on. The walkthrough below assumes a clean backdoor Roth with no other IRA balances.

Lines 1 Through 5: Your Basis

Line 1 is your non-deductible contribution for the tax year. If you contributed $7,500 and didn’t claim a deduction, enter $7,500.⁶

Line 2 is your total basis carried forward from prior years. First backdoor Roth? Enter $0. Otherwise, use line 14 from your last Form 8606.

Line 3 adds lines 1 and 2. In the clean example, $7,500.

Lines 4 and 5 handle a timing quirk. If you made the contribution between January 1 and April 15 of the following year (designating it for the prior tax year) and it happened after a conversion that took place during the tax year, those late-arriving dollars don’t count toward the non-taxable portion of that year’s conversion. Line 4 captures that exclusion; line 5 gives you the adjusted basis available for the conversion calculation. For most straightforward backdoor Roths, line 4 is $0 and line 5 equals line 3.

Lines 6 Through 13: The Pro-Rata Calculation

Line 6 is the most consequential line on the form. Enter the total value of all your traditional, SEP, and SIMPLE IRAs as of December 31, plus any outstanding rollovers.² If you contributed $7,500 and converted the full amount before year-end, the balance is $0. Any other traditional IRA with pre-tax money also goes on line 6. This is where the pro-rata rule lives.

Line 7 is any distributions you took from traditional IRAs during the year that were not conversions. For a backdoor Roth with no other distributions, $0.

Line 8 is the total amount you converted to a Roth during the year. Take this from your 1099-R. In a clean backdoor, $7,500.

Line 9 adds lines 6, 7, and 8. This is the total IRA “universe” the IRS uses as the denominator in the pro-rata fraction. In the clean example, $0 + $0 + $7,500 = $7,500.

Line 10 divides line 5 by line 9. This is the ratio that tells you what percentage of your IRA pool was after-tax money. With $7,500 on both lines, the ratio is 1.000, meaning 100% of the pool is after-tax. If you had $93,000 of pre-tax IRA money on line 6, the denominator would be $100,500 and the ratio would drop to about 0.075.

Line 11 multiplies line 8 by line 10. This is the non-taxable portion of the conversion. In the clean scenario, $7,500 × 1.000 = $7,500. None of the conversion is taxable.

Lines 12 and 13 do the same math for any non-conversion distributions (line 7 × line 10) and add that to line 11. With no other distributions, line 13 equals line 11.

Line 14: Basis Carried Forward

Line 14 subtracts line 13 from line 3. This is the leftover basis you carry into next year’s Form 8606. In a clean backdoor Roth where the entire contribution was converted, line 14 is $0. If you contributed but didn’t convert in the same year, line 14 preserves your basis for the year you do convert.⁶

Part II: Reporting the Conversion

Part II is short. It takes the Part I results and applies them to the conversion to determine how much, if any, is taxable.

Line 16: If you completed Part I, enter the amount from line 8. Otherwise, enter the net conversion amount. For a backdoor Roth, this matches your conversion total.

Line 17: The non-taxable portion of the conversion. If you completed Part I, use line 11. In the clean scenario, $7,500.

Line 18: Line 16 minus line 17. This is the taxable portion of the conversion, and it flows to line 4b of your Form 1040.⁶ For a clean backdoor Roth, $7,500 − $7,500 = $0. That zero is the entire point.

If line 18 shows a positive number, either the pro-rata rule captured some pre-tax money in the conversion, or the traditional IRA earned investment gains between the contribution and the conversion. Small gains of a few dollars are common, taxable, and usually trivial. To minimize them, convert as quickly after contributing as your custodian allows, ideally within days.

Part III of Form 8606 covers distributions out of a Roth IRA, not conversions into one. You do not need to complete it in the year you do the backdoor conversion.

When the Contribution and Conversion Fall in Different Tax Years

The cleanest backdoor Roth happens inside a single calendar year: contribute in March, convert in March, report everything on one Form 8606. Timing doesn’t always cooperate.

If you make a non-deductible contribution in January 2026 designated for the 2025 tax year (allowed up to the April 15 filing deadline) and then convert those funds later in 2026, the contribution goes on line 1 of the 2025 Form 8606 and the conversion goes on the 2026 Form 8606.⁷ You file Form 8606 twice: once to establish the basis, once to report the conversion. The basis from the 2025 filing carries to line 2 of the 2026 form.

The split doesn’t change the tax outcome. It does mean you need to file the first Form 8606 even though there’s no conversion to report that year. Skip it and the IRS has no record of your basis when you convert the following year, and you’ll pay tax on dollars you already paid tax on.

Filing the Form

Form 8606 attaches to your Form 1040. Tax software usually generates it automatically when you enter IRA contributions and conversions, but review the output: software sometimes mishandles the December 31 balance or fails to aggregate multiple IRA accounts on line 6.²

If you’ve already filed your 1040 and realize you left out Form 8606, you can file it as a standalone document. Sign page 2, include your address on page 1, and mail it to the IRS service center where you’d normally file your return. The deadline matches your regular return deadline, typically April 15, with extensions applying.

Fixing Past Mistakes and Late Filings

If you made non-deductible IRA contributions in prior years and never filed Form 8606, that basis still exists. You just haven’t told the IRS about it. You can file Form 8606 retroactively for prior tax years to establish the record. Use the version that matches each year you’re claiming, and complete Part I for each year to build the basis chain from line 14 forward.

The stakes are real. Without a filed Form 8606, the IRS assumes every dollar in your traditional IRA was deducted and is therefore fully taxable on conversion or withdrawal. If you’ve been doing backdoor Roth conversions for several years without filing, you could face an unexpected tax bill covering all those prior conversions. Filing the form late carries a $50 penalty, and overstating your non-deductible contributions carries a $100 penalty, both small compared to losing your basis documentation.⁸

Keep every Form 8606 you file, along with your year-end IRA statements and 1099-R forms, until you’ve taken every last dollar out of your IRAs. The IRS instructions specifically say to retain these records until all distributions are made, which could be decades away.

1
Internal Revenue Service. Retirement Topics – IRA Contribution Limits
2
Internal Revenue Service. Instructions for Form 8606 (2025)
3
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
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Internal Revenue Service. Instructions for Forms 1099-R and 5498 (2025)
5
Internal Revenue Service. About Form 8606, Nondeductible IRAs
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Internal Revenue Service. Form 8606 – Nondeductible IRAs
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Internal Revenue Service. Publication 590-A (2025)
8
Office of the Law Revision Counsel. 26 U.S. Code 6693 – Failure to Provide Reports on Certain Tax-Favored Accounts or Annuities