Form 8606: Tracking Nondeductible IRA Basis

Key Takeaways

• Form 8606 tracks nondeductible IRA contributions, the after-tax basis in your IRA.
• It applies the pro-rata rule, splitting each withdrawal between taxable and tax-free dollars.
• The form is required for nondeductible contributions, Roth conversions, and certain distributions.
• Skipping it can cause your after-tax dollars to be taxed a second time.

What Form 8606 Nondeductible IRA Basis Tracking Is

Most traditional IRA contributions are deductible, but high earners covered by a workplace plan often cannot deduct them. Those nondeductible contributions create basis, money that was already taxed. Form 8606 keeps a running record of that basis.

You file Form 8606 when you make a nondeductible contribution, convert a traditional IRA to a Roth, or take a distribution from an IRA that holds any basis. The form computes the taxable and nontaxable portions so each is reported correctly.

The Intuition

The IRS does not let you cherry-pick. If your traditional IRA holds both pre-tax and after-tax money, you cannot withdraw only the after-tax dollars to avoid tax. The law treats every withdrawal as a blend.

Form 8606 enforces that blend through the pro-rata rule. It looks at your total basis compared to your total IRA balance and applies that ratio to every distribution or conversion. Without the form, the IRS assumes you have no basis and taxes the full amount.

How It Works

The pro-rata calculation drives the form:

Nontaxable percentage = total basis / total IRA balance at year-end (plus distributions)
Tax-free portion      = distribution x nontaxable percentage
Taxable portion       = distribution - tax-free portion

All your traditional, SEP, and SIMPLE IRAs are treated as one combined account for this test. You cannot isolate a single account. Roth IRAs are kept separate.

For a Roth conversion, the same ratio applies. The basis portion converts tax-free; the pre-tax portion is taxable in the year of conversion. Your remaining basis carries forward to the next Form 8606.

This is the engine behind the backdoor Roth, where someone contributes nondeductible money to a traditional IRA and converts it. The strategy is tax-efficient only if there is little or no other pre-tax IRA balance, because the pro-rata rule otherwise makes most of the conversion taxable.

Worked Example

You contribute $7,000 of nondeductible money to a traditional IRA. You also have $63,000 of pre-tax money in other traditional IRAs, so your total balance is $70,000 with $7,000 of basis.

You convert $7,000 to a Roth, hoping it is tax-free. The pro-rata rule says otherwise:

Nontaxable percentage = $7,000 / $70,000 = 10%
Tax-free portion      = $7,000 x 10% = $700
Taxable portion       = $7,000 - $700 = $6,300

Only $700 of the conversion is tax-free. The other $6,300 is taxable, and you still carry $6,300 of basis forward. The large pre-tax balance defeated the clean backdoor result.

Had you held no other pre-tax IRA money, the full $7,000 conversion would have been tax-free.

Common Mistakes

1. Not filing the form. If you never report nondeductible contributions, the IRS treats your whole IRA as pre-tax and taxes it all on withdrawal. You lose credit for money already taxed.
2. Ignoring the pro-rata rule. Many backdoor Roth attempts fail because a large rollover IRA sits in the background, making most of the conversion taxable.
3. Forgetting to carry basis forward. Basis not used in a year carries to the next Form 8606. Restarting from zero overstates future taxable amounts.
4. Counting workplace plan balances. Only IRAs enter the pro-rata test. A 401(k) balance does not, which is why some people roll a pre-tax IRA into a 401(k) before doing a backdoor Roth.
5. Filing one form for a couple. Form 8606 is per person. Spouses each file their own because IRAs are individual accounts.

Frequently Asked Questions

What is Form 8606 nondeductible IRA basis tracking in simple terms? Form 8606 nondeductible IRA basis tracking records the after-tax money in your traditional IRA. That record stops the IRS from taxing those same dollars again when you withdraw or convert.

How does Form 8606 affect investment decisions? It determines how much of a Roth conversion is taxable, which shapes the timing and size of conversions. The worked example shows how a large pre-tax balance can make most of a conversion taxable.

What is a real-world example of Form 8606? Someone with $7,000 of basis but $70,000 in total IRA balances converts $7,000 to a Roth, and the pro-rata rule makes only $700 tax-free while $6,300 is taxed.

How can investors use Form 8606 effectively? File it every year you make a nondeductible contribution or conversion, and consider rolling pre-tax IRA balances into a 401(k) first to make a backdoor Roth cleaner.

How is Form 8606 different from Form 5498? You file Form 8606 with your tax return to track basis. Form 5498 is filed by the IRA custodian to report contributions and balances; you keep it for your records.

Sources

1. IRS. Instructions for Form 8606 (2025). https://www.irs.gov/instructions/i8606
2. IRS. About Form 8606, Nondeductible IRAs. https://www.irs.gov/forms-pubs/about-form-8606
3. IRS. Topic no. 451, Individual Retirement Arrangements (IRAs). https://www.irs.gov/taxtopics/tc451
4. Cornell Legal Information Institute. 26 U.S.C. 408. https://www.law.cornell.edu/uscode/text/26/408

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.