IRS Form 8606 is the form you file with your 1040 to track your nondeductible (after-tax) IRA contributions and report Roth conversions. It is the only thing that tells the IRS which dollars in your traditional IRA have already been taxed — so without it, the IRS treats your whole balance as pre-tax and taxes the same money twice. Anyone doing a backdoor Roth, making nondeductible contributions, or converting a traditional IRA to a Roth needs it.

Jump to your situation:

• Why can't I keep my after-tax money separate? (the pro-rata trap)
• What happens if I lose track of my basis year to year?
• How can a December rollover ruin a clean March conversion?
• How does Form 8606 flow to my Form 1040?
• What if I never filed Form 8606 in past years?

You finally crossed the income limit where you can no longer contribute directly to a Roth IRA. So, you fund a traditional IRA with after-tax money and immediately convert it to a Roth. It feels like a clean, simple workaround.

But the IRS does not see a "Backdoor Roth." They see two distinct, reportable tax events: a nondeductible contribution to a traditional IRA, and a subsequent conversion to a Roth IRA.

The bridge between those two events—and the only thing stopping the IRS from taxing that money a second time—is IRS Form 8606.

Form 8606 tracks your "basis." In tax terms, basis is the money you have already paid taxes on. Because traditional IRAs are designed for pre-tax money, injecting after-tax money into the system requires a tracking mechanism. Form 8606 is that mechanism. If you fail to file it, or if you file it incorrectly, the IRS will assume every dollar in your traditional IRA is pre-tax, and they will tax you again when you convert or withdraw it.

Here is the brutal reality of Form 8606, how it actually works, and the traps that catch high-earning taxpayers off guard.

The Rule of Aggregation: The Pro-Rata Trap

• The core rule: Under IRC § 408(d)(2), the IRS views all of your non-Roth IRAs—Traditional, SEP, and SIMPLE IRAs—as one single, giant bucket.

The costly mistake here: Taxpayers mistakenly believe they can keep their new, after-tax contribution separated from their old, pre-tax money. You cannot.

Imagine you have $93,000 of pre-tax money sitting in an old SEP-IRA from your freelancing days. You open a brand new, empty traditional IRA and drop in a $7,000 nondeductible contribution, then immediately convert that specific $7,000 to a Roth.

Form 8606 forces you to do the math on your aggregate balances. It takes your total after-tax basis ($7,000) and divides it by the total value of all your IRAs combined ($100,000). Your "tax-free percentage" is only 7%. When you convert that $7,000, 93% of it ($6,510) is taxable income. This is the Pro-Rata Rule. Form 8606 is where this painful math is executed.

The Carryforward Trap: Losing Track of Your Basis

• The core rule: Form 8606 is a cumulative, historical document. The basis you establish in one year must be carried forward to the next.

Where people screw this up: If you make a nondeductible contribution in Year 1, but wait until Year 2 to do the conversion, you generate a Form 8606 for Year 1 that calculates your total basis on Line 14.

When Year 2 rolls around, you must transfer that exact number from last year's Line 14 onto this year's Line 2. If you switch tax software, hire a new CPA, or simply forget to carry that number forward, the IRS system loses your basis. When you convert the funds, they will treat the entire amount as taxable.

The December 31st Balance Trap

• The core rule: The pro-rata calculation on Form 8606 (specifically Line 6) looks at the value of your IRAs as of December 31st of the year you do the conversion.

Where taxpayers go wrong: The timeline can retroactively ruin your strategy. Let's say you execute a perfect, clean Backdoor Roth in March. Your traditional IRA balance goes to zero. You think you are safe.

In October, you leave your corporate job and decide to roll your $500,000 401(k) into a traditional IRA. When December 31st hits, Form 8606 takes a snapshot of your accounts. Because that $500,000 is sitting in a traditional IRA on the final day of the year, it gets thrown into the pro-rata denominator. Your "clean" $7,000 conversion from March is now almost entirely taxable. The IRS does not care what your balance was on the day you converted; they only care what it was on New Year's Eve.

How Form 8606 Flows to Your Form 1040

Form 8606 acts as an attachment, but its calculations directly impact the core numbers on your tax return:

• Line 1: This is where you declare your current-year nondeductible contribution. It does not flow to your 1040 because it isn't a deduction. It stays here to build your basis.
• Line 13: The non-taxable portion of your conversion. This is the money you successfully shielded. It does not get reported as income.
• Line 18: The taxable portion of your Roth IRA conversion. This is the painful number. It flows directly to Form 1040, Line 4b (IRA distributions, taxable amount), driving up your Adjusted Gross Income (AGI).

Need to Untangle Your Basis?

If you have executed a Backdoor Roth while holding other pre-tax IRAs, or if you failed to file Form 8606 in previous years, it is incredibly easy for your basis to slip through the cracks. The good news is that this math can be reconstructed and corrected.

Our tax strategists can rebuild your historical basis, file corrective 8606 forms, and structure your accounts to isolate your after-tax money. Book a consultation with our office today to get your accounts properly aligned and ensure you never pay tax twice on the same dollar.

Form 8606 is usually one piece of a larger move. For the strategy it supports, see our overview of Roth conversion and backdoor Roth tax planning, or the related guides to the pro-rata rule and fixing a backdoor Roth mistake.