Form 8606 and Roth Conversion Basis Tracking

Form 8606, filed annually with the tax return, is the IRS mechanism for tracking whether dollars moved into a Roth IRA consist of after-tax or pre-tax contributions. Part II requires you to account for the entire balance of all your traditional and SEP IRAs every time you convert, determining what portion of the conversion is taxable. This calculation can be counterintuitive—even if you transfer money marked “after-tax” from a traditional IRA, the IRS calculates taxability across all IRA accounts combined.

What Form 8606 Is For

Form 8606 has two halves. Part I handles non-deductible contributions to traditional IRAs—those you make after hitting income limits for deductions. Part II handles conversions: when you move money from a traditional, SEP, or SIMPLE IRA into a Roth IRA. Any Roth conversion in any year requires filing Form 8606 that year, even if the conversion is small or even if you’ve filed the form in prior years without a conversion.

The core purpose is enforcement. The IRS wants to prevent a tax-free arbitrage: putting pre-tax money into a traditional IRA, converting it to Roth, and never paying tax on the growth. Form 8606 forces you to account for your total pre-tax IRA balance whenever you convert.

The Pro-Rata Rule and Why You Can’t Cherry-Pick

Many taxpayers assume they can convert only the after-tax portion of a traditional IRA and leave the pre-tax portion behind untouched. The pro-rata rule prevents this. On the date of conversion, the IRS treats all of your traditional, SEP, and SIMPLE IRA balances as a single pool. The fraction of that pool that consists of pre-tax dollars determines what fraction of your conversion is taxable.

Here’s the formula that Part II walks through:

Taxable conversion = Conversion amount × (Pre-tax IRA balance / Total IRA balance)

Suppose you have $100,000 in a traditional IRA (all pre-tax employer rollovers) and $10,000 in a separate savings vehicle. If you convert $10,000, you cannot declare it all after-tax. The calculation is:

• Total IRA balance = $100,000 (all pre-tax)
• Non-taxable basis (after-tax contributions already tracked) = $0
• Taxable portion ratio = $100,000 / $100,000 = 100%
• Taxable amount from your $10,000 conversion = $10,000 × 100% = $10,000

You owe tax on the full $10,000, even though you may have intended to convert only “new” after-tax dollars.

Part II Calculation, Year by Year

Form 8606 Part II requires you to report four key numbers at the end of the year:

1. Traditional, SEP, or SIMPLE IRA balance at year-end (not counting Roth balances). This includes all pre-tax rollovers, employer contributions, and earnings.
2. Non-taxable basis in those accounts. This is cumulative: it includes any non-deductible contributions you’ve made over all prior years, plus any basis carried forward from the previous year’s conversion.
3. Conversion amount during the year.
4. Taxable portion of the conversion, calculated as: [(Total IRA balance − Non-taxable basis) / Total IRA balance] × Conversion amount.

The tricky part: the non-taxable basis figure is a cumulative sum. If you made a non-deductible $5,000 contribution last year, and this year you make another $5,000 and convert $10,000, your basis carryforward is $5,000, your new contribution is $5,000, so your total non-taxable basis starts at $10,000. Then you subtract nothing for the conversion (basis doesn’t “burn” during the conversion; instead, it reduces the taxable fraction). If your total IRA balance is $110,000, the taxable portion of the $10,000 conversion is:

[(110,000 − 10,000) / 110,000] × 10,000 = (100,000 / 110,000) × 10,000 ≈ $9,091

So $9,091 is taxable, and $909 is tax-free (the after-tax portion).

Common Mistakes and Filing Requirements

Not filing for all accounts. Many people think Form 8606 applies only if they’ve made non-deductible contributions. In fact, any Roth conversion—regardless of the conversion amount or account history—requires Form 8606. The IRS cross-checks against 1099-R forms issued by custodians. Filing late or not at all triggers accuracy-related penalties.

Mismatching conversions across custodians. If you have IRAs at multiple institutions, you must aggregate their balances for the pro-rata calculation, even if you physically convert from only one. The IRS doesn’t care which custodian holds the money; it cares about total IRA value under your Social Security Number.

Overestimating basis carryforward. Part II requires a precise calculation of non-taxable basis, usually found in your prior year’s Form 8606, line 14. If you’ve filed the form multiple times, only the most recent basis figure carries forward.

Forgetting basis on non-deductible contributions. If you made non-deductible contributions to a traditional IRA but never filed Form 8606 to claim them, your basis is lost unless you can reconstruct the contributions with Form 8948 (although technically you’d amend prior years’ returns using Form 8606). The IRS assumes all contributions are deductible unless proved otherwise.

Why This Matters for Multi-Year Conversions

Some taxpayers execute “Backdoor Roth” strategies—converting non-deductible contributions immediately each year. Form 8606 simplifies if there’s zero pre-tax balance:

• Year 1: Contribute $7,000 non-deductible, convert $7,000 immediately. IRA balance ends at $0; no taxable portion; $7,000 is entirely after-tax in the Roth.
• Year 2: Repeat. Form 8606 shows $0 pre-tax IRA balance again.

But if you have a $500,000 traditional IRA from an old employer 401(k) rollover, the pro-rata rule forces you to recognize tax on almost the entire conversion. The form doesn’t care that the $7,000 you’re converting came from a “fresh” non-deductible contribution; it taxes you based on the ratio of pre-tax to after-tax across all accounts. This is why high-income earners who hold old rollovers face a substantial tax hit on backdoor conversions.

Filing and Documentation

Form 8606 is filed with your Form 1040 each year you convert. You’ll receive a 1099-R from your custodian showing the conversion amount and any tax withheld. The 1099-R code identifies it as a Roth conversion. IRS Form 5498 (the information return from your Roth IRA custodian) shows the contribution amount but not the conversion; Form 8606 is the required tie-in.

Keep all prior years’ completed Form 8606 pages with your tax records. The cumulative non-taxable basis figure carries forward across years and decades. If you cannot locate prior forms, you may request transcripts from the IRS, although reconstructing basis from old 1040s and 8606 pages is tedious.

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