Non-Deductible IRA Contributions and RMDs: Avoiding Double Tax with Form 8606

If you made after-tax (non-deductible) contributions to a traditional IRA at any point in your working years — years when your income was too high to claim a deduction — part of every distribution you take, including required minimum distributions, is tax-free. The mechanism for claiming that exemption is IRS Form 8606. Many retirees don't realize they have basis in their IRAs, or lost the records to prove it. Here's how the rules work and what to do about it.

The double-tax problem

Traditional IRA contributions are normally deductible — you put pre-tax money in, it grows tax-deferred, and you pay ordinary income tax on distributions (including RMDs). That's the standard structure.

But there's a wrinkle: if you or your spouse participated in a workplace retirement plan (401(k), 403(b), pension) during years when your income exceeded certain IRS thresholds, your IRA contributions were not deductible. You contributed anyway — tax-deferred growth is still valuable even without the upfront deduction — but those dollars came from after-tax income.

Those after-tax dollars represent your "basis" — money the IRS has already taxed once. When you take distributions, including RMDs, the basis portion is a return of capital, not taxable income. But if you don't document and claim that basis on Form 8606, the IRS assumes your entire distribution is ordinary income. You pay tax a second time on money you already paid tax on.

Form 8606: the document that protects your basis

IRS Form 8606 is required whenever you make a non-deductible IRA contribution. It serves a single essential function: tracking the cumulative total of after-tax dollars in your traditional IRA pool across your lifetime.¹ Each year you make a non-deductible contribution, Form 8606 adds that amount to a running total. Each year you take a distribution that partially recovers basis, the form reduces the total by the tax-free amount. The remaining basis carries forward from year to year indefinitely.

Without this paper trail, you have no documented claim to tax-free treatment on any portion of your distributions. The IRS default is 100% taxable.

The pro-rata rule: how each RMD is split between taxable and non-taxable

The IRS does not allow you to selectively withdraw after-tax dollars first. Instead, every distribution — including RMDs — is proportionally split between taxable and non-taxable amounts based on the ratio of your total basis to the total value of your entire traditional IRA pool. This is the pro-rata rule.²

The calculation (Form 8606, Part I) works as follows:

1. Identify your total basis. Cumulative non-deductible contributions minus amounts already recovered tax-free in prior distributions. This is the carryforward figure from your most recent Form 8606.
2. Calculate the denominator. Add the fair market value of all traditional, SEP, and SIMPLE IRAs on December 31 to the total distributions taken during the year (including the RMD itself). The distribution amount is included because you're measuring the pool that existed before the withdrawal.
3. Determine the non-taxable fraction. Divide total basis by the denominator.
4. Apply the fraction. Multiply the non-taxable fraction by your total distributions for the year. The result is the tax-free return-of-basis amount.
5. Carry the remaining basis forward. Subtract the tax-free amount from your total basis. This smaller number goes on next year's Form 8606.

The aggregation trap: you cannot isolate basis in one account

This is the most important point to understand about the pro-rata rule: it applies to all your traditional IRAs combined — including SEP IRAs and SIMPLE IRAs — not just the account where you made non-deductible contributions.² You cannot open a separate "basis IRA" and take distributions exclusively from that account to get 100% tax-free treatment. The IRS aggregates every dollar in every traditional IRA into a single pool.

The practical implication: if your overall traditional IRA pool is large relative to your basis, the non-taxable fraction is small.

Denominator simplified for illustration (assumes year-end value equals pool value before distribution).

This is why retirees who rolled over large pre-tax 401(k) balances into a traditional IRA often find that their non-deductible basis becomes a smaller and smaller fraction of the pool — even though the dollar amount of basis is unchanged. The rollover inflates the denominator, diluting the tax-free percentage.

What to do if you never filed Form 8606

This is a common situation. Many retirees contributed non-deductibly to IRAs during high-income working years but never filed — or lost — their Form 8606. The situation is fixable but requires documentation work:

1. Gather your tax history. Request prior-year tax transcripts from IRS.gov (available online for recent years; Form 4506-T for older years). Look for any Form 8606 already in your IRS file. If prior returns show non-deductible contributions on the relevant line, that establishes your basis for those years.
2. Reconstruct contribution history. Old bank statements, IRA custodian records, and annual IRA contribution notices can document the amounts and years of non-deductible contributions. IRA custodians typically maintain records going back 5–7 years; for earlier years, personal records are essential.
3. File late Forms 8606. The IRS allows you to file Form 8606 for prior years. There is a $50 penalty per required-but-unfiled form under IRC § 6693(b)(2), but it is waivable for reasonable cause — and the value of documenting basis over a multi-decade RMD horizon typically far exceeds the penalty cost.³
4. Work with a tax professional. Reconstructing missing Form 8606 history is an accounting task — an enrolled agent or CPA who handles IRA basis issues can calculate the correct cumulative totals, file amended or standalone 8606s for prior years, and determine whether you can amend recent returns to recapture overpaid taxes.

After-tax 401(k) contributions: a related but separate situation

Some 401(k) plans permit voluntary after-tax (non-Roth) contributions above the standard pre-tax deferral limit. When you roll a 401(k) to a traditional IRA, these after-tax amounts either:

• Roll into the traditional IRA, adding to your IRA basis tracked on Form 8606. You then use the pro-rata rule on distributions as described above. If your IRA pool is large, this dilutes the tax-free fraction.
• Roll directly to a Roth IRA — the preferred option for most people. Under IRS Notice 2014-54, you can split a single 401(k) rollover: send pre-tax amounts to a traditional IRA and send after-tax amounts directly to a Roth IRA, with no tax owed on the after-tax portion going to Roth.⁴ This effectively converts after-tax 401(k) money to Roth without any tax cost and avoids the pro-rata complexity entirely going forward.

If you rolled your 401(k) to a traditional IRA in a lump sum years ago and did not track the after-tax portion separately, those after-tax dollars are in your IRA pool as basis — but reconstructing the amount requires records from the plan administrator showing the after-tax sub-account balance at the time of rollover.

Inherited IRAs with basis

When you inherit a traditional IRA from someone who had documented Form 8606 basis, you inherit that basis too. The pro-rata rule applies to distributions from the inherited IRA using the same formula — but only against the inherited IRA pool, calculated separately from your own traditional IRAs. Your IRAs and inherited IRAs are never combined for basis recovery purposes.²

The decedent's final Form 8606 (or the last one filed before death) establishes the basis that passes to beneficiaries. Executors should preserve and transfer this documentation to the inheriting beneficiary. If the decedent never filed Form 8606, the same late-filing reconstruction process applies to the inherited account.

How basis interacts with Roth conversions

Roth conversions draw from the same pro-rata pool as RMDs. If you have traditional IRA basis and convert a portion of your IRA to Roth, each conversion is also partly non-taxable based on the same pro-rata fraction. This means:

• Converting does accelerate basis recovery — each dollar you convert partially returns after-tax basis tax-free.
• But conversions also reduce the denominator (the remaining IRA pool), which increases the non-taxable fraction for future distributions — a beneficial compounding effect as the pool shrinks.
• A coordinated strategy of Roth conversions plus RMDs can recover basis faster while also reducing future RMDs. An advisor can model the interaction across your specific account balances.

Once your traditional IRA pool reaches zero — either through distributions, conversions, or both — your basis is fully recovered and there is nothing left to apply the pro-rata rule to. At that point, Roth assets and taxable accounts carry no further RMD obligations (for the original owner).

Sources

1. IRS — About Form 8606, Nondeductible IRAs. Form 8606 is required when making non-deductible traditional IRA contributions, taking distributions from an IRA with basis, or converting to Roth. It establishes and tracks cumulative after-tax basis and calculates the tax-free portion of each distribution using the pro-rata rule.
2. IRS Publication 590-B — Distributions from Individual Retirement Arrangements. Pro-rata rule: all traditional, SEP, and SIMPLE IRAs are aggregated for basis recovery calculations. Non-taxable fraction = total basis ÷ (December 31 fair market value of all traditional IRAs + total distributions taken during the year). Inherited IRA basis is tracked separately from the beneficiary's own IRA pool.
3. 26 U.S.C. § 6693(b)(2) — Failure to File Form 8606. The penalty for failure to file Form 8606 when required is $50 per unfiled return. The IRS accepts late Forms 8606 for prior years; the penalty is waivable for reasonable cause. Filing retroactively is generally preferable to forfeiting documented basis.
4. IRS Notice 2014-54 — Guidance on Rollovers of After-Tax Contributions from Workplace Plans. After-tax (non-Roth) contributions in a 401(k) or 403(b) can be rolled directly to a Roth IRA in the same distribution event that sends pre-tax amounts to a traditional IRA. No ordinary income is recognized on the after-tax portion going to Roth. The IRS confirmed this "split rollover" treatment in Notice 2014-54.

Pro-rata rule and Form 8606 mechanics reflect IRS Publication 590-B and established IRS guidance. This area of tax law is stable and has not changed materially in recent years. Penalty amounts under § 6693 and contribution phase-out thresholds are subject to IRS adjustment. Consult a tax professional for Form 8606 reconstruction if you have missing or incomplete records. Values verified as of May 2026.