Form 8606

Form 8606 is the IRS form on which an individual reports nondeductible contributions to a traditional IRA and, in later years, calculates how much of a distribution or conversion comes out tax-free as a result. The IRS named the form after the concept rather than the transaction, which is why "Form 8606" and "nondeductible IRA contribution" are used almost interchangeably. The underlying idea is basis. When a contribution is deductible, the government has not yet taxed that money, so every dollar coming out later is taxable. When a contribution is nondeductible, tax has already been paid, and that dollar should not be taxed again. Nothing in the IRA itself records the difference, and no custodian statement tracks it. Form 8606 is the record, filed with the return and kept indefinitely.

Where basis comes from. The most common source is a contribution made by someone whose income exceeds the traditional IRA deduction phase-out range, or who elects not to deduct a contribution that would otherwise be deductible. Contributing to a traditional IRA is always allowed if you have enough earned income; deductibility is a separate question. When the deduction is unavailable, the contribution simply becomes basis, and the backdoor Roth strategy is built on exactly that step.

Where basis does not come from, and this boundary is stated on the form itself. The instructions to Part I say that nondeductible contributions to a traditional IRA "do not include employer contributions (including contributions made pursuant to a salary reduction arrangement) made to a SEP IRA under a SEP arrangement or to a SIMPLE IRA under a SIMPLE IRA plan." The parenthetical is the part that surprises people: a SIMPLE participant's own salary deferrals are employer contributions for this purpose, so they create no basis either. That is easy to get wrong, because the form also states that "traditional IRA" includes traditional SEP IRAs and traditional SIMPLE IRAs, and that "Roth IRA" includes Roth SEP IRAs and Roth SIMPLE IRAs. So those accounts are inside the definitions for every other purpose on the form, including the balance used in the pro-rata calculation, while the employer money that funded them creates no basis. Employer contributions were either excluded from income going in, in which case they are fully taxable coming out, or made on a Roth basis, in which case Roth rules govern.

Basis comes out proportionally, never first. A reader's intuition is usually that a withdrawal takes the after-tax money out first, or that the after-tax contribution can be isolated and converted on its own. Neither is true. All traditional, SEP, and SIMPLE IRAs a person owns are treated as one pool, and each distribution or conversion carries the same ratio of basis to pre-tax money as the pool itself. Form 8606 performs that arithmetic. Someone with a large rollover IRA and a small nondeductible contribution therefore recovers very little basis on any given conversion, which is why a backdoor Roth contribution behaves so differently for two people who made an identical contribution.

The three parts. Part I handles nondeductible contributions to traditional IRAs and distributions from them, and is where the basis carryforward lives. Part II reports conversions from traditional, SEP, or SIMPLE IRAs to Roth IRAs. Part III reports distributions from Roth IRAs, applying the ordering rules that bring contributions out before conversions and conversions before earnings. A married couple files one form each, because basis is personal; there is no joint IRA and no joint basis. The form can also be filed on its own by someone with no filing requirement, which matters for a non-working spouse making spousal IRA contributions.

The penalty is small and the consequence is not. The statute imposes a $50 penalty for failing to file the form and a $100 penalty for overstating nondeductible contributions, each excused if the failure was due to reasonable cause. Nobody's planning turns on $50. The damage is that the IRS has no other record of the after-tax money, so a distribution years later is treated as fully taxable, and reconstructing basis for a decade of unfiled years means pulling old returns, old Forms 5498, and old custodian statements. Anyone who has ever made a nondeductible contribution should keep every Form 8606 permanently, alongside the return rather than in it.

In the year of a nondeductible contribution, you enter the amount on Part I, add it to the basis carried forward from your last filed Form 8606, and the resulting total becomes the new basis. In a year with no distribution and no conversion, that is the whole exercise. In a year with a distribution or a conversion, Part I computes the nontaxable portion by comparing your basis against the total value of all your traditional, SEP, and SIMPLE IRAs, measured as the year-end balance plus amounts distributed or converted during the year. Whatever basis is not recovered stays on the form and carries forward again.

A hypothetical example. Over twelve years Owen made $40,000 of nondeductible contributions, each one reported on Form 8606, and his traditional IRAs now total $200,000 on that measure. He withdraws $20,000. His basis is 20% of the pool, so $4,000 of the withdrawal is tax-free and $16,000 is ordinary income, and his basis drops to $36,000 to carry into next year. Had Owen never filed the form, he would have no documented basis, and the entire $20,000 would ordinarily be treated as taxable, costing him the tax on $4,000 he had already paid tax on once.

Pros

• It is the only mechanism that keeps after-tax IRA money from being taxed a second time, and it costs nothing to file.
• Basis carries forward indefinitely, so the record survives job changes, custodian changes, and decades of inactivity.
• It makes the backdoor Roth contribution reportable and defensible rather than something reconstructed under audit.
• It can be filed by itself by someone with no other filing obligation.

Cons

• Nothing prompts you to file it. No custodian issues a form that says basis was created, so the obligation is easy to miss for years at a time.
• Reconstructing missed years is genuinely difficult and sometimes impossible, and the burden of proving basis sits with the taxpayer.
• It only tracks the number. It does not make basis come out any faster, because recovery is strictly proportional.
• Tax software will happily complete a return without it if you never say a nondeductible contribution was made.