Form 8606 tracks every dollar of after-tax basis you ever contributed to a traditional IRA. Without it, the IRS treats every distribution as fully taxable - even amounts you already paid tax on. Basis tracking is the single most overlooked component of retirement planning. The IRS estimates failure rates of 60-70% for taxpayers required to file. The backdoor Roth strategy depends entirely on Form 8606. The pro-rata rule under IRC §408(d)(2) decides whether a conversion is mostly tax-free or mostly taxable.
Part I (Nondeductible Contributions and Distributions). Required whenever you (1) make nondeductible contributions to a traditional IRA, (2) take a distribution from any traditional, SEP, or SIMPLE IRA and you have basis, or (3) convert any of those to a Roth in the same year a nondeductible contribution was made.
Part II (Conversions). Required whenever you convert a traditional, SEP, or SIMPLE IRA to a Roth IRA.
Part III (Roth Distributions). Required whenever you take a non-qualified distribution from a Roth IRA before age 59 1/2 or before the 5-year clock has run.
Nondeductible IRA contributions arise primarily in two situations. The first is the income-related rule: an active participant in an employer retirement plan whose AGI exceeds the deduction phase-out can still contribute, but the contribution is nondeductible. The second is a deliberate strategy: contributing nondeductibly with the intent to later convert to Roth (the backdoor strategy).
| Filing Status | Phase-Out Begins | Fully Phased Out |
|---|---|---|
| Single or HoH, active participant | $79,000 MAGI | $89,000 MAGI |
| MFJ, both spouses active | $126,000 MAGI | $146,000 MAGI |
| MFJ, one spouse active (active spouse) | $126,000 MAGI | $146,000 MAGI |
| MFJ, one spouse active (non-active spouse) | $236,000 MAGI | $246,000 MAGI |
| MFS, active participant | $0 MAGI | $10,000 MAGI |
Line 14 of Form 8606 reports your cumulative nondeductible basis carried forward to the next year. This is the number that follows you through your retirement lifetime. Every Form 8606 you file updates line 14. Lose it, and you lose the basis.
| Line | What It Reports |
|---|---|
| Line 1 | Current-year nondeductible contributions |
| Line 2 | Total basis in traditional IRAs from prior years (line 14 of prior year) |
| Line 3 | Total of lines 1 and 2 (cumulative basis before this year's distributions) |
| Line 4 | Contributions made between Jan 1 and Apr 15 attributable to next tax year |
| Line 5 | Adjusted basis (line 3 minus line 4) |
| Line 6 | Value of all traditional, SEP, SIMPLE IRAs at Dec 31 |
| Line 7 | Distributions taken in the year (excludes rollovers, QCDs, recharacterizations) |
| Line 8 | Amount converted to a Roth IRA |
| Line 9 | Total of lines 6, 7, and 8 |
| Line 10 | Basis ratio: line 5 divided by line 9 (rounded to at least 3 decimal places) |
| Line 11 | Nontaxable portion of conversion (line 8 times line 10) |
| Line 12 | Nontaxable portion of distributions (line 7 times line 10) |
| Line 13 | Total nontaxable amount recovered (line 11 plus line 12) |
| Line 14 | Basis remaining for future years (line 3 minus line 13) |
| Line 15 | Taxable portion of distributions (line 7 minus line 12) |
The pro-rata rule says that every distribution from any traditional, SEP, or SIMPLE IRA is treated as part basis recovery and part taxable distribution, in the same ratio as basis to total IRA value. You cannot isolate the after-tax dollars and convert just those. The IRS treats all your IRAs - across every custodian, every account - as one aggregated pool for this calculation.
Facts: Sarah contributes $7,000 nondeductibly to a new traditional IRA in 2026, intending a backdoor Roth conversion. She already has $93,000 in a SEP-IRA from her freelance years.
Pro-rata math: Total IRA value = $100,000. Basis = $7,000. Basis ratio = 7%.
If she converts the full $7,000: Only 7% ($490) is nontaxable. The other $6,510 is taxable income at her marginal rate. The remaining $6,510 of basis stays in line 14 and only matters when she takes another distribution.
The Roth grows tax-free, but Sarah just paid tax on $6,510 to get $7,000 into a Roth. That defeats the entire purpose of the backdoor strategy.
The pro-rata rule applies to traditional, SEP, and SIMPLE IRAs - but not to employer plans like 401(k)s. The standard workaround: roll your existing pre-tax IRA balance into an employer 401(k) before December 31 of the conversion year. The Form 8606 line 6 calculation captures the year-end balance. If the pre-tax balance is gone by December 31, the basis ratio approaches 100% and the conversion is nearly tax-free.
| Strategy | How It Works |
|---|---|
| 401(k) reverse rollover | Roll pre-tax IRA to current employer's 401(k); 401(k) balances are excluded from the §408(d)(2) calculation. Plan must accept incoming rollovers. |
| Solo 401(k) | Self-employed taxpayer establishes a solo 401(k), rolls pre-tax IRA into it. Same exclusion as employer 401(k). |
| Convert all pre-tax IRA | Pay the tax on the existing balance once, in a lower-bracket year, then enjoy clean backdoor conversions every future year. |
Conversions report on lines 16-18. The amount converted shows on line 16, the basis portion (from line 11) shows on line 17, and the taxable conversion amount shows on line 18. Line 18 flows to Form 1040 line 4b as taxable income.
Important: there is no income limit on Roth conversions. The income limit applies only to direct Roth contributions under IRC §408A(c)(3). The Tax Increase Prevention and Reconciliation Act of 2005 (TIPRA) eliminated the conversion income cap effective 2010. Any taxpayer at any income level can convert any amount.
The backdoor Roth is a two-step transaction. Step one: contribute up to the annual IRA limit nondeductibly to a traditional IRA. Step two: convert that traditional IRA to a Roth. When the strategy works, basis equals contribution and there is no taxable conversion income.
| 2026 Backdoor Roth Limits | Amount |
|---|---|
| Annual contribution (under age 50) | $7,000 |
| Annual contribution (age 50+) | $8,000 |
| Direct Roth contribution phase-out (single) | $150,000 - $165,000 MAGI |
| Direct Roth contribution phase-out (MFJ) | $236,000 - $246,000 MAGI |
The strategy is most useful when the taxpayer's MAGI exceeds the direct Roth contribution phase-out. A married couple at $300,000 MAGI cannot contribute directly to a Roth, but they can each contribute $7,000 nondeductibly to a traditional IRA and convert it - effectively getting $14,000 per year into a Roth that grows tax-free.
Roth distributions taken before the account is qualified (before age 59 1/2 AND before the 5-year clock) follow strict ordering rules under IRC §408A(d)(4). Withdrawals come from (1) regular contributions first - always tax-free and penalty-free, (2) conversion contributions next - in order, oldest first, then (3) earnings - taxable and subject to 10% penalty if under 59 1/2.
| Roth Five-Year Clocks - Two Distinct Tests | What It Determines |
|---|---|
| 5-year contribution clock | Whether earnings are tax-free at distribution. Starts January 1 of the first year ANY contribution was made to ANY Roth IRA. |
| 5-year conversion clock | Whether the 10% penalty under §72(t) applies to a withdrawal of converted principal before age 59 1/2. Separate clock for EACH conversion. |
The two clocks are independent. A taxpayer can have a satisfied 5-year contribution clock but still owe penalty on a recent conversion withdrawal. The clocks always start on January 1 of the relevant year - a contribution made on December 15, 2026 starts the clock retroactively to January 1, 2026.
The default statute of limitations is three years from the date the return was filed. For nondeductible IRA issues, that period applies only if Form 8606 was timely filed for each year a contribution was made or basis was used. Without Form 8606, the assessment period for IRA-related tax is effectively indefinite - the IRS can come back 10 or 20 years later to challenge basis claims.
| Filing Pattern | SOL on IRS Assessment |
|---|---|
| Form 8606 filed every year contribution or basis used | 3 years from return filing |
| Form 8606 not filed in year of nondeductible contribution | Open until form filed; basis claim weak |
| Form 8606 not filed in distribution year | 6 years if substantial understatement (IRC §6501(e)) |
Taxpayers regularly arrive with no Form 8606 history but insist they made nondeductible contributions in past years. The IRS allows late Form 8606 filings without amending the original return - file each missing year's Form 8606 standalone with the $50 penalty under IRC §6693(b), or attach an explanation requesting reasonable-cause waiver.
Evidence to gather: prior-year Forms 5498 from each custodian (these show contributions but not deductibility), prior-year Forms 1040 (to verify whether the contribution was claimed as a deduction on Schedule 1), prior-year W-2 box 13 (showing active participation status), and bank statements showing the contribution. The taxpayer's own claim alone is insufficient.
Each spouse files their own Form 8606. Husband's IRA balance and basis do not aggregate with wife's. Spousal IRAs do not appear on each other's pro-rata calculations. This is one of the few times the IRS treats spousal accounts strictly separately.
An inherited traditional IRA is not aggregated with the taxpayer's own IRAs for §408(d)(2) purposes. The pro-rata calculation on Form 8606 considers only the taxpayer's own traditional, SEP, and SIMPLE IRAs - not inherited accounts. Inherited IRA basis (if any) is tracked on a separate Form 8606 for the inherited account.
If a taxpayer makes a nondeductible contribution AND a Roth conversion in the same year, line 1 of Form 8606 includes the new contribution, which flows into line 3 (current-year basis) before the pro-rata calculation. The new contribution's basis is recovered pro-rata in the same year - this is the cleanest scenario and the heart of the backdoor strategy.
The §408(d)(2) aggregation rule includes SEP IRAs and SIMPLE IRAs alongside traditional IRAs. A taxpayer with $4,000 of nondeductible basis and $200,000 in a SEP from years of self-employment has a 2% basis ratio. Many practitioners miss the SEP/SIMPLE inclusion and overstate the tax-free portion of conversions. Always pull all 12/31 IRA values from every custodian.
The TCJA eliminated the ability to recharacterize a Roth conversion back to a traditional IRA effective for conversions made after December 31, 2017 (IRC §408A(d)(6)(B)(iii)). Recharacterization of regular contributions between traditional and Roth IRAs - and vice versa - is still permitted. Once a conversion is done, it is permanent.
Primary authority: IRC §408 (traditional IRAs), §408(d)(2) (pro-rata aggregation rule), §408A (Roth IRAs), §408A(c)(3) (direct Roth contribution income limits), §408A(d)(4) (Roth distribution ordering), §408A(d)(6)(B)(iii) (no conversion recharacterization post-2017), §6501 (statute of limitations), §6693(b)(2) ($50 penalty for missed Form 8606), §72(t) (10% early distribution penalty). Tax Increase Prevention and Reconciliation Act of 2005 (TIPRA), removing Roth conversion income limits effective 2010. IRS Form 8606 Instructions (2025), IRS Publication 590-A (Contributions to Individual Retirement Arrangements), IRS Publication 590-B (Distributions from Individual Retirement Arrangements), Rev. Proc. 2025-32 (2026 inflation adjustments).