Form 8606 is how many people report nondeductible traditional IRA contributions and track the basis that can reduce tax on a later Roth conversion—the paperwork spine of a typical backdoor Roth. Pro-rata math across all non-Roth IRAs: Roth conversion pro-rata basics. Conversion mechanics: Roth conversions basics. Filing orientation: Filing taxes for beginners.
This page is Form 8606 hygiene for backdoor steps—not a green light to run the strategy and not mega-backdoor 401(k) plan design.
What Form 8606 is doing in a backdoor year
| Piece | Typical role in a backdoor sequence |
|---|---|
| Nondeductible traditional IRA contribution | Often Part I: builds basis when you could not (or chose not to) deduct |
| Roth conversion | Often Part II: reports the conversion; taxable amount depends on basis vs pre-tax IRA balances |
| Running basis carryforward | Prior-year 8606 basis must be tracked so you do not “lose” nondeductible dollars |
| Pro-rata overlay | If other traditional / SEP / SIMPLE IRA pre-tax money exists, conversion tax is usually blended—not “basis only” |
Custodians such as Fidelity, Vanguard, Charles Schwab, and E*TRADE issue contribution and conversion confirmations (and often Form 1099-R for conversions). They do not file Form 8606 for you. Missing 8606 years is a common reason people later overpay tax on conversions.
Common Form 8606 mistakes
- Skipping 8606 in the contribution year. You made a nondeductible contribution but never recorded basis—later conversion looks fully taxable on paper.
- Ignoring pro-rata. You convert “just the new $7,000” while a rollover IRA still holds pre-tax money (pro-rata basics).
- Mismatched years. Contribution in late December, conversion in January—two tax years, two forms; basis and 1099-R timing must match reality.
- Double-counting basis. Reusing the same nondeductible dollars on multiple conversions without reducing carryforward.
- Confusing excess contribution fixes with basis. Removing an excess has its own path (IRA excess contribution removal)—do not improvise on 8606 lines.
Worked example (illustrative only)
Casey earns above the Roth IRA direct-contribution phase-out and uses a backdoor at Vanguard.
- Nondeductible traditional IRA contribution (Year 1): $7,000 → should appear on Form 8606 with basis $7,000 (assuming no prior basis).
- No other IRA balances.
- Conversion a few days later of $7,010 (contribution plus tiny earnings): Form 8606 / conversion reporting generally treats most of the $7,000 as nontaxable return of basis; roughly $10 of earnings may be taxable.
- Contrast: if Casey also held a $63,000 pre-tax rollover IRA, pro-rata would make most of a $7,000 conversion taxable even with $7,000 of basis on Form 8606.
Casey keeps PDF copies of every 8606 and the 1099-R. Software (TurboTax, H&R Block, Free File options when eligible) still needs correct IRA balance inputs—garbage in, garbage out.
Process habits that keep basis alive
- Confirm whether a direct Roth IRA contribution is allowed before engineering a backdoor (Roth IRA vs 401(k) starter).
- Inventory traditional / SEP / SIMPLE IRA balances on December 31 of the conversion year for pro-rata.
- File Form 8606 for every year you add nondeductible basis or convert—even if the taxable conversion amount is small.
- Store prior-year 8606 PDFs with your tax returns; basis is a multi-year ledger.
- Read the current-year IRS Form 8606 instructions; line numbers and IRA limits change.
Checklist
- Know whether this year’s traditional IRA dollars are deductible, nondeductible, or mixed.
- Enter nondeductible contributions on Form 8606 so basis is not invisible.
- Report Roth conversions with the matching 1099-R and 8606 parts.
- Apply pro-rata when other pre-tax IRA money exists—do not invent a “basis-only” conversion.
- Carry forward unused basis; do not restart at zero each January without records.
- Use a tax professional when balances, excess contributions, or multi-year gaps are messy.
Educational only. Not tax, legal, or investment advice. Not a recommendation to use a backdoor Roth. IRS forms, contribution limits, and conversion rules change; confirm with IRS.gov Form 8606 instructions and a qualified tax professional.