When you convert traditional IRA dollars to a Roth IRA, the IRS generally applies a pro-rata (pro rata) rule if you have a mix of pre-tax and after-tax (nondeductible) money across your non-Roth IRAs. You cannot usually convert “only the nondeductible slice” while ignoring pre-tax balances in other traditional, SEP, or SIMPLE IRAs. That surprise is why a backdoor Roth can still create a tax bill.
Conversion orientation: Roth conversions basics. Ladder timing is separate: Roth conversion ladder basics. Deduction phaseouts on traditional IRAs: Traditional IRA deduction phaseouts. Wrapper taxes: Roth vs traditional IRA taxes.
What the pro-rata rule is aggregating
| Piece | What counts (orientation) |
|---|---|
| Numerator (for taxable share) | Your pre-tax IRA amounts |
| Denominator | All traditional / SEP / SIMPLE IRA balances (pre-tax + nondeductible basis), typically measured under year-end aggregation rules on Form 8606 |
| Result | Each conversion is treated as partly taxable and partly nontaxable in proportion—not “basis first” cherry-picking |
Workplace 401(k) balances are usually outside that IRA aggregation (a common planning lever is rolling pre-tax IRA money into a 401(k) that accepts rollovers—plan rules vary; confirm before you move dollars). Custodians such as Fidelity, Vanguard, and Schwab will process the conversion; they do not erase Form 8606 math for you.
Return mechanics: Filing taxes for beginners.
Why backdoor Roth shoppers care
A classic backdoor Roth is: nondeductible traditional IRA contribution → convert to Roth. If your only IRA dollars are that fresh nondeductible contribution, the conversion may be mostly nontaxable (earnings, if any, still taxable). If you also hold $80,000 of old deductible IRA money, the pro-rata rule generally makes most of the conversion taxable even though you “meant” to convert only the new $7,000.
Worked example: $7,000 nondeductible + $63,000 pre-tax
Alex wants a backdoor Roth at Vanguard:
- New nondeductible traditional IRA contribution: $7,000 (basis tracked on Form 8606).
- Existing rollover IRA (all pre-tax): $63,000.
- Year-end combined non-Roth IRA balance for the ratio (illustrative, ignoring earnings for simplicity): $70,000.
- Alex converts $7,000 the same year.
Illustrative taxable fraction:
| Slice | Math |
|---|---|
| Pre-tax share of IRAs | $63,000 ÷ $70,000 = 90% |
| Nondeductible share | $7,000 ÷ $70,000 = 10% |
| Taxable part of $7,000 conversion | ~$6,300 ordinary income |
| Nontaxable part | ~$700 (basis recovered) |
Alex still owes tax on roughly $6,300 at ordinary rates (federal and possibly state)—not $0—despite the “backdoor” label. Paying that tax from a taxable cash sleeve at Schwab beats withholding from the IRA when Alex wants the full converted amount to compound.
If Alex instead rolled the $63,000 pre-tax IRA into a current employer 401(k) that accepts IRA rollovers before year-end (when that is allowed and wise), the IRA denominator might shrink so a later nondeductible-and-convert is cleaner. That is plan- and timing-specific—get current IRS and plan documents, not blog guarantees.
Practical habits
- Inventory every traditional, SEP, and SIMPLE IRA (all custodians) before you assume a tax-free conversion.
- Read the current Form 8606 instructions; keep prior-year basis statements.
- Estimate the taxable percentage before you click convert at Fidelity / Vanguard / Schwab.
- Do not confuse pro-rata conversion math with the conversion ladder five-year clocks.
- Confirm whether a workplace plan accepts IRA rollovers if you are evaluating that cleanup path.
- Pay conversion tax from non-IRA cash when you can (Roth conversions basics).
Checklist
- List all non-Roth IRA balances and any nondeductible basis.
- Sketch the pro-rata taxable fraction for the conversion size you want.
- Decide whether a same-year backdoor still makes sense after that tax.
- Save contribution and conversion confirmations as PDFs.
- File Form 8606 when required; do not skip basis tracking.
- Re-check IRA totals each December if you convert near year-end.
How Form 8606 records nondeductible basis beside pro-rata math: Backdoor Roth Form 8606 basics.
Educational only. Not tax, investment, or legal advice. Pro-rata aggregation, Form 8606, and plan-rollover rules are fact-specific and change; confirm on IRS.gov and with a qualified tax professional before you contribute or convert.