The backdoor Roth pro-rata rule prevents you from choosing only your after-tax IRA dollars for a conversion. For the ordinary calculation, your nontaxable percentage is eligible after-tax basis ÷ (adjusted December 31 IRA value + that year’s Roth conversions + other counted distributions). Basis is the documented after-tax IRA amount; the basis section below explains which part is eligible for this year’s allocation. Opening a separate IRA does not isolate the new contribution.
For example, convert a $7,000 nondeductible contribution in the same calendar year while $93,000 of pre-tax IRA money remains at year-end, with no prior basis or other distributions: $490 is nontaxable and $6,510 is taxable. The unused $6,510 of basis carries forward. The examples below use the final 2025 Form 8606.
How the backdoor Roth pro-rata rule works
The rule allocates after-tax and pre-tax dollars across your counted IRA pool. It does not follow the account from which you request the conversion. That is why an old rollover IRA can make a conversion partly taxable even when the new contribution was nondeductible.
A $7,000 conversion with a $93,000 pre-tax IRA
Suppose you contribute $7,000 for 2025, claim no deduction, and convert the full $7,000 during 2025. You have no prior basis, no following-year contribution designated for 2025, no other counted distributions and no gains or losses. Your adjusted December 31 IRA value is $93,000, all pre-tax.
The counted total is $93,000 + $7,000 = $100,000. The nontaxable percentage is $7,000 ÷ $100,000 = 7%. Multiplying the conversion by 7% gives $490 nontaxable; the other $6,510 is taxable income. Only $490 of the $7,000 basis was used, so $6,510 remains for a later distribution or conversion. That remaining basis is an after-tax record, not an additional account balance.
On the 2025 form, lines 1 and 8 are $7,000, lines 2, 4 and 7 are zero, and line 6 is $93,000. Line 10 is 0.070000; lines 11 and 17 are $490.00; lines 14 and 18 are $6,510.00.
Same $7,000 conversion. Different taxable amounts.
Each bar totals $7,000. Only the pre-tax IRA balance left at year-end changes.
Green: nontaxable · Navy: taxable
The examples follow the final 2025 Form 8606. For a conversion in another year, use that year’s final form and instructions; the line mapping here has not been verified for final 2026 forms.
If income limits may prevent a full deduction for your traditional IRA contribution for the same tax year, check whether you need the taxable distribution amount to determine modified adjusted gross income (MAGI). If both conditions apply, use Publication 590-B, Worksheet 1-1 before the ordinary calculation. Inherited IRAs, special repayments, disaster distributions and death-year reporting also require their applicable instructions.
A contribution can be designated for the prior tax year, but a Roth conversion belongs to the calendar year in which it actually occurs. For example, contributing for 2025 in early 2026 and converting that money in 2026 does not create a 2025 conversion.
Which IRA balances count—and which do not
Include your own traditional IRAs, traditional SEP IRAs and traditional SIMPLE IRAs, across custodians. A rollover IRA is still an IRA. Do not add balances still held in a 401(k), 403(b), Roth IRA or your spouse’s IRA to your own ordinary traditional-IRA pool. If workplace-plan money is rolled into a traditional IRA, its treatment changes and it may enter the IRA calculation.
Use the adjusted December 31 value for the conversion year—not the balance on the day you converted. For a 2025 conversion, that date is December 31, 2025. For a 2026 conversion, track December 31, 2026 values and check the final 2026 form and instructions before filing; the line numbers here are verified for 2025 only. Outstanding rollovers and other required adjustments can change the reported amount. See Form 8606 instructions, line 6.
Inherited IRAs require separate treatment rather than automatic inclusion in your own pool. A surviving spouse’s election to treat an inherited IRA as their own can change that treatment. Follow the inherited-IRA and separate-form instructions.
Basis is the documented after-tax amount in your traditional IRAs: dollars that should not be taxed again when allocated to a distribution. The amount available for this year’s allocation can differ from your total basis. The form’s lines 1 through 9 separate those amounts before calculating a percentage.
On a narrow screen, scroll the table horizontally to see every column.
| Form line | Amount to identify | Records and qualifications |
|---|---|---|
| 1 | Nondeductible traditional IRA contributions designated for 2025. | Contribution records and the applicable deduction calculation, including eligible following-year contributions. Employer SEP or SIMPLE contributions are not nondeductible contributions for this line. |
| 2 | Prior traditional IRA basis, with required adjustments. | Prior Forms 8606 and supporting worksheets or statements. An unavailable basis record is unknown, not evidence that basis is zero. |
| 4 | The applicable part of line 1 contributed from January 1 through April 15, 2026. | The contribution date, designated year and deductible/nondeductible treatment. This amount is already included in line 1. |
| 6 | Aggregate counted traditional, traditional SEP and traditional SIMPLE IRA value on December 31, 2025, with the form’s adjustments. | Year-end account values; include applicable outstanding rollovers and recharacterizations, and subtract specified repayments as instructed. |
| 7 | Counted 2025 traditional IRA distributions other than Roth conversions. | Distribution records, reconciled to the exclusions and repayment rules below. |
| 8 and 16 | The net amount converted from traditional IRAs to Roth IRAs in 2025. | Conversion and Form 1099-R records. In this ordinary path the amount on line 8 is also entered on line 16. |
The counted IRA pool combines the individual’s adjusted year-end IRA values, counted other distributions and net conversions. It is not just the balance of the account that received the latest contribution. Spouses prepare separate Forms 8606 when required; even on a joint return, their inputs are not combined. The 2025 instructions explain individual reporting and the line-specific adjustments.
For line 7, exclude Roth conversions, qualifying rollovers, qualified charitable distributions (QCDs), a one-time distribution to fund a health savings account (HSA), certain returned contributions and recharacterizations. Transfers incident to divorce and certain repaid qualified distributions also have specified treatment. Include the distributions covered by the instructions’ special repayment rules even if they were later repaid; this does not mean adding the repayment itself as another distribution. Reconcile each amount to the line 7 instructions, and do not count a conversion on both lines 7 and 8.
The final 2025 form includes applicable nondeductible contributions designated for 2025 and made from January 1 through April 15, 2026, on line 1. The applicable portion made in that period also goes on line 4, which removes it from the amount available to allocate against 2025 distributions.
Add line 1 and line 2 to get total basis on line 3. Subtract line 4 to get eligible basis on line 5. The late designated contribution remains part of line 3 even though it is excluded from line 5 for this allocation. The denominator, line 9, is line 6 plus line 7 plus line 8.
Before filing, reconcile Forms 8606, 1099-R and 5498 with account records by tax year. Prior forms and worksheets establish carried basis; contribution statements establish dates and designation; year-end statements establish values; distribution records establish conversions and other withdrawals. The IRS record-retention instructions identify these supporting records. Where they disagree or are missing, the unresolved input needs verification before the arithmetic can answer your question.
Calculate the taxable conversion and remaining basis
For the ordinary path with a counted distribution or conversion, divide eligible basis on line 5 by the counted total on line 9. That percentage determines the nontaxable share of both the conversion and other counted distributions. It must be calculated from the applicable year’s inputs, rather than carried over from a previous year’s percentage.
On a narrow screen, scroll the table horizontally to see every column.
| Form line | Calculation | What it tells you |
|---|---|---|
| 10 | Line 5 ÷ line 9, capped at 1.000. | The nontaxable share. The form requires a decimal rounded to at least three places. |
| 11 and 17 | Line 8 × line 10. | Basis allocated to the Roth conversion: its nontaxable portion. |
| 12 | Line 7 × line 10. | Basis allocated to other counted distributions. |
| 13 | Line 11 + line 12. | Total basis used by the conversion and other counted distributions. |
| 14 | Line 3 − line 13. | Remaining traditional IRA basis after both allocations. |
| 18 | Line 16 − line 17. | The taxable dollar amount of the Roth conversion. |
The mapping follows Form 8606, Parts I and II. Taxable conversion means the amount included in taxable income, not the income-tax bill. This calculation does not apply a marginal tax rate or estimate lifetime tax savings.
Remaining basis comes from line 3, not line 5. It includes basis that was unavailable for allocation because of the contribution date, less the portions actually used by both kinds of distribution. Subtracting only the conversion’s nontaxable amount would overlook basis allocated to other withdrawals. Two checks keep the outputs connected: line 13 plus line 14 equals line 3, and line 17 plus line 18 equals line 16.
The examples round the ratio half up to six decimal places, capped at 1.000, and round each allocation separately to cents. The IRS form allows at least three decimal places. If you use a different permitted precision or whole-dollar filing convention, recalculate the dependent amounts consistently; the examples are not a tax-software certification.
In the simple first-year case, the newest nondeductible contribution supplies all eligible basis. Dividing it by adjusted year-end IRA value plus the conversion therefore agrees with the full calculation. That shortcut assumes no prior basis, no later designated contribution and no other counted distribution. Once those inputs change, use the full form calculation.
If there was no counted distribution and no Roth conversion, the form directs line 3 straight to line 14 and stops the rest of Part I. No ratio is needed. In the hypothetical example, nondeductible contributions designated for 2025 are $7,000, all made in early 2026, and prior basis is $3,000. The adjusted year-end IRA value is $10,000, and counted distributions and conversions are both zero. Enter $7,000 on line 1 and $3,000 on line 2; line 3 and remaining basis on line 14 are both $10,000. Do not fill in lines 4 through 13 for this branch; the additional amounts are example inputs, not instructions to complete those lines.
Carry forward the remaining basis and its supporting records, not this year’s percentage. A later conversion uses that year’s basis, balances and distributions.
The worked example below shows how contribution dates and another distribution change the allocation.
Worked example: prior basis and a later contribution
In this hypothetical 2025 case, $7,000 of nondeductible contributions belong to 2025, but $2,000 of that amount was contributed in the applicable early-2026 period. Prior basis is $3,000. The adjusted year-end IRA value is $23,000, other counted distributions are $5,000, and the Roth conversion is $7,000. Those are the six inputs from the earlier table, with the ordinary-path conditions already screened.
On a narrow screen, scroll the table horizontally to see every column.
| Step | Reconciliation |
|---|---|
| Total basis, line 3 | $7,000 + $3,000 = $10,000 |
| Eligible basis, line 5 | $10,000 − $2,000 = $8,000 |
| Counted total, line 9 | $23,000 + $5,000 + $7,000 = $35,000 |
| Nontaxable share, line 10 | $8,000 ÷ $35,000 = 0.228571 after six-place rounding |
| Conversion basis, lines 11 and 17 | $7,000 × 0.228571 = $1,600.00 after cent rounding |
| Other-distribution basis, line 12 | $5,000 × 0.228571 = $1,142.86 after cent rounding |
| Total allocated basis, line 13 | $1,600.00 + $1,142.86 = $2,742.86 |
| Remaining basis, line 14 | $10,000 − $2,742.86 = $7,257.14 |
| Taxable conversion, line 18 | $7,000 − $1,600.00 = $5,400.00 |
Using the full $10,000 of line 3 as the numerator on these same inputs would produce a six-place ratio of 0.285714 and a taxable conversion of $5,000.00. That result allocates basis that the contribution date excludes from this year’s numerator. The applicable Form 8606 lines 4 and 5 instead leave $8,000 eligible. The late contribution still belongs in total basis when calculating what remains.
Where the $10,000 of basis goes
In the detailed example above, $2,742.86 is used this year and $7,257.14 remains.
On a narrow screen, scroll the table horizontally to see every column.
| Inputs that distinguish the case | Taxable conversion | Remaining basis |
|---|---|---|
| Zero basis: lines 1, 2 and 4 are zero; line 6 is $10,000, line 7 is $2,000 and line 8 is $5,000. | $5,000.00 | $0.00 |
| All basis converted: line 1 is $7,000; lines 2, 4, 6 and 7 are zero; line 8 is $7,000. | $0.00 | $0.00 |
| Ratio cap with loss: line 1 is $7,000; lines 2, 4, 6 and 7 are zero; line 8 is $6,500. The ratio is capped at 1. | $0.00 | $500.00 |
In the zero-basis example, the displayed Part I amounts are arithmetic inputs, not instructions to complete unnecessary form lines. With no nondeductible contributions or basis and no other applicable Part I condition, report the conversion directly in Part II: line 16 is $5,000, line 17 is zero and line 18 is $5,000. Follow the 2025 form’s Part I applicability conditions and Part II instructions.
The no-distribution branch is shown in the calculation section. The download includes the inputs and results for all six examples.
Keep your basis records and check special cases
Can moving pre-tax IRA money to a 401(k) change the result?
Potentially. An eligible workplace plan that accepts the rollover may receive the otherwise-taxable IRA amount, while after-tax basis stays outside that rollover. Under the IRS special rule, the amount left in your IRAs or not rolled over must be at least equal to your basis. Plans are not required to accept incoming rollovers, and SIMPLE IRAs have additional timing restrictions. The Publication 590-A rollover rules explain these conditions.
For line 6, the IRS excludes an outstanding traditional-IRA-to-qualified-plan rollover; line 7 also excludes qualifying distributions rolled into a qualified plan. A pending transaction label alone does not establish either treatment. Check the transaction and receiving plan before relying on the exception. The existing backdoor Roth guide covers the planning choices and tradeoffs.
Retain the year’s Form 8606 and supporting statements and worksheets alongside the contribution, value and distribution records that explain its inputs. Keep the relevant tax-return pages too. The IRS record instructions call for these records until all distributions are made. A line 14 amount without its basis history may be hard to verify later; an absent history leaves an unresolved amount, not a demonstrated zero.
The manual record worksheet provides blank spaces for the tax year, contribution dates and designation, six named inputs, source records, adjustments, ordinary or special path, precision convention, remaining basis and verification notes. Enter amounts and calculations yourself. Record the line 14 carryforward with its supporting evidence and any required adjustments. The CSV contains no formulas and does not calculate or certify a tax return.
Download the manual record worksheet (CSV)
When taxable distributions and the IRA deduction affect each other through MAGI, use the separate worksheet. In Publication 590-B’s Rose Green example, prior basis is $300, total current contributions are $2,000, year-end IRA value is $20,000 and the Roth conversion is $5,000. Worksheet 1-1 produces a $460 nontaxable conversion and a $4,540 taxable conversion. The illustrated Form 8606 assumes a final nondeductible contribution of $500, leaving $340 of basis; it does not treat the entire $2,000 contribution as nondeductible.
That worksheet and the deduction calculation interact before the final form entries are determined. Substituting $500 and $300 into the ordinary allocation would not reproduce the illustrated $460 result. Follow the applicable Worksheet 1-1 reporting instructions for that path. Unknown basis, inherited accounts, repayments, disaster distributions and death-year reporting likewise need verified records or their special instructions before the ordinary worksheet is used.
Sources and calculation conventions
For the prior-basis example, the ratio is $8,000 ÷ $35,000. Rounding it to three places gives 0.229, rather than 0.228571: conversion basis is $1,603.00, other-distribution basis is $1,145.00, the taxable conversion is $5,397.00 and remaining basis is $7,252.00. The six-place convention gives $5,400.00 taxable and $7,257.14 remaining; the difference reflects permitted ratio precision, so compare rounding conventions before concluding that software is wrong.
The IRS online instructions for line 4 pair January 1, 2026 with April 15, 2025, which appears to be a typo on the IRS page. Lines 1 and 4 of the final 2025 form both say April 15, 2026; this article follows the form. The download includes the source record for comparison.
Download the inputs, results, Python code and source records for all six ordinary examples, the rounding and shortcut comparisons, and the separate IRS worksheet example. The code reproduces these cases and checks the listed invalid inputs; it is not a complete tax-return engine.
Download all six cases, exact code and source records (ZIP)
To rerun the examples, extract the ZIP and run python reproduce_pre.py from its folder. The accompanying README explains the files and source checks.
For contribution amounts, use the combined traditional and Roth IRA contribution limits. For direct Roth contribution eligibility, use the Roth IRA MAGI calculation guide. After converting, the Roth IRA five-year rules address a different question: withdrawing converted money and earnings. For setup and rollover planning, continue to the backdoor Roth IRA guide; its simplified estimator has a narrower scope than this form-input reconciliation.
Form-year scope
- 2025 Form 8606 2025 tax year Scope
The worked examples follow the final 2025 Form 8606. Check the applicable final form for another tax year.