The backdoor Roth is two transactions, not a special account

A typical backdoor Roth strategy begins with a nondeductible contribution to a traditional IRA and then converts traditional IRA assets to a Roth IRA. The contribution creates after-tax basis when it is properly reported. The conversion moves assets and can be partly taxable. There is no separate “backdoor” account label that protects the contributed dollars from Form 8606.

For 2026, the regular IRA contribution limit is $7,500, with an additional $1,100 catch-up for an eligible person age 50 or older. Contribution eligibility also requires sufficient compensation and coordination across traditional and Roth IRA contributions. A conversion itself is not limited to $7,500 and does not require the taxpayer to be eligible for a direct Roth contribution.

The tax result depends on all traditional IRA value, not on whether the nondeductible contribution and conversion occur in a newly opened account. Moving money between IRA custodians does not escape aggregation.

How the Form 8606 denominator creates the pro-rata percentage

The denominator generally combines year-end value of all traditional IRAs with traditional IRA distributions and Roth conversions during the year. For this purpose, “traditional IRA” generally includes traditional SEP and SIMPLE IRAs. The numerator is cumulative after-tax basis: prior-year basis plus current nondeductible contributions, adjusted for relevant items.

If basis exceeds the denominator because of an input mismatch, the calculator caps the nontaxable percentage at 100%. Official Form 8606 sequencing and outstanding rollover rules must be used for the filed return. Values should include accrued earnings and assets in transit that the instructions treat as year-end IRA value.

Accounts included and excluded from the aggregation

An inherited IRA can have special basis and aggregation rules depending on whether it came from a spouse or nonspouse. Do not add every retirement account on a net-worth statement without following the form instructions.

December 31 matters even when conversion happened in January

A taxpayer might contribute and convert $7,500 early in the year when a new traditional IRA appears empty, yet still own $100,000 in a SEP IRA on December 31. The year-end SEP value enters the denominator and makes most of the conversion taxable. Transaction order does not isolate basis.

Conversely, an eligible workplace plan may accept a rollover of pretax IRA dollars before year-end, potentially reducing the IRA denominator while leaving after-tax basis behind. That strategy requires plan acceptance, correct separation of basis, completion before year-end, and careful reporting. This calculator shows arithmetic, not permission or execution.

A rollover initiated near year-end can remain outstanding. Form 8606 instructions include rules for amounts rolled over after year-end or in transit. Obtain custodian statements rather than entering only the balance displayed in one online account.

A four-stop Form 8606 workflow

File Form 8606 even when the conversion appears fully nontaxable. Failure to report nondeductible contributions can lose the basis record and cause later double taxation. Preserve each year’s form indefinitely with IRA statements.

Example: $7,500 contribution beside a $100,000 IRA

Assume no prior basis, a $7,500 nondeductible 2026 contribution, a $7,500 Roth conversion, no other distributions, and $100,000 remaining across traditional IRAs on December 31. Total basis is $7,500 and the denominator is $107,500.

The nontaxable percentage is about 6.98%. Approximately $523 of the conversion is nontaxable and $6,977 is taxable. At a 24% federal marginal rate, the estimated federal tax is about $1,674 before state tax and return interactions. Approximately $6,977 of basis remains for later years.

If all pretax IRA value were zero at year-end and no other distributions existed, the denominator would be $7,500 and the conversion could be fully nontaxable, apart from earnings between contribution and conversion. That outcome depends on actual year-end facts, not the label applied to the transaction.

Other distributions share the same basis pool

If the taxpayer also withdraws money from a traditional IRA, basis is allocated across both the withdrawal and conversion. The nontaxable conversion cannot use all basis while the cash withdrawal is treated as entirely pretax. This calculator shows total basis used across entered activity and the conversion’s own nontaxable share.

Early distributions can also generate a 10% additional tax on the taxable portion unless an exception applies. Conversion income itself is generally not subjected to that additional tax merely because the taxpayer is under 59½, but withholding taken from converted funds or a later Roth distribution can create separate issues.

Do not withhold conversion tax from the IRA without considering lost Roth principal and early-distribution consequences. Paying tax from outside funds often preserves more retirement money, but household liquidity matters.

Contribution year and conversion year may be different

A traditional IRA contribution made between January 1 and the federal filing deadline can be designated for the prior tax year, while a Roth conversion is reported only in the calendar year it actually occurs. For example, a contribution made in March 2027 for 2026 and converted in March 2027 can require a 2026 Form 8606 to establish basis and a 2027 Form 8606 to allocate that basis to the conversion. The custodian’s Forms 5498 and 1099-R will not necessarily appear in the same filing season.

That timing does not let basis bypass the aggregation rule. The conversion-year form uses the taxpayer’s total available basis, conversion-year distributions, and December 31 value for that conversion year. Carry the prior form’s ending basis forward exactly; do not enter the contribution again merely because the conversion happened later.

Deductibility must also be resolved before calling a contribution “after-tax.” If the traditional IRA contribution is deducted on Schedule 1, it does not create Form 8606 basis. Covered-by-plan status, filing status, and modified AGI can phase out the deduction, while a spouse’s workplace coverage can affect a noncovered spouse under a different range. The calculator assumes the entered contribution is nondeductible.

Conversion earnings deserve separate attention. If $7,500 becomes $7,560 before conversion, the extra $60 is normally pretax value and may be taxable even when no older IRA exists. Enter the actual gross conversion rather than the intended contribution amount. Small earnings are not a failed strategy; they are ordinary conversion income to report.

Questions investors ask about the pro-rata rule

References

Official aggregation, conversion, and basis reporting: IRS Instructions for Form 8606. IRA contribution and conversion background: IRS Publication 590-A and IRS Publication 590-B.