Every IRA is one pool. The conversion draws from all of it.

The facts, in one place.

Six quotable sentences on the pro-rata rule, 2026 figures.

1. A direct Roth IRA contribution phases out between $153,000 and $168,000 of income single and $242,000 to $252,000 joint for 2026. A conversion has had no income limit since 2010, which is why the backdoor exists: a nondeductible contribution, then a conversion.
2. The 2026 IRA limit is $7,500, plus $1,100 from age 50. A nondeductible contribution creates basis, reported on Form 8606, that comes out tax-free.
3. The pro-rata rule (§ 408(d)(2)) treats every traditional, SEP, and SIMPLE IRA a person owns as one pool. A conversion carries out basis and pre-tax dollars in the pool's proportion, measured at December 31 with the year's conversions added back; the converter cannot choose to move only the basis.
4. Example: a $7,500 nondeductible contribution converted the same year, with $92,500 in other pre-tax IRAs, is 7.5% basis and 92.5% taxable: $6,938 of income on a $7,500 conversion, and $6,938 of basis stranded in the pool.
5. Employer plans are not in the pool. Rolling pre-tax IRA money INTO a 401(k) that accepts it empties the pool and makes the next conversion clean; a spouse's IRAs are a separate pool.
6. Nothing here turns on the calendar month: a contribution for a year can be made until the following April, but the pool is measured at the December 31 of the year of the conversion.

The same conversion, four pools.

How the taxable share of one conversion rises with the rest of the pool.

How the pro-rata rule works.

A Roth IRA cannot be contributed to directly above the income phase-out, but a traditional IRA can be contributed to at any income, without a deduction, and then converted. The nondeductible contribution is basis, and basis comes out tax-free. The catch is § 408(d)(2): every traditional, SEP, and SIMPLE IRA the person owns is treated as one contract, and a distribution or conversion carries out basis and pre-tax dollars in the pool's proportion, measured at the end of the year with the year's conversions added back.

So the backdoor is clean only when the pool is nothing but basis. With other pre-tax IRA money in the pool, most of the conversion is taxable and most of the basis stays behind, to be recovered a little at a time over every future distribution. The usual cure is to move the pre-tax dollars out of the pool first, into a 401(k) that accepts rollovers, before December 31 of the conversion year.

Methodology.

1. Inputs. The amount converted this year, the total basis (Form 8606 line 2 plus this year's nondeductible contribution), every other traditional, SEP, and SIMPLE IRA balance on December 31, and a marginal rate for the tax figure.
2. The pool. Year-end balances plus the conversion (Form 8606 line 9); distributions other than the conversion are not modeled.
3. The split. Basis over the pool, at most one, is the nontaxable share (line 10); applied to the conversion it gives the nontaxable part (line 17) and, by subtraction, the taxable part (line 18). The basis left is the total basis less the part used (line 14).
4. Tax. The stated marginal rate on the taxable part. No bracket arithmetic: the rate is the user's.
5. Validation. Four pinned cases: the clean backdoor, a 7.5 percent basis share, a one-tenth share, and no conversion; and checks that the parts sum to the conversion and the basis carried forward stays in range. A transcription error fails the build.
6. Not modeled. Distributions in the same year, inherited IRAs (a separate pool), a spouse's IRAs (another), rollovers into a 401(k) (which change the December 31 balance), earnings on the contribution before conversion, and state tax. Educational, not advice.

Sources.

1. 1. United States Code (Cornell LII), 26 U.S.C. § 408(d)(2) — Individual retirement accounts: aggregation of accounts and distributions. That all of a person's individual retirement plans are treated as one contract, all distributions in a year as one distribution, and the value figured at the end of the year with distributions added back: the pro-rata rule. Retrieved September 5, 2026; verified September 5, 2026.
2. 2. Internal Revenue Service, Instructions for Form 8606, Nondeductible IRAs. The line-by-line computation: total basis (line 5) over the sum of year-end IRA values, distributions, and conversions (line 9) gives the nontaxable share (line 10) applied to the conversion (line 16 to 18). Retrieved September 5, 2026; verified September 5, 2026.
3. 3. United States Code (Cornell LII), 26 U.S.C. § 408A(c)(3) and (d)(3) — Roth IRA contribution limits by income; conversions. Why the backdoor exists: direct Roth contributions phase out by income while conversions have had no income limit since 2010, and a conversion is taxed as a distribution. Retrieved September 5, 2026; verified September 5, 2026.
4. 4. Internal Revenue Service, Notice 2025-67 — 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living. Every plan, IRA, SIMPLE, catch-up, compensation, phase-out, and QCD figure in the retirement-plans section. Retrieved September 4, 2026; verified September 4, 2026.

Revision history.

The record's history.

September 5, 2026

First release: the Form 8606 pro-rata computation for a conversion, with the taxable share, the basis carried forward, and the tax at a chosen marginal rate.

Canonical address: https://consideratecapital.com/tools/backdoor-roth-pro-rata