Backdoor Roth Pro-Rata Calculator 2026: Methodology and Sources

Core calculation used

1. Total basis equals prior nondeductible basis plus current nondeductible contribution.
2. Total distributions equal the Roth conversion plus any other traditional IRA distributions.
3. The denominator equals year-end value of all traditional, SEP, and SIMPLE IRAs plus total distributions.
4. Nontaxable ratio equals total basis divided by that denominator.
5. Nontaxable conversion amount equals conversion multiplied by the nontaxable ratio.
6. Taxable conversion amount equals conversion minus the nontaxable amount.

Assumptions and limits

• The calculator is designed for planning, not final Form 8606 prep.
• Users must enter year-end IRA values and prior basis accurately.
• The tool does not model every special timing or recharacterization scenario.
• The calculator assumes one aggregated IRA pool for traditional, SEP, and SIMPLE IRAs under the pro-rata rule.

Primary sources

• IRS Form 8606 and instructions
• IRS IRA contribution and basis guidance
• IRS Roth conversion reporting rules

Related pages: calculator and guide.

Update policy

We update this page when the IRS changes Form 8606 instructions, basis-tracking guidance, or Roth conversion reporting rules in a way that affects how the pro-rata calculation should be described for planning purposes.

Scope note

This calculator is intentionally focused on the main pro-rata rule. It does not attempt to cover every recharacterization issue, every timing nuance, or every special reporting situation. The goal is to make the pooled-balance math and basis logic transparent enough for planning without pretending to replace the return itself.

Why the denominator matters

The most important part of the pro-rata rule is the denominator. It is not just the IRA used for the conversion. It includes the year-end value of all traditional, SEP, and SIMPLE IRAs plus distributions for the year. That pooled approach is what causes a relatively small nondeductible contribution to produce a mostly taxable conversion when larger pre-tax IRA balances still exist elsewhere.

What this page does not model

This methodology does not try to cover every special reporting twist involving recharacterizations, inherited IRA rules, or unusual timing facts. It is built to explain the main Form 8606-style basis ratio that drives most backdoor Roth planning conversations. Accurate year-end values and prior basis records still matter more than any amount of general explanation.

Interpretation note

The calculator is designed to answer one planning question clearly: how much of the conversion is likely to be taxable if the IRA pool stays in its current shape through year-end. That makes it useful for strategy comparison even though the final return still depends on accurate basis records and Form 8606 reporting.