Backdoor Roth IRA Pro-Rata Calculator

Backdoor Roth IRA introduction

Backdoor Roth IRA rules are easy to describe but easy to misread. The strategy exists because high earners can be barred from making a direct Roth IRA contribution. The common workaround is to place money into a traditional IRA as a nondeductible contribution and then convert that amount to Roth. The catch is that the IRS does not look only at the dollars you meant to convert. It blends every traditional, SEP, and SIMPLE IRA you own when it decides how much of the conversion is taxable.

This backdoor Roth IRA calculator estimates that blend before you move any money. Enter your total IRA balance, your nondeductible basis, the amount you plan to convert, and your marginal federal tax rate. The calculator then applies the pro-rata rule to estimate the taxable and non-taxable portions of the conversion, the likely federal tax on the taxable amount, and the basis left for later years. If you are trying to understand why a seemingly simple Roth conversion can still create taxable income, this page walks through the math in plain language.

Backdoor Roth IRA basics

A backdoor Roth IRA is a two-step strategy used when direct Roth contributions are unavailable because of income limits. The usual pattern is simple:

1. Make a nondeductible contribution to a traditional IRA.
2. Convert that amount from the traditional IRA to a Roth IRA.

Because Roth IRAs can offer tax-free growth and qualified tax-free withdrawals, many investors use the backdoor route to get new money into Roth treatment. The IRS, however, does not let you cherry-pick only the after-tax dollars if you still have pre-tax money in any IRA. That is where the pro-rata rule enters the picture.

How the backdoor Roth pro-rata rule works

The backdoor Roth IRA pro-rata rule treats your traditional, SEP, and SIMPLE IRAs as one combined pool. Every Roth conversion from that pool is considered to contain pre-tax and after-tax money in the same proportion as the pool itself.

For pro-rata purposes, the IRS looks at the combined IRA balance on December 31 of the conversion year. Employer plans such as 401(k)s, 403(b)s, and 457(b)s are excluded. That difference matters because some people move pre-tax IRA money into an employer plan before doing a backdoor Roth conversion in order to reduce the taxable share.

Think of the account as a blended bucket. If 20% of the bucket is after-tax basis, then roughly 20% of the conversion is non-taxable and the rest is taxable. The calculator below follows that same fraction.

Backdoor Roth IRA formulas used in this calculator

This backdoor Roth IRA calculator uses the standard pro-rata fraction to estimate how much of a conversion is taxable and how much basis remains afterward.

Let:

• T = total value of all traditional, SEP, and SIMPLE IRAs (pre-tax + after-tax) at the time of conversion
• B = total nondeductible (after-tax) basis in those IRAs
• C = amount you convert to a Roth IRA

The IRS defines the nontaxable portion of your conversion as:

The taxable portion is then:

Substituting gives:

In plain terms, the calculator multiplies the conversion by your basis share of the entire IRA pool. The remainder is taxable. It then multiplies the taxable portion by your marginal tax rate to estimate the federal tax cost of the conversion.

Backdoor Roth IRA input meanings

For a backdoor Roth IRA estimate, each input needs to reflect the full IRA picture rather than only the account you plan to convert. Total IRA balance should include the combined value of all traditional, SEP, and SIMPLE IRAs that fall under the pro-rata rule. Nondeductible basis is the after-tax amount you have tracked on Form 8606. Conversion amount is the dollar amount you expect to move to Roth during the year. Marginal tax rate is your top federal rate, which the calculator uses as a planning shortcut for the taxable portion.

Notice what is not included. Roth IRAs are excluded from the pro-rata pool, and standard employer plans such as 401(k)s, 403(b)s, and 457(b)s are not counted either. That is why some investors first move pre-tax IRA money into an employer plan before using the backdoor Roth strategy.

Interpreting your backdoor Roth IRA results

When you run a backdoor Roth IRA scenario, the result tells you how the IRS blend splits the conversion. The taxable portion is the amount expected to show up as ordinary income. The non-taxable portion is the piece covered by your basis. Estimated tax owed applies your marginal rate to the taxable amount. Remaining IRA basis shows the after-tax amount still available for future Form 8606 tracking. Finally, the remaining pre-tax IRA balance is a planning aid that shows the pre-tax dollars left after the modeled conversion.

A few patterns matter more than the exact dollar values. If basis is a large share of the total IRA pool, the conversion is more tax-efficient. If pre-tax IRA money dominates, most of the conversion will be taxable even when the conversion amount is small. If you are in a higher marginal bracket, the same taxable conversion will produce a larger estimated tax bill.

Backdoor Roth IRA worked example

Here is a backdoor Roth IRA worked example using numbers that follow the same pro-rata logic as the calculator.

• Total IRA balance (T): $100,000
• Nondeductible basis (B): $20,000
• Conversion amount (C): $10,000
• Marginal tax rate: 24%

First, compute the non-taxable share of the conversion. The after-tax basis is 20% of the total IRA balance because $20,000 divided by $100,000 equals 0.20. That means 20% of the conversion is non-taxable: $10,000 × 20% = $2,000.

Next, compute the taxable portion. If $2,000 of the $10,000 conversion is covered by basis, the remaining $8,000 is taxable. Then estimate the federal tax by multiplying $8,000 by 24%, which gives $1,920. Finally, reduce the basis by the amount used in the conversion. Since $2,000 of basis was consumed, the remaining basis is $18,000 for future Form 8606 tracking.

This example shows why the pro-rata rule matters so much in backdoor Roth planning. Even though the conversion amount was only $10,000, the larger pool of pre-tax IRA money caused most of the conversion to be taxable.

Backdoor Roth IRA scenario outcomes table

These backdoor Roth IRA scenarios show how the taxable share changes when basis makes up a larger or smaller slice of the IRA pool.

In the first row, 40% of the IRA is after-tax basis, so only 60% of the conversion is taxable. In the second row, basis is only 6% of the total account, so almost the entire conversion is taxable. In the third row, the larger conversion amount still leads to a meaningful tax bill because the pre-tax portion of the IRA remains dominant.

How to use this backdoor Roth IRA calculator

To use this backdoor Roth IRA calculator, start with the year-end combined balance of all traditional, SEP, and SIMPLE IRAs. Then enter the after-tax basis already tracked on Form 8606. After that, enter the amount you want to convert and your marginal federal rate. The calculator applies the same pro-rata fraction to your conversion that exists in the IRA pool itself, which makes it useful for quick scenario testing and for checking whether a backdoor Roth still makes sense when pre-tax IRA assets are already in the picture.

For practical planning, it can help to run the calculator more than once. Try one case with your current IRA balance, then another with pre-tax IRA dollars moved into an employer plan if that is available to you. The difference between those runs often shows why the pro-rata rule is such a big part of the backdoor Roth conversation.

Backdoor Roth IRA assumptions and limitations

This backdoor Roth IRA calculator is for education and quick planning, not for preparing a tax return. It makes several simplifying assumptions:

• Marginal rate only: The calculation uses the single marginal tax rate you enter. It does not model full tax brackets, phase-outs, credits, the Net Investment Income Tax, the alternative minimum tax, or other federal rules.
• IRA types included: It assumes your total IRA balance includes all traditional, SEP, and SIMPLE IRAs that are subject to the pro-rata rule. Employer plans are not included.
• Timing of balances: The total balance is treated as your year-end, December 31 IRA value for the tax year of the conversion, which is how the IRS applies the pro-rata rule.
• No penalties shown: The calculator does not account for early distribution penalties, excess contribution penalties, or any other penalties that might apply if a transaction is structured incorrectly.
• No state or local tax: State and local income taxes are ignored. Some states treat IRA basis and Roth conversions differently from federal rules, which can change your actual tax cost.
• Single-year view: The results focus on one tax year and do not project future tax savings or long-term investment performance.
• Accurate basis required: The accuracy of the results depends on entering the correct nondeductible basis. If your Form 8606 history is incomplete or incorrect, the calculator cannot repair it for you.

That last point is especially important. Many backdoor Roth problems are not mathematical; they come from missing or inconsistent basis records. If your historical filings are messy, treat this calculator as a guide and compare your numbers against official IRS instructions or professional advice.

Tracking backdoor Roth basis with IRS Form 8606

Every time you make a nondeductible contribution to a traditional IRA or complete a Roth conversion that involves after-tax amounts, you generally need to file IRS Form 8606, Nondeductible IRAs. This form tracks your running total of nondeductible contributions, the portion of each distribution or conversion that is non-taxable, and the basis that carries into later years.

The calculator's estimate of remaining basis is meant to mirror that logic. It shows how much basis remains after the modeled conversion, which is one of the most useful planning outputs because it affects the tax treatment of future conversions and distributions.

When a backdoor Roth IRA may or may not make sense

A backdoor Roth IRA can be especially attractive when you are above the income limit for direct Roth contributions, expect to be in the same or a higher tax bracket later, and have little or no existing pre-tax IRA money. In that case, the pro-rata rule may have only a small impact or no impact at all.

It may be less attractive when large pre-tax IRA balances cause most of the conversion to be taxable right away, when you expect to be in a much lower bracket in retirement, or when state tax treatment makes conversions more expensive. Sometimes the right conclusion is not simply yes or no, but rather not yet. Investors occasionally wait until they can reduce pre-tax IRA balances before revisiting the strategy.

Backdoor Roth IRA disclaimer

This backdoor Roth IRA calculator provides a simplified estimate of the taxable portion of a conversion under the pro-rata rule. It does not give personalized tax, investment, or legal advice, and it may not reflect every rule that applies to your situation. Before making significant contributions, conversions, or withdrawals, consider reviewing your plan with a qualified tax professional or financial advisor.