Backdoor Roth IRA Tax Calculator

Introduction

A backdoor Roth IRA can sound more mysterious than it really is. In plain language, it is a legal workaround used by people whose income is too high for a direct Roth IRA contribution. Instead of contributing straight to a Roth IRA, they first make a nondeductible contribution to a traditional IRA and then convert that money to a Roth IRA. The strategy is simple on paper, but the tax result is often misunderstood because the IRS looks at all of your traditional, SEP, and SIMPLE IRAs together when it decides how much of a conversion is taxable.

This calculator helps you estimate that tax result before you convert. Enter your total IRA balance, your nondeductible basis, the amount you plan to convert, and your marginal federal tax rate. The tool 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 that would remain for future years. If you have ever wondered why a small backdoor Roth conversion can still trigger tax when you also hold large pre-tax IRA balances, this page is built to make that interaction clear.

What is a backdoor Roth IRA?

A backdoor Roth IRA is a two-step strategy that lets high-income earners move money into a Roth IRA even when their income is too high to contribute directly. The basic idea 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 grow tax-free and qualified withdrawals are tax-free, many investors use the backdoor Roth to shift money into an account with more favorable long-term tax treatment. However, the IRS does not let you choose to convert only your after-tax dollars when you have other pre-tax IRA money. Instead, it requires the pro-rata rule.

How the pro-rata rule works

The pro-rata rule says that every Roth conversion from your IRAs is made from a mix of pre-tax and after-tax money, in proportion to how much of each you hold across all of your traditional, SEP, and SIMPLE IRAs. You cannot isolate just the nondeductible, after-tax contributions for a tax-free conversion if you still have pre-tax balances in any IRA.

For pro-rata purposes, the IRS looks at your total IRA balances as of December 31 of the tax year of the conversion. Employer plans like 401(k)s and 403(b)s are not included in this total. That distinction matters because some investors roll pre-tax IRA money into an employer plan before doing a backdoor Roth, specifically to reduce or eliminate the pre-tax IRA balance that would otherwise make the conversion taxable.

A useful way to think about the rule is this: the IRS treats your IRA money like one blended pool. If 20% of that pool is after-tax basis, then 20% of your conversion is treated as non-taxable and the remaining 80% is treated as taxable. The calculator below follows that logic directly.

Formulas used in this calculator

This calculator uses a straightforward version of the IRS pro-rata formula to estimate what portion of your Roth conversion is taxable and how much after-tax basis remains.

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 words, the non-taxable share of the conversion equals the after-tax basis divided by the total IRA balance, multiplied by the conversion amount. The taxable share is whatever remains. The calculator then multiplies the taxable portion by your marginal tax rate to estimate the federal tax hit from the conversion.

What each input means

The quality of the estimate depends on the quality of the inputs, so it helps to slow down here. Total IRA balance should include the combined value of all traditional, SEP, and SIMPLE IRAs subject to the pro-rata rule. Nondeductible basis is the after-tax amount you have tracked on Form 8606. Conversion amount is the dollar amount you plan to move into a Roth IRA during the year. Marginal tax rate is your top federal rate, which the calculator uses as a shortcut for estimating tax on the taxable portion.

Notice what is not included. Roth IRAs are not part of the pro-rata pool. Standard employer plans such as 401(k)s, 403(b)s, and 457(b)s are not included either. That exclusion is why some people explore moving pre-tax IRA money into an employer plan before using the backdoor Roth strategy.

Interpreting your results

When you run the calculator, you will typically see five numbers. The taxable portion shows the amount expected to be taxed as ordinary income. The non-taxable portion shows how much of the conversion is covered by your basis. The estimated tax owed applies your marginal rate to the taxable amount. Remaining IRA basis tells you how much after-tax basis should still be tracked for the future. Finally, the remaining pre-tax IRA balance is a planning aid that shows the pre-tax dollars left after the modeled conversion.

There are a few common patterns worth noticing. If your basis is a large share of your total IRA balance, the conversion becomes more tax-efficient. If your pre-tax IRA money is much larger than your basis, most of the conversion will be taxable even if the conversion amount is relatively small. And if you are in a higher marginal bracket, the same taxable conversion amount will create a larger estimated tax bill.

Worked example

Assume the following:

• 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 also explains why many people feel surprised by the pro-rata rule. Even though the conversion amount was only $10,000, the existence of a much larger pool of pre-tax IRA money caused most of the conversion to be taxable.

Example outcomes table

The table below shows how different IRA balances and basis amounts affect the taxable share of a conversion.

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 calculator

Start with your best estimate of the year-end combined balance of all traditional, SEP, and SIMPLE IRAs. Then enter the after-tax basis you have already tracked on Form 8606. After that, enter the amount you want to convert and your marginal federal rate. The calculator instantly applies the same proportion to the conversion that exists in the IRA pool itself. That makes it useful both for quick scenario testing and for checking whether a backdoor Roth still makes sense when you already have pre-tax IRA assets.

For practical planning, it can help to run the calculator more than once. Try one case with your current IRA balance, then another case in which pre-tax IRA dollars have been 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.

Assumptions and limitations

This tool is designed 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 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.

Important disclaimer

This calculator provides a simplified estimate of the taxable portion of a backdoor Roth IRA 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.