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Backdoor Roth IRA Tax Calculator

Estimate the taxable portion of a backdoor Roth conversion under the IRS pro-rata rule.

Result

Taxable Portion of Conversion
$0.00
Estimated Tax Owed
$0.00
Tax-Free Percentage
100%

The IRS pro-rata rule aggregates all your traditional, SEP and SIMPLE IRA balances - if you hold any pre-tax IRA money anywhere, part of every backdoor Roth conversion becomes taxable, even if the contribution itself was nondeductible.

About the Backdoor Roth IRA

High earners who exceed the Roth IRA income limits often use a two-step workaround: contribute to a traditional IRA with no deduction, then convert it to a Roth. It sounds simple until the IRS pro-rata rule enters the picture. Our Backdoor Roth IRA Tax Calculator estimates exactly how much of that conversion actually ends up taxable.

How It Works

The calculator adds your nondeductible contribution to any existing pre-tax IRA balance you already hold, then applies the IRS pro-rata rule to determine what fraction of the conversion is treated as taxable pre-tax money versus tax-free after-tax basis, multiplying the taxable portion by your marginal tax rate to estimate the tax owed.

Taxable portion = Contribution x (Existing Pre-Tax Balance / (Contribution + Existing Pre-Tax Balance)) Tax Owed = Taxable Portion x Marginal Tax Rate

Formula & Methodology

The pro-rata rule treats all of your traditional, SEP, and SIMPLE IRA money as one combined pool for tax purposes. You can't choose to convert only the nondeductible after-tax dollars while leaving the pre-tax dollars behind. Instead, every dollar converted is deemed to carry the same pre-tax-to-total ratio as your entire IRA pool. The calculator implements this exactly: it divides your existing pre-tax balance by the total of that balance plus your new nondeductible contribution, then applies that ratio to the amount being converted to find the taxable portion. If you hold no other pre-tax IRA money at all, the ratio is zero and the entire conversion is tax-free.

Step-by-Step: Calculating It By Hand

  1. 1Add your nondeductible contribution to any existing pre-tax IRA balance to find the total combined balance.
  2. 2Divide the existing pre-tax balance by that total to find the pre-tax ratio.
  3. 3Multiply the contribution amount being converted by that ratio to find the taxable portion.
  4. 4Multiply the taxable portion by your marginal tax rate to estimate the tax owed on the conversion.

Examples

No existing pre-tax IRA

A $7,000 nondeductible contribution converted with $0 in other pre-tax IRA balances has a taxable portion of $0, meaning the entire backdoor conversion is tax-free.

Existing pre-tax balance

The same $7,000 contribution alongside a $63,000 existing pre-tax IRA balance means only 10% of the total pool is after-tax money, so roughly $6,300 of the conversion becomes taxable at your marginal rate.

Advantages

  • Applies the actual IRS pro-rata formula instead of assuming the conversion is automatically tax-free
  • Shows the tax-free percentage clearly alongside the dollar tax owed
  • Helps decide whether rolling pre-tax IRA money into a 401(k) first is worth doing before converting
  • Useful for high earners locked out of direct Roth contributions by income limits

Common Mistakes

  • Assuming a backdoor Roth is automatically tax-free without checking for existing pre-tax IRA balances
  • Forgetting that SEP and SIMPLE IRA balances count toward the pro-rata calculation, not just traditional IRAs
  • Failing to file Form 8606 to document the nondeductible contribution's basis
  • Converting immediately without realizing any growth between contribution and conversion is also taxable

Edge Cases to Watch For

  • The pro-rata rule aggregates all traditional, SEP, and SIMPLE IRAs across every account you own as of December 31 of the conversion year, even ones unrelated to the backdoor contribution.
  • Rolling existing pre-tax IRA balances into an employer 401(k) that accepts incoming rollovers, before converting, removes them from the pro-rata calculation entirely and makes the conversion fully tax-free.
  • Any investment growth on the nondeductible contribution between contributing and converting is also taxable, even with zero pre-tax balance elsewhere.
  • Filing Form 8606 correctly every year is essential to prove the nondeductible basis - without it, the IRS has no record that any part of the conversion was already taxed.

Common Use Cases

  • Estimating the tax bill before executing a backdoor Roth conversion
  • Deciding whether to roll pre-tax IRA balances into a 401(k) before converting
  • Understanding how existing IRA balances affect a planned conversion strategy
  • Comparing the after-tax outcome across different existing balance scenarios
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

How do I avoid the pro-rata rule entirely?

Roll any existing pre-tax IRA balances into an employer 401(k) plan (if it accepts incoming rollovers) before converting, so your IRA holds only the nondeductible contribution and the conversion is fully tax-free.

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Conclusion

The backdoor Roth strategy only delivers a fully tax-free conversion when there's no pre-tax IRA money sitting in the same pool. Running the numbers here before converting can save an unpleasant tax surprise, and often points toward rolling existing balances into a 401(k) first if that option is available.

References