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Backdoor Roth pro-rata calculator

Completes Form 8606 lines 1 to 18 with your numbers, counts traditional, SEP and SIMPLE IRAs together as the form requires, and shows the tax avoided by rolling pre-tax IRA money into a workplace plan first.

In short

Under the pro-rata rule, a Roth conversion is taxed in proportion to the pre-tax money in all your traditional, SEP and SIMPLE IRAs on December 31. Form 8606 line 10 divides your basis by those balances plus the conversion. With no other pre-tax IRA money, a backdoor conversion is generally tax-free.

  • Form 8606 lines 1 to 18
  • Roth limit check
  • Worksheet
  • PNG image
  • Runs in your browser

Private by design. Nothing you type is uploaded or saved. Add this tool to your phone or computer for one-tap access.

What do you need?

This year’s contribution and conversion

Add up every such IRA, plus any outstanding rollover. Roth IRAs and 401(k)s do not count.

Other lines (optional)

Last year’s line 14.

Contributions made by the filing deadline.

What if you roll pre-tax money into a workplace plan?

Your plan must accept roll-ins; the plan decides, not this tool.

Enter your nondeductible contribution (line 1), or your basis from earlier years.

A planning worksheet, not an IRS form. It follows the 2025 Form 8606 instructions and the 2026 limits, and it is not for filing. Deductible contributions, disaster distributions, inherited IRAs and employer-plan conversions are not covered. Tax law can change, so check for new rules and ask a tax professional before you convert.

Pro-rata worksheet
Based on IRS Form 8606, Parts I and II
Taxable
—

Enter your nondeductible contribution (line 1), or your basis from earlier years.

An estimate using the 2025 IRS Form 8606 instructions and the 2026 limits. A planning worksheet, not an IRS form and not for filing. Not tax advice.

How to use it

  1. On Pro-rata, enter your nondeductible contribution and the amount you converted.
  2. Enter the value of all traditional, SEP and SIMPLE IRAs on December 31, plus any outstanding rollover. Do not include Roth IRAs or a 401(k).
  3. Add basis from earlier years, distributions and any contribution made after year-end if they apply.
  4. Optionally enter pre-tax money you could roll into a workplace plan, and your tax rate, to see the saving.
  5. On Can I contribute directly, enter your modified AGI to see the maximum direct Roth contribution.

How it calculates

All of your traditional, SEP and SIMPLE IRAs are treated as one pot on December 31. Line 10 divides your basis by that pot plus the conversion and any distributions, rounded to three decimals and never above 1.000, and that fraction of the conversion is tax-free. Employer plan balances such as a 401(k) are not in the pot, which is why rolling pre-tax IRA money into a plan first can change the result.

For example, someone who contributes $7,500 nondeductible and converts it, with $50,000 left in a rollover IRA on December 31, has line 9 of $57,500 and a line 10 ratio of 0.130. The tax-free part of the conversion is $975, so $6,525 is taxable and $6,525 of basis carries forward. With no other pre-tax IRA money the ratio would be 1.000 and the conversion tax-free. The figures are made up; yours come from what you type.

Who should use it

  • High earners who make nondeductible IRA contributions and convert them to Roth
  • People with an old rollover IRA who want to see how it affects a conversion
  • Anyone filling in Form 8606 who wants the lines worked out to check against

Who should not use this

  • Anyone who needs a form to file; this is a planning worksheet, not an IRS form
  • People with deductible contributions, inherited IRAs, qualified disaster distributions or Roth distributions to report on Part III
  • Anyone doing conversions inside an employer plan (the so-called mega backdoor), which follows different rules

Limitations

  • It follows the 2025 Form 8606 instructions and the 2026 limits. Tax law can change, so check for new legislation before you convert.
  • The roll-in what-if assumes your plan accepts it and it is done before December 31; the plan decides, not this tool.
  • The taxable conversion can also raise the taxable part of your Social Security and your Medicare premiums two years later.
  • Your custodian’s year-end forms and a tax professional decide the numbers you file.
  • Nothing is uploaded or stored on a server. Clearing your browser data also removes the saved profile.

Sources and review

Reviewed on 18 Oct 2026 against the published sources below. Rules and prices change, so check the current position before you rely on a figure.

Common questions

How do I calculate the pro rata rule?
Add your after-tax basis (Form 8606 line 5) and divide it by the December 31 value of all traditional, SEP and SIMPLE IRAs plus the amount you converted and any distributions (line 9). Round to at least three decimals. That fraction is the tax-free share of your conversion.
Do I get taxed on a backdoor Roth conversion?
Only on the pre-tax share. If you have no other pre-tax IRA money on December 31, the conversion is generally tax-free apart from any growth between the contribution and the conversion. If you do, the pro-rata rule taxes part of it as ordinary income.
How much are the taxes on a $50,000 Roth conversion?
It depends on your basis and your bracket. With no basis, the whole $50,000 is added to your taxable income and taxed at your marginal rates, which may push you into a higher bracket. With basis, the pro-rata rule makes part of it tax-free. The taxable amount is line 18 of Form 8606.
Is a backdoor Roth still allowed in 2026?
The IRS Form 8606 instructions for 2025 returns and Publication 590-A still describe conversions and the pro-rata computation. Tax law can change, so check for new legislation and IRS guidance before you convert.
Do 401(k) balances count in the pro-rata rule?
No. The Form 8606 pot is made up of traditional, SEP and SIMPLE IRAs. Employer plan balances such as a 401(k) are not counted, which is why rolling pre-tax IRA money into a plan before December 31 can reduce the taxable part.
What is Form 8606 line 10?
Line 10 is the ratio of your basis to the total of your IRA balances, distributions and conversions. It is rounded to at least three decimals and capped at 1.000, and it sets the tax-free share of both the conversion and any distributions.

Guides that explain the rules

Backdoor Roth pro-rata rule

Related tools

IRMAA calculatorSocial Security taxable benefits calculatorRMD calculator

This tool produces a document from the information you type. It does not give legal, tax or accounting advice, and it does not check that your figures or rates are right. See the disclaimer. Found a mistake? Report an error in a calculation.