Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Taxable amount of conversion
$6,140
Tax-free amount of conversion
$860
Nontaxable ratio (Form 8606)
12.3%
Basis remaining after conversion
$6,140
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How to use this
- 1Enter total nondeductible basis (all iras) ($).
- 2Enter total traditional/sep/simple ira balance (year-end, incl. this contribution) ($).
- 3Enter amount you plan to convert ($).
- 4Read your taxable amount of conversion on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
The backdoor Roth strategy — contribute nondeductible dollars to a traditional IRA, then convert to Roth — only works cleanly if you have zero other pretax IRA balances. If you hold any pretax traditional, SEP, or SIMPLE IRA money, the IRS pro-rata rule forces you to treat every conversion as a proportional mix of pretax and after-tax dollars across all your IRAs, not just the account you just funded. This calculator applies IRS Form 8606 logic: it totals all your traditional-type IRA balances (aggregated), figures out what fraction is your basis (nondeductible contributions), and shows exactly how much of your intended conversion is taxable income this year versus tax-free return of basis. Getting this wrong is one of the most common backdoor Roth mistakes, and it usually surprises people who forgot about an old rollover IRA sitting from a prior employer's 401(k).
Worked example
Using the values the calculator loads with:
Inputs
- Total nondeductible basis (all IRAs): 7000 $
- Total traditional/SEP/SIMPLE IRA balance (year-end, incl. this contribution): 57000 $
- Amount you plan to convert: 7000 $
Results
- Taxable amount of conversion: $6,140
- Tax-free amount of conversion: $860
- Nontaxable ratio (Form 8606): 12.3%
- Basis remaining after conversion: $6,140
What each field means
Inputs
- Total nondeductible basis (all IRAs) ($)
- The total nondeductible basis (all iras) used in the calculation, measured in $. Starts at 7000 $ so you have a working example on load.
- Total traditional/SEP/SIMPLE IRA balance (year-end, incl. this contribution) ($)
- The total traditional/sep/simple ira balance (year-end, incl. this contribution) used in the calculation, measured in $. Starts at 57000 $ so you have a working example on load.
- Amount you plan to convert ($)
- The amount you plan to convert used in the calculation, measured in $. Starts at 7000 $ so you have a working example on load.
Results
- Taxable amount of conversion
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Tax-free amount of conversion
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Nontaxable ratio (Form 8606)
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Basis remaining after conversion
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Does the pro-rata rule count my Roth IRA balance?
No. Only traditional, SEP, and SIMPLE IRAs are aggregated for pro-rata purposes. Roth IRA balances and 401(k)/403(b) balances at an employer plan are excluded from this calculation entirely.
How do people avoid the pro-rata problem?
The common fix is a 'reverse rollover': move existing pretax traditional IRA money into your current employer's 401(k) if the plan accepts incoming rollovers, leaving your traditional IRA balance at zero before you do the backdoor Roth. Once no pretax IRA balance exists, conversions are basis-only and fully tax-free.
What if I forget an old IRA and get this wrong?
The IRS will tax the pro-rata share as income regardless of which account you think you converted, and you'll likely need to amend Form 8606. Check every 5498 and old employer rollover before doing a backdoor Roth.
Does the pro-rata calculation use my balance before or after the conversion?
It uses the year-end balance across all traditional IRAs, which by IRS convention includes any amount converted during the year added back in for the ratio, per the Form 8606 worksheet. This calculator's 'total balance' input should be your year-end balance before subtracting the conversion.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Backdoor Roth Pro-Rata Tax Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/backdoor-roth-pro-rata
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/backdoor-roth-pro-rata" target="_blank" rel="noopener">Backdoor Roth Pro-Rata Tax Calculator — RevenueLab</a> (2026).</p>
Source: [Backdoor Roth Pro-Rata Tax Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/backdoor-roth-pro-rata) (2026).