Backdoor Roth Calculator
Calculate your backdoor Roth conversion tax with the pro-rata rule, find your mega backdoor 401(k) space, and walk through Form 8606 line by line. 2026 limits and OBBBA rules built in.
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How to Use This Calculator
Backdoor Roth tab
The default tab. Enter your MAGI, filing status, age, and existing Traditional IRA balance. The calculator determines whether you need a backdoor Roth (based on income limits), computes the pro-rata rule tax on conversion, and projects your Roth value at retirement versus a taxable brokerage account. If your existing IRA balance is $0, you get a "clean" backdoor with zero conversion tax.
Mega Backdoor tab
For high earners with generous 401(k) plans. Enter your salary, employer match, and 401(k) contribution. The calculator subtracts your employee + employer contributions from the 415(c) limit ($72,000) to show how much after-tax space remains. Both after-tax contributions and in-service conversions must be allowed by your plan — check with HR if unsure.
Form 8606 tab
Walks through the actual IRS Form 8606 line by line. Enter your total Traditional IRA balance (Dec 31), prior year basis, current contribution, and conversion amount. The calculator shows the pro-rata fraction, taxable and nontaxable portions, and remaining basis carried forward. Use this to prepare your tax return.
Share your result
Every input is encoded in the URL. Click Share to send your exact scenario to a spouse, financial advisor, or CPA.
The Formula
The backdoor Roth strategy involves two steps: a nondeductible Traditional IRA contribution followed by a Roth conversion. The pro-rata rule determines how much of the conversion is taxable.
Nontaxable Portion = Conversion Amount × Pro-Rata Ratio
Taxable Portion = Conversion Amount − Nontaxable Portion
Conversion Tax = Taxable Portion × (Federal Rate + State Rate)
Mega Backdoor Space = 415(c) Limit − Employee Deferrals − Employer Match
The pro-rata rule (IRC 408(d)(2)) aggregates all Traditional, SEP, and SIMPLE IRA balances. You cannot isolate just the nondeductible portion for conversion. If you have $0 in existing IRAs, the entire conversion is nontaxable — the ideal "clean" backdoor.
For mega backdoor, the 415(c) limit ($72,000 in 2026) caps total contributions: employee elective deferrals + employer match + after-tax contributions. The remaining space is your mega backdoor opportunity.
Example
David — Software Engineer, 38, San Francisco CA
David earns $250,000 (single), has $0 in existing Traditional IRAs, and his employer offers a 401(k) with 4% match, after-tax contributions, and in-service conversions. California state tax: 9.3%.
Backdoor Roth tab
With no existing IRA balances, David pays $0 tax on the conversion. His $7,500 annual backdoor Roth contribution grows to $544K tax-free by age 65.
Mega Backdoor tab
David can contribute $39,000/year in after-tax 401(k) and convert to Roth. Combined with his regular backdoor Roth, that is $46,000/year into Roth accounts — growing to over $3.5M tax-free.