Mega Backdoor Roth Calculator
Calculate your after-tax 401(k) contribution room and compare Roth vs. taxable growth over time.
e.g. 50% means employer matches $0.50 per $1.00
Max % of salary the employer will match on
After-Tax Contribution Room
$44,000
per year available for mega backdoor Roth
Employee Deferral
$20,000
Pre-tax/Roth 401(k)
Employer Match
$6,000
Free money
Total Tax-Advantaged
$70,000
Per year
Roth Advantage
$474,253
Over 25 years vs. taxable
How to Use This Calculator
First, confirm your 401(k) plan allows after-tax contributions and in-plan Roth conversions. Then enter your salary, deferral rate, and employer match details. The calculator shows your available after-tax contribution room and projects the long-term advantage of investing that amount in a Roth vs. a taxable brokerage account.
What Is the Mega Backdoor Roth?
The standard 401(k) employee deferral limit is $23,500 in 2025. But the IRS §415(c) total contribution limit — which includes employee deferrals, employer match, and after-tax contributions — is $70,000. The gap between your deferrals plus match and the $70,000 ceiling is your mega backdoor Roth opportunity.
By making after-tax contributions to your 401(k) and immediately converting them to Roth (either in-plan or by rolling to a Roth IRA), you effectively get Roth treatment on contributions far beyond the normal limits. The key requirement is that your plan must support both after-tax contributions and in-plan Roth conversions or in-service distributions.
How It Works
The calculation has three components. First, your employee deferral: your salary times your deferral rate, capped at the §402(g) limit. Second, your employer match: the employer's matching contribution on the portion of salary they match. Third, the after-tax room: the §415(c) total limit minus your deferral minus the employer match.
The chart compares investing that after-tax room in a Roth account (tax-free growth, tax-free withdrawals) versus a taxable brokerage account (annual capital gains tax drag of ~0.5%, plus 15% LTCG at withdrawal). Over decades of compounding, the tax-free growth advantage is substantial.
Frequently Asked Questions
What is a mega backdoor Roth?
A strategy using after-tax 401(k) contributions (beyond the standard limit) converted to Roth, letting you contribute up to the §415(c) total limit ($70,000 in 2025) to tax-advantaged accounts.
How much can I contribute?
Your room = §415(c) limit ($70,000) minus employee deferrals minus employer match. With $23,500 deferrals and $6,000 match, that's $40,500 in after-tax room.
Does my plan support this?
Your plan needs after-tax contributions and either in-plan Roth conversions or in-service distributions. Check with your plan administrator.
What's the advantage over taxable?
Roth grows tax-free. Taxable accounts face ~0.5%/year tax drag plus 15% LTCG at withdrawal. Over 25-30 years this compounds into hundreds of thousands in savings.
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Methodology & Assumptions
Uses 2025 IRS contribution limits. The taxable projection applies 0.5% annual tax drag (capital gains distributions) and 15% LTCG tax on gains at withdrawal. Roth grows with no tax drag and no withdrawal tax.
This tool provides estimates for educational purposes. It is not financial advice. Consult a fee-only financial planner for your specific situation.