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Mega Backdoor Roth Calculator,
2026 limits.
This mega backdoor Roth calculator finds how much after-tax money you can add to your 401(k) in 2026. It subtracts your deferrals and employer contributions from the $72,000 annual additions limit, then estimates the conversion tax and Roth growth.
Under 50: no catch-up.
The $72,000 limit cannot exceed 100% of pay. Plans count at most $360,000 of pay for percentage formulas.
2026 deferral limit $24,500. Catch-up dollars sit outside the $72,000 limit.
Employer contributions (match, profit sharing)
Include the match, any profit sharing and forfeiture allocations. They all use up the same limit.
Many plans cap after-tax money at a percent of pay. Leave blank if you do not know or there is no cap.
$37,500
$72,000 limit, minus $24,500 of your deferrals, minus $10,000 from your employer.
How the room is found
All-in ceiling for your age: $72,000 (the $72,000 limit). With your numbers about $72,000 goes into the plan this year.
What it means per paycheck
After-tax contributions come out of take-home pay, so check that your paycheck can cover them. Contributing less early in the year means a larger share per paycheck later.
Converting to Roth and growth
The after-tax dollars you contribute are already taxed, so converting them costs nothing; only the growth that built up before the conversion is taxed. The projection assumes a steady 6% return, year-end contributions and flat limits, and it ignores the tax on the converted growth and fees. Real returns vary.
Ask your plan administrator before counting on this
- Does the plan accept after-tax (non-Roth) contributions, and is there a cap?
- Does it offer an in-plan Roth conversion or an in-service withdrawal of after-tax money, and is conversion automatic?
- Does the nondiscrimination (ACP) test limit or refund contributions for higher earners?
Field guide
How the mega backdoor Roth works.
A 401(k) has two limits. The first caps what you can defer from your own paycheck. The second caps everything that goes into your account in a year, from you and from your employer. For many people the gap between the two is large, and a plan that accepts after-tax contributions lets you fill it. Move those dollars into Roth and they can grow tax-free. That two-step move is the mega backdoor Roth.
The 2026 limits
- Employee deferrals: $24,500, pre-tax and Roth combined.
- Catch-up, age 50 or over: $8,000. Ages 60 to 63 get $11,250 instead.
- Annual additions limit: $72,000, or 100% of pay if that is lower. It counts your deferrals, the match, profit sharing and your after-tax contributions.
- Pay a plan may count: $360,000 for formulas based on a percent of pay.
Catch-up contributions are outside the $72,000, so the all-in total is $80,000 for most people 50 and over and $83,250 at ages 60 to 63. Each of these figures comes from IRS Notice 2025-67, which raised the deferral limit from $23,500 and the annual additions limit from $70,000.
The formula
Only the part of your deferrals up to $24,500 counts, because catch-up dollars sit outside the limit. Employer money means the match, any profit sharing and forfeitures reallocated to you.
Example: age 42
Dana is 42, earns $200,000, defers the full $24,500 and receives $10,000 from her employer. Her room is $72,000 − $24,500 − $10,000 = $37,500. Over 26 paychecks that is about $1,442 each, or 18.75% of pay.
Example: age 61
Sam is 61 and defers $35,750, which is the $24,500 limit plus the $11,250 catch-up. His employer adds $12,000. Only $24,500 of his deferrals counts, so his room is $72,000 − $24,500 − $12,000 = $35,500. In total $83,250 goes into his account.
Step two: getting the money into Roth
After-tax contributions are not Roth until you convert them. Some plans allow an in-plan Roth rollover, which moves the money to the plan's Roth account (IRS Notice 2013-74). Others allow an in-service withdrawal that you roll to a Roth IRA. When the pre-tax and after-tax pieces of one distribution go to different places, IRS Notice 2014-54 sets how the pieces are assigned. Your contributions were already taxed, so only earnings that built up before the conversion are taxable. The calculator estimates that tax from the months you wait and your return.
Plan rules that can shrink your number
The room is a legal ceiling, not a promise. Your plan must offer after-tax contributions and a conversion route, may cap after-tax money at a percent of pay, and must pass the actual contribution percentage test. That test can limit contributions or trigger refunds for highly compensated employees, a group that includes people who earned more than $160,000 the year before. Contributions that exceed your room are an excess addition that has to be corrected.
What this calculator does not model
It does not know your plan, so it cannot confirm that after-tax contributions or conversions are available. It leaves out state tax, the five-year and age rules for withdrawing converted money, the employer deduction limit and any pre-tax balance you also move. If your 2025 Social Security wages from this employer were over $150,000, your 2026 catch-up must be Roth; that changes how those dollars are taxed but not your after-tax room. Growth figures are illustrations at a steady return.
Disclaimer
This calculator is for informational planning purposes only and does not constitute tax, legal or financial advice. It estimates after-tax 401(k) room using the 2026 federal limits published by the IRS, and it does not account for your plan's terms, nondiscrimination testing, state taxes or the other items listed above. Whether you can contribute after-tax money or convert it depends on your employer's plan. Confirm the details with your plan administrator and consult a CPA or tax professional before relying on these results.
The 2026 limits used here are the section 415(c) annual additions limit of $72,000, the section 402(g) deferral limit of $24,500 and the catch-up amounts of $8,000 and $11,250, as published in IRS Notice 2025-67. Rates, thresholds, and formulas are checked against IRS — Notice 2025-67, 2026 Amounts Relating to Retirement Plans and IRAs and updated when the underlying rules change.