After-Tax 401(k) Calculator: Estimate Mega Backdoor Roth Room
Use this guide to understand the assumptions, inputs, results, and next steps behind the calculator.
Created by Luis Gonzalez · Last updated June 25, 2026
After-tax 401(k) contributions are employee contributions made beyond the amount deferred as traditional or Roth salary deferrals. If a plan allows in-plan Roth conversions or in-service rollovers, those contributions may support a “mega backdoor Roth” strategy. This calculator estimates the available contribution space, employer match, and long-term balance under its plan-limit assumptions.
With the defaults, the calculator reports $43,500 of after-tax contribution space, $72,000 in total annual contributions, and a projected balance of $7,040,802 at age 65. It assumes a $150,000 salary, a 15% employee deferral, a 4% employer match, a 6% match limit, a 7% annual return, and annual contributions across 30 years.
Check your actual plan document before contributing. Plans can restrict after-tax deposits, conversions, or rollovers, and the applicable annual IRS limits depend on the tax year and age.
Enter salary, plan terms, and timeline
Enter annual salary, employee deferral percentage, employer match percentage, and the salary percentage eligible for matching. Add current age, retirement age, expected return, current 401(k) balance, and any intended after-tax contribution. The conversion toggle indicates whether the modeled after-tax deposit is available for an in-plan Roth conversion.
A zero after-tax contribution is treated as “use the full available space” by this calculator. To cap the modeled deposit, enter a positive amount. Compare results with the 401(k) contribution calculator, 401(k) max contribution calculator, and 401(k) calculator.
Read the default estimate carefully
The default employee deferral is $22,500: $150,000 × 15% = $22,500. The calculator estimates the employer match as $6,000, or 4% of salary. It then calculates after-tax space as $43,500, with total modeled contributions of $72,000:
Employee deferral = salary × deferral percentage
$150,000 × 15% = $22,500
After-tax space = modeled total plan limit − employee deferral − employer match
$72,000 − $22,500 − $6,000 = $43,500
Total annual contribution = employee deferral + employer match + after-tax deposit
$22,500 + $6,000 + $43,500 = $72,000
The result card calls the $43,500 contribution “Mega Backdoor Roth” potential because in-plan Roth conversion is set to yes. It does not mean the IRS automatically permits that amount in every plan or year.
There is a limit discrepancy to verify. The provided 2026 planning data lists a $24,500 employee elective deferral limit and a $72,000 total defined-contribution limit, while the calculator’s precomputed result uses a $72,000 total limit. The default employee deferral of $22,500 is below the listed $24,500 limit. Confirm the tax year’s official limit and your plan’s calculation before acting. The IRS explains contribution limits in its 401(k) contribution limits guidance.
The projection reaches $7,040,802 at age 65. Its final row reports $2,301,000 in cumulative contributions and $4,739,802 in growth. That is a smooth 7% annual-return projection, not a guaranteed balance. Test the same inputs at 4% and 5% returns in the 401(k) compound interest calculator or 401(k) future value calculator.
What “mega backdoor Roth” does—and does not—mean
A mega backdoor Roth usually involves after-tax 401(k) contributions followed by conversion to a Roth 401(k) or rollover to a Roth IRA, when the plan permits the transaction. The contribution is not the same as a regular Roth salary deferral. After-tax contributions are made from income that has generally already been taxed; investment earnings may be taxable if they are not converted or distributed under appropriate rules.
A plan may allow after-tax contributions but not in-service Roth conversions. In that case, the calculator’s “Mega Backdoor Roth” result may overstate what you can actually move into Roth status now. Ask the plan administrator about contribution sources, conversion frequency, fees, vesting, and whether conversions can happen automatically.
The regular employee elective deferral limit is distinct from the total plan limit, which can include employee deferrals, employer contributions, and eligible after-tax contributions. For 2026, the provided federal reference values are $24,500 for employee deferrals and $72,000 for total contributions, before applicable age-based catch-ups. Workers age 50 or older may have a 401(k) catch-up limit of $8,000, and ages 60–63 may qualify for a higher $11,250 catch-up under SECURE 2.0. Eligibility and plan implementation matter. Check current IRS publications and your plan rules.
The calculator does not model every limit or eligibility rule in federal law, and its displayed $72,000 limit conflicts with the supplied 2026 reference. Treat the output as a scenario calculation, not legal confirmation. A Roth 401(k) calculator can help compare Roth salary deferrals, while the 401(k) withdrawal tax calculator can help estimate future tax on traditional withdrawals.
How the balance projection works
For each year, the calculator adds the modeled employee deferral, employer match, and after-tax amount, then applies the expected return to the existing balance:
Annual employee deferral =
minimum(salary × deferral percentage, applicable employee limit)
After-tax contribution =
minimum(amount entered, total plan limit − employee deferral − employer match)
When the after-tax input is zero, the calculator instead fills all calculated remaining space.
Next year's balance =
current balance × (1 + expected annual return) + annual contributions
The default projection begins at age 35 with a $0 current balance, compounds annual deposits at 7%, and ends at age 65. It assumes contributions continue each modeled year and uses the current year’s salary and plan settings; it does not incorporate future salary increases in the provided calculation. Use a lower return scenario to account for market uncertainty.
Compare contribution choices
The table illustrates how contribution choices can change the amount eligible for a Roth conversion. These are arithmetic scenarios, not outputs from the default run. Each uses the calculator’s $72,000 modeled plan limit; that assumption must be checked against the legal limit for the relevant year.
| Salary | Employee deferral | Employer match | Modeled remaining space |
|---|---|---|---|
| $120,000 | $18,000 (15%) | $4,800 (4%) | $49,200 |
| $150,000 | $22,500 (15%) | $6,000 (4%) | $43,500 |
| $180,000 | $24,500 (capped) | $7,200 (4%) | $40,300 |
| $150,000 | $15,000 (10%) | $6,000 (4%) | $51,000 |
The higher deferral in the third example is capped at the 2026 employee limit supplied for this content. The table’s remaining-space calculation still uses the $72,000 modeled total, so do not interpret it as a validated contribution recommendation.
Frequently asked questions
Is an after-tax 401(k) contribution the same as a Roth 401(k) contribution?
No. A Roth 401(k) contribution is an elective salary deferral made to a designated Roth account. An after-tax contribution is a separate contribution source that may be converted to Roth if the plan permits. The Roth 401(k) calculator can help compare the contribution types.
Does every 401(k) plan allow mega backdoor Roth contributions?
No. The plan must allow after-tax contributions and a conversion or rollover route. Ask the administrator whether the plan permits in-service conversion, automatic conversion, or distributions while employed.
Should I prioritize this strategy or increase pre-tax contributions?
It depends on your current and expected future tax rates, cash flow, employer match, and plan fees. First evaluate capturing the full employer match. Then compare the tax treatment of traditional and Roth savings using the 401(k) paycheck impact calculator and Roth 401(k) calculator.
Can the projected balance be treated as guaranteed?
No. The projection applies a constant return. Actual returns vary, and contributions or plan terms can change. Compare more conservative returns in the 401(k) future value calculator.
Do after-tax contributions reduce my current taxable income?
Generally, after-tax contributions do not receive the same current-year income-tax exclusion as traditional salary deferrals. The result depends on the contribution source and plan reporting. For distribution considerations, see the IRS retirement plan distributions overview and the IRA withdrawal tax calculator.
Next steps
Verify the applicable annual limits, your plan’s after-tax contribution feature, and the conversion procedure. Then compare a smaller after-tax deposit with the full modeled space and test lower investment returns. For retirement withdrawal planning, review RMD strategies and the RMD tax calculator.