Mega Backdoor Roth Calculator

Enter your salary, employee contribution percentage, employer match rate and match limit, age, expected return, current Roth balance, and annual raise assumption. The calculator uses the 2026 contribution limits in its estimate. Its default assumptions produce:

• Mega Backdoor Space: $45,000
• Total 401(k) Limit: $72,000
• Projected Roth Balance: $3,984,234
• Tax-Free Income: $159,369 per year, based on a 4% withdrawal estimate

The projection assumes the remaining contribution room goes into the Roth each year and earns the expected return. It is a scenario, not a forecast: limits, plan rules, investment returns, and your contributions can change.

With a $150,000 salary and a 15% employee contribution, the employee contribution is $22,500:

Employee contribution = salary × contribution rate
$150,000 × 15% = $22,500

The employer matches 50% of contributions up to 6% of salary. Because the $22,500 contribution exceeds the $9,000 matchable salary amount, the estimated match is $4,500:

Matchable salary = $150,000 × 6% = $9,000
Employer match = $9,000 × 50% = $4,500

The calculator subtracts both the employee contribution and employer match from the $72,000 total limit:

Mega backdoor space = $72,000 − $22,500 − $4,500
$72,000 − $27,000 = $45,000

With a current Roth balance of $25,000, age 35, retirement age 65, 7% expected return, and 3% annual raises, the displayed projected Roth balance is $3,984,234. The calculator’s 4% income estimate is:

Estimated annual income = projected Roth balance × 4%
$3,984,234 × 0.04 = $159,369.36
Rounded result = $159,369 per year

The projection models 31 ages, from 35 through 65. It starts with the current Roth balance at age 35 and adds projected annual mega backdoor contributions and growth in later years. The final contribution shown at age 65 is $35,577, reflecting the calculator’s salary-growth and contribution-limit logic.

For 2026, the regular 401(k) employee elective deferral limit is $24,500. The calculator uses a $72,000 total annual limit for employee and employer contributions before catch-up contributions. At age 50 or older, its inputs apply a $8,000 catch-up to the employee limit and use a higher total limit. SECURE 2.0 provides a larger catch-up amount of $11,250 for eligible participants ages 60 through 63; the calculator’s simplified age-based rule does not separately model that age band.

The annual limit includes traditional and Roth employee deferrals plus employer contributions and after-tax contributions. See the IRS explanation of 401(k) contribution limits and verify current-year limits with your plan.

A mega backdoor Roth is different from a regular backdoor Roth contribution. The regular backdoor strategy generally involves a nondeductible traditional IRA contribution followed by a Roth IRA conversion. A mega backdoor Roth uses an employer 401(k) plan’s after-tax contribution feature. It is also distinct from electing Roth contributions directly in a Roth 401(k).

These examples hold the match formula and $72,000 limit constant to isolate the effect of employee contributions. Actual matching policies vary. Also compare the Roth 401(k) contribution calculator and Roth IRA tax calculator before choosing an account type.

The calculator estimates the current-year amount first, then projects the Roth balance through the retirement age. Its core calculations are:

Employee contribution = smaller of (salary × contribution percentage) or employee limit

Employer match = smaller of (employee contribution, salary × match limit) × match percentage

Mega backdoor space = larger of (0, total 401(k) limit − employee contribution − employer match)

Projected Roth balance = prior Roth balance + annual Roth contribution + investment growth

The tool treats the entire remaining contribution room as mega backdoor Roth money. It does not ask how much after-tax money you actually plan to contribute, model plan fees, calculate conversion taxes on investment gains, or test whether your plan permits in-service conversions. To compare account growth under different contribution choices, use a Roth 401(k) calculator or Roth 403(b) calculator.

Mega Backdoor Space is an estimated contribution ceiling under the calculator’s assumptions, not a statement that your plan will accept that amount. If you already make after-tax contributions or receive other employer contributions, those can reduce the room available.

Projected Roth Balance is a future-dollar estimate built from fixed annual return assumptions and modeled contributions. It does not include market volatility, taxes on taxable investments used to fund contributions, or future changes in contribution limits. Try different returns and retirement dates rather than treating one projection as certain.

Tax-Free Income is not a guaranteed withdrawal or an IRS-prescribed amount. It is simply 4% of the projected balance. Qualified Roth distributions generally require the applicable five-year period and a qualifying event; see the IRS Roth account distribution rules. The estimate does not check whether a future withdrawal meets those requirements.

A mega backdoor Roth may make sense when you have already considered emergency savings, high-interest debt, and other retirement contributions. Compare potential taxable-account tradeoffs and future withdrawal needs with a Roth 401(k) withdrawal calculator or Roth IRA withdrawal calculator. For broader tax planning, review Roth versus traditional IRA tradeoffs and Roth conversion ladders.

Does every 401(k) plan allow mega backdoor Roth contributions?

No. Your plan must allow after-tax employee contributions and provide a way to convert those contributions to Roth or roll them into a Roth account while you are still employed. Confirm the exact options, limits, timing, and fees with the plan administrator.

Is the $45,000 estimate the amount I can contribute to any Roth account?

No. It is the calculator’s estimate of space under the modeled total 401(k) limit. It does not mean you can contribute $45,000 directly to a Roth IRA. The plan’s terms and the annual combined workplace-plan limit control this estimate.

Are after-tax 401(k) contributions taxed twice?

The original contribution is made from after-tax income, so it is not taxed again as contribution basis when distributed. Investment earnings that are converted to Roth may be taxable if they have accrued before conversion. Ask the plan how quickly contributions can be converted and review the applicable IRS rules.

Should I prioritize a mega backdoor Roth or a traditional 401(k) contribution?

It depends on your current and expected future tax rates, cash flow, plan features, and need for current deductions. Traditional contributions may reduce taxable income now; Roth contributions trade that deduction for the possibility of qualified tax-free withdrawals later. Compare the effect with a Roth 401(k) calculator and a Roth IRA tax calculator.

Does the calculator guarantee $159,369 of tax-free retirement income?

No. That figure is 4% of the projected balance, not a guaranteed income stream. Investment results, inflation, withdrawal timing, taxes, and Roth distribution rules all affect what you can spend. A withdrawal-planning tool such as the Roth 401(k) withdrawal calculator can help test different drawdown assumptions.

What should I check before making after-tax contributions?

Ask whether the plan accepts after-tax contributions, whether automatic or frequent Roth conversions are available, whether gains are taxable at conversion, and how employer contributions count toward the limit. Then compare your plan options with a backdoor Roth estimate and your broader retirement plan.

Confirm your employer plan’s after-tax and conversion features, then rerun the estimate with your actual contribution election and match formula. Compare the result with other Roth options, including a Roth 457 plan or Roth 403(b), before treating the modeled contribution room as part of your plan.