Mega Backdoor Roth Calculator
Calculate how much extra Roth savings you can build using after-tax 401(k) contributions immediately converted to Roth — and compare it to a taxable account over time.
Roth advantage over taxable
$145,733
After 20 years: Roth $1,229,865 vs taxable $1,084,132.
Final Roth balance
$1,229,865
Final taxable (after tax)
$1,084,132
Total contributions
$600,000
- After-tax contributions are converted to Roth immediately (in-plan Roth conversion or roll to Roth IRA). Each converted dollar grows fully tax-free from the date of conversion.
- The taxable account receives the same annual contribution. Annual tax drag is approximated at 30% of the return taxed at the ordinary income rate (modeling dividend income and short-term turnover). Capital gains tax (15%) is applied on withdrawal.
- The 2024 combined 401(k) limit is $69,000 (including employer contributions). This calculator does not validate that your after-tax contribution fits within that limit — check with your plan administrator.
- The 5-year rule on Roth conversions: each year's converted amount must season for 5 years before you can withdraw it penalty-free before age 59½. The calculator models the final account value, not a withdrawal schedule.
- Returns are nominal (not inflation-adjusted) and assumed constant each year.
What is the mega backdoor Roth?
The mega backdoor Roth is a strategy that lets high-income earners contribute after-tax dollars to a 401(k) — on top of the normal pre-tax or Roth contribution limits — and then immediately convert those after-tax contributions to a Roth account. This can add $30,000–$46,000 per year of additional tax-free savings beyond the standard $23,500 employee limit.
The mechanism works like this: the total 401(k) contribution limit (employee pre-tax + Roth + after-tax + employer match) is $70,000 in 2025 ($77,500 if 50+). If your employer match is $5,000 and you contribute the maximum pre-tax $23,500, you have roughly $41,500 of remaining room for after-tax contributions. Converting these to Roth means that growth is entirely tax-free — something no other account type allows at this scale.
Why it matters to your money
For high earners, the mega backdoor Roth is one of the most powerful tax-advantaged savings strategies available. The tax-free growth on $30,000–$46,000 per year over decades can result in hundreds of thousands of dollars of additional retirement savings compared to a taxable brokerage account. And unlike a Roth IRA, there's no income limit — anyone with access to a qualifying 401(k) plan can use it.
Read the full explainer on backdoor Roth strategies for details on eligibility, plan requirements, and the step-by-step conversion process.
Rules of thumb
- Check your plan first: Your 401(k) must allow after-tax contributions AND either in-service withdrawals or in-plan Roth conversions. Many plans don't support both.
- Convert immediately: To minimize tax liability on earnings between contribution and conversion, convert as often as your plan allows (ideally every payroll period).
- The 2025 total limit is $70,000 ($77,500 if 50+): This includes all employee and employer contributions. Subtract your match and pre-tax contributions to see how much room remains for after-tax.
Frequently asked questions
- What is the mega backdoor Roth?
- The mega backdoor Roth is a strategy that lets you contribute up to ~$46,500 of after-tax dollars to a 401k (on top of the normal $23,500 pre-tax limit) and then convert them to a Roth — either within the plan or by rolling them out to a Roth IRA.
- Who can use the mega backdoor Roth?
- Only employees whose 401k plan allows after-tax contributions AND either in-service withdrawals or in-plan Roth conversions. Many large employers' plans support it, but many smaller company plans do not. Check your Summary Plan Description.
- What is the 2025 total 401k contribution limit?
- The 2025 total limit (employee + employer + after-tax) is $70,000 ($77,500 if age 50+). Subtracting the $23,500 pre-tax limit and any employer match leaves the remainder available for after-tax contributions — the source for the mega backdoor Roth.