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What Is a Mega Backdoor Roth?
The mega backdoor Roth is a strategy that lets employees pour tens of thousands of additional dollars per year into Roth retirement accounts beyond the standard 401(k) and Roth IRA limits. It works by exploiting two features built into some employer 401(k) plans: after-tax (non-Roth) contributions and in-service or in-plan Roth conversions. When both features are available, you contribute after-tax dollars into the 401(k) and immediately convert them to a Roth account, where they grow and are withdrawn tax-free in retirement. The mega backdoor Roth calculator above quantifies exactly how much you can contribute and what those contributions become at retirement.
The strategy is called a "mega" backdoor Roth because it dwarfs the regular backdoor Roth IRA. The regular backdoor Roth IRA caps contributions at the annual IRA limit, $7,000 in 2024, or $8,000 with catch-up. The mega backdoor Roth can move up to roughly $46,000 per year for those under 50, or as much as $46,000-plus for those 50 and over, depending on your employer match. For high earners who already max out a traditional 401(k) deferral and a backdoor Roth IRA, the mega backdoor Roth IRA calculator reveals what is often the single largest additional source of tax-advantaged savings available. Combined with our 401k calculator and our Roth IRA calculator, it gives you a complete picture of your retirement savings ceiling.
Eligibility Requirements for the Mega Backdoor Roth
Two specific plan features must be present for the mega backdoor Roth to work. First, your 401(k) must permit after-tax contributions distinct from both pre-tax deferrals and Roth deferrals. After-tax contributions are made with money that has already been taxed, much like a Roth contribution, but the earnings grow tax-deferred and are taxed as ordinary income on withdrawal. Without this feature, there is no after-tax bucket to convert. According to Investopedia, only about half of large employer plans offer after-tax contributions, and the share is lower at small employers.
Second, your plan must allow either in-service withdrawals to a Roth IRA or in-plan Roth conversions. These features let you move the after-tax dollars into a Roth account quickly, ideally the same paycheck, so any taxable earnings on the after-tax balance are minimal. Without conversion access, the after-tax bucket still grows but loses its tax-free Roth treatment because earnings are eventually taxed as ordinary income. The Bogleheads wiki maintains a detailed reference on plan-level mechanics and edge cases worth reading before you set up your contributions.
Beyond plan rules, there are no income limits for the mega backdoor Roth. The Roth IRA phaseouts that cut off direct Roth contributions above $146,000 (single) or $230,000 (married filing jointly) in 2024 do not apply to in-plan Roth conversions or to after-tax 401(k) contributions. This is precisely why the strategy is so attractive to high earners who are completely shut out of direct Roth IRA contributions.
2024 Contribution Limits and the 415(c) Cap
The mega backdoor Roth lives or dies by the IRC Section 415(c) total contribution limit, which the IRS sets at $69,000 for 2024 for those under 50 and $76,500 for those 50 and over with the catch-up contribution. This cap covers all contributions to a single 401(k) plan in a year: employee pre-tax deferrals, employee Roth deferrals, employer match, employer profit-sharing, and after-tax employee contributions. The standard employee deferral cap is $23,000 ($30,500 with catch-up), and the after-tax bucket fills the gap between your employee deferral plus employer match and the overall 415(c) ceiling.
A concrete example: a 35-year-old earning $200,000 maxes the employee deferral at $23,000. Her employer contributes a 4% match of $8,000. Her mega backdoor Roth room is therefore $69,000 − $23,000 − $8,000 = $38,000. Each year she can contribute $38,000 in after-tax dollars and convert it to Roth. Over 25 years at a 7% growth rate, that single annual $38,000 stream compounds to roughly $2.4 million in tax-free retirement assets. The full details on these limits live in IRS Publication 590-A, which the agency updates annually. Always confirm the current year's limits before running final numbers.
One important constraint built into the mega backdoor Roth IRA calculator: after-tax contributions cannot exceed your eligible compensation for the year. If your salary is below the 415(c) ceiling, your salary becomes the binding constraint. Pair this calculator with our Roth IRA contribution limit calculator to confirm how much you can also place into a direct Roth IRA on top of the mega backdoor amount.
Mega Backdoor Roth vs Regular Backdoor Roth
The regular backdoor Roth IRA is a separate, smaller strategy that high earners use to sidestep Roth IRA income limits. You make a nondeductible contribution to a traditional IRA (up to $7,000 in 2024, or $8,000 if 50 or older) and then convert it to a Roth IRA. Because the contribution was nondeductible, the conversion has no tax cost (assuming no other pre-tax IRA balances trigger the pro-rata rule). The regular backdoor Roth is available to anyone with earned income and does not require any specific employer plan features.
The mega Roth conversion calculator here addresses a different strategy that lives inside your 401(k) plan and can move five to seven times more money per year into Roth status. The two strategies stack: a single high earner can do both, putting up to $46,000-plus through the mega backdoor and another $7,000 to $8,000 through the regular backdoor Roth IRA. Both strategies sidestep the Roth IRA income phaseouts, but they use entirely different mechanisms. Use our Roth conversion calculator to compare full traditional-to-Roth conversions, and use the mega backdoor calculator on this page for the after-tax 401(k) route. They are complementary tools.
Step-by-Step Execution Guide for the Mega Backdoor Roth
Once the mega backdoor Roth calculator confirms the strategy is available and the dollar amounts make sense, execution follows a predictable sequence. First, contact your plan administrator and confirm in writing that the plan allows after-tax contributions and either in-service withdrawals or in-plan Roth conversions. Ask whether automatic in-plan conversion is supported on a per-paycheck basis. This is the cleanest option because it converts contributions immediately, preventing taxable earnings from accumulating in the after-tax bucket.
Second, log into your 401(k) provider and adjust your contribution election to include the after-tax line. Plans typically display three separate sliders or percentages: pre-tax, Roth, and after-tax. Set your after-tax percentage so that, when multiplied by your remaining paychecks for the year and added to your existing deferrals and employer match, the total lands as close to the 415(c) limit as possible without exceeding it. Some plans will auto-stop contributions at the limit; others will not, so monitor your year-to-date totals.
Third, enable in-plan Roth conversion or schedule periodic in-service withdrawals to move the after-tax balance to Roth. If your plan supports automatic per-paycheck conversion, turn it on. If you must request conversions manually, do so at least quarterly to minimize taxable earnings. Finally, document each conversion for your tax records; the broker will issue a Form 1099-R showing the conversion, and you will report it on your federal return. For a complete retirement plan that incorporates the mega backdoor alongside Social Security, 401(k), and IRA strategies, review all of our retirement planners. The cluster works together to give you a coherent, optimized retirement strategy.