Mega Backdoor Roth Calculator (401(k) After-Tax Contribution Room)
Enter your age and your employee and employer 401(k) contributions to estimate the after-tax contribution room available for a mega backdoor Roth, based on the IRS Section 415 annual limit.
What Is a Mega Backdoor Roth?
A mega backdoor Roth is a strategy available when your employer's 401(k) plan allows "after-tax contributions" (a separate contribution category from pre-tax and Roth deferrals) and also allows in-plan Roth conversion or in-service withdrawal. IRS Section 415 caps the combined total of employee contributions, employer contributions, and after-tax contributions for a single 401(k) plan. Any remaining room after your employee contribution (capped at $24,500 for 2026) and employer match can potentially be filled with after-tax contributions.
While a regular backdoor Roth (money.wealth.backdoor_roth_calculator) goes through a Traditional IRA, a mega backdoor Roth goes through your employer's 401(k) plan, which can allow for a much larger amount ($72,000 under the Section 415 annual limit, compared to roughly $7,000 for a regular IRA). However, only a limited number of 401(k) plans allow in-plan Roth conversion, so whether this is actually available to you depends on your plan's rules.
How to Use
- Enter your age Used to determine whether you qualify for catch-up contributions (age 50+ or age 60-63)
- Enter this year's employee contribution Enter the total of your pre-tax and Roth 401(k) contributions from your paycheck
- Enter this year's employer contribution Enter the total employer matching and profit-sharing contributions, and the potential after-tax contribution room is calculated automatically
Tips for getting more out of it
- Making after-tax contributions without your plan supporting in-plan Roth conversion or in-service withdrawal just leaves post-tax money sitting inside your 401(k) — you won't get the Roth benefit.
- The typical order is to first max out your employee deferral (pre-tax + Roth), then use any remaining Section 415 room for after-tax contributions.
- The more your employer contributes (e.g., a generous match), the less room remains for after-tax contributions.
- This tool provides only a simplified estimate. Always confirm the actual contribution amount and conversion process with your plan documents and HR or plan administrator.
Use Cases
High earners maximizing retirement account contributions
Check how much additional room you may have beyond your 401(k) employee deferral limit and IRA income limits
Preliminary research before talking to HR or your plan administrator
Understand the Section 415 room first, then follow up by confirming whether your plan allows in-plan Roth conversion
Combining with a regular backdoor Roth (money.wealth.backdoor_roth_calculator)
Useful for planning contributions that combine an IRA-based backdoor Roth with a 401(k)-based mega backdoor Roth
Glossary
- Mega Backdoor Roth
- A strategy that converts after-tax 401(k) contributions to a Roth IRA or Roth 401(k), allowing you to build Roth assets well beyond the normal Roth contribution limits.
- After-tax Contribution
- A 401(k) contribution category separate from pre-tax and Roth deferrals, funded with money that has already been taxed. Not all 401(k) plans allow it.
- Section 415 (IRC §415(c))
- An IRS rule that caps the combined total of employee contributions, employer contributions, and after-tax contributions for a single 401(k) plan.
- In-plan Roth Conversion
- A feature that lets you convert after-tax contributions to Roth 401(k) within the plan itself. A mega backdoor Roth is not possible if your employer's plan does not offer this.
- In-service Withdrawal
- A plan feature that allows withdrawals from a 401(k) while still employed, sometimes used as a route to move after-tax contributions to a Roth IRA.
- Catch-up Contribution
- An additional contribution allowed for participants age 50 and over. For 2026, it is $8,000 for ages 50-59 and 64+, and $11,250 for ages 60-63 under SECURE 2.0.
Frequently Asked Questions
Side Note — Why "Mega"?
The term "mega backdoor Roth" was coined by financial planners and bloggers to distinguish it from the regular backdoor Roth, which goes through an IRA. While a regular backdoor Roth is limited to the annual IRA contribution cap (roughly $7,000 as of 2026), a mega backdoor Roth taps into a 401(k) plan's Section 415 limit ($72,000), which can allow for an order of magnitude more Roth savings when the right conditions are met — hence the "mega."
The strategy gained widespread attention in the mid-2010s, when several large tech companies began actively offering after-tax contributions paired with in-plan Roth conversion as part of their benefits packages. Because engineers in that industry often earned enough to be phased out of direct Roth IRA contributions, the technique spread quickly through Silicon Valley's tech worker community.
Even today, relatively few 401(k) plans offer in-plan Roth conversion, largely because many employers are wary of the added plan administration costs. Larger companies tend to offer it more often than smaller ones, which means whether your own employer supports it is, in many cases, simply a matter of luck.