Mega Backdoor Roth After-Tax 401(k) Calculator
Mega backdoor Roth lets high earners stuff $30-50K+ extra into Roth via after-tax 401(k) contributions converted to Roth. 2026 limit: $70K annual additions.
| Employee pre-tax contribution | — |
| Age 50+ catch-up | — |
| Employer match | — |
| Maximum after-tax contribution (mega backdoor) | — |
| Annual additions limit (2026) | — |
| Total 401(k) contributions if maxed | — |
The mega backdoor Roth is the highest-impact tax-free wealth strategy available to W-2 employees in 2026. Take your $23,500 employee 401(k) contribution + employer match, then fill the remaining space up to the $70,000 annual additions limit ($77,500 with age-50 catch-up, $81,250 at age 60-63) with after-tax 401(k) contributions. Convert those to Roth via in-service conversion or in-service rollover. Result: $30,000-$50,000+ in extra Roth contributions per year.
The $70K Annual Additions Limit
IRC Section 415(c) caps total 2026 contributions to a single 401(k) plan at $70,000 (the 'annual additions limit'). This includes employee pre-tax, employee Roth, employer match, employer profit-sharing, and employee after-tax. The mega backdoor Roth uses the gap between this limit and the typical employee/employer total. A high earner with $23,500 employee + $10,000 match = $33,500 has $36,500 of remaining space for after-tax contributions, which can then be converted to Roth.
Plan Must Support After-Tax + In-Service Conversion
Two plan features are required: (1) The plan must accept after-tax contributions (different from Roth — these are post-tax dollars that go into a separate after-tax bucket). (2) The plan must allow either in-service conversion to Roth 401(k) or in-service rollover to a Roth IRA. Without both features, the after-tax contributions sit and accumulate taxable growth, making conversion later costly. Check your Summary Plan Description or ask HR specifically about 'after-tax contributions' and 'in-service Roth conversion' or 'in-plan Roth rollover.'
Convert Quickly to Minimize Growth
Convert after-tax money to Roth as quickly as possible — ideally same day as contribution. Any earnings on the after-tax balance (before conversion) become taxable when you convert. Best-case scenario: contribution and conversion same day = $0 taxable earnings. Worst case: leave after-tax for years and pay tax on substantial growth. Many plans support automatic same-day in-service conversion, sometimes called 'automatic Roth conversion' or 'Roth-in-plan rollover.'
Last updated May 2026. Sources: IRS 401(k) Limits.