Mega Backdoor Roth: How High Earners Save Up to $46,500 More Per Year
If your employer's 401(k) plan allows it, the mega backdoor Roth is the largest single Roth contribution most people will ever make. It lets high earners stash up to $46,500 a year (2025) of after-tax dollars into a Roth account that grows tax-free forever.
How it works in three steps
- Max your regular 401(k): Put in the $23,500 employee deferral first (pre-tax or Roth, your choice).
- Make after-tax (not Roth) contributions: Most plans that support this strategy let you contribute additional dollars on an after-tax basis, up to the $70,000 415(c) total.
- Convert immediately: Roll the after-tax balance into the Roth side of the same 401(k) (an "in-plan Roth conversion") or to a Roth IRA. Doing it quickly avoids taxes on growth.
The math
2025 415(c) cap is $70,000. Subtract:
- $23,500 regular employee deferral
- Your employer match (let's say $0–$10,000)
What is left is your after-tax / mega backdoor headroom. With no match, that is $46,500 a year of additional Roth contributions — almost 7× the regular $7,000 Roth IRA limit.
Three things your plan must allow
- After-tax contributions (separate from Roth deferrals)
- In-plan Roth conversions, or in-service withdrawals of after-tax dollars to a Roth IRA
- No "anti-discrimination" testing limits that cap you below the IRS limit
If your plan does not allow after-tax contributions or in-plan conversions, you cannot use this strategy. Many large-company plans (Microsoft, Meta, Google, Amazon among them) do allow it. Smaller employers often do not.
How to ask your plan administrator
Email or call HR and ask exactly: "Does our 401(k) plan permit after-tax employee contributions above the standard 402(g) elective deferral limit? And does it permit in-plan Roth conversions of those after-tax balances?" If both answers are yes, you can run the strategy.
Common mistakes
- Confusing after-tax with Roth. They are different. After-tax contributions sit in a separate sub-account that must be converted to become Roth.
- Waiting too long to convert. Earnings on after-tax dollars are taxable when converted. Convert quickly (ideally automatically) so there is little to no growth in between.
- Forgetting the match counts. Employer match eats into your $70,000 cap, reducing your after-tax headroom.
Curious how big your mega backdoor opportunity is? RetirementCheck101 calculates it automatically based on your income, employer match, and plan features.