The Mega Backdoor Roth is one of the most powerful legal loopholes in the U.S. retirement system. It lets high earners shovel up to $46,500 of after-tax money (2026 figure) into a Roth vehicle each year — on top of the normal $23,500 elective deferral limit. But it only works if your employer’s 401(k) plan supports two specific features.
The Two Plan Features You Need
| Feature | Why It Matters |
|---|---|
| After-tax (non-Roth) contributions | Lets you contribute beyond the $23,500 deferral cap, up to the $70,000 total annual addition limit. |
| In-plan Roth conversion or in-service withdrawal | Lets you move the after-tax money into a Roth bucket immediately, before earnings accrue and become taxable on conversion. |
2026 Contribution Math
- Total annual 401(k) addition limit: $70,000
- Your elective deferral (traditional or Roth): up to $23,500
- Employer match: varies (call it $10,000)
- Remaining room for after-tax contributions: $70,000 − $23,500 − $10,000 = $36,500
That $36,500 is the amount you can funnel through the Mega Backdoor Roth at this example employer. If your match is smaller, the room is larger — up to roughly $46,500.
Which Employers Are Known to Allow It
Plan documents change, so verify with your own plan summary (SPD) before assuming. As of recent filings, plans at large tech and finance firms commonly support the feature. Plans at small private companies, many startups, and most government 457(b) plans generally do not. The fastest way to check: search the SPD for the phrase “after-tax contributions” and separately for “in-plan Roth” or “in-service distribution.”
Execution Checklist
- Confirm both features in your plan’s SPD.
- Enroll in after-tax contributions through payroll.
- Set automatic in-plan Roth conversion if offered (ideal: same-day sweep).
- If only in-service withdrawal is offered, schedule a quarterly rollover to a Roth IRA.
- Track the conversion on Form 1099-R at year-end to ensure after-tax basis is reported correctly.
Tax Trap: Earnings on After-Tax Dollars Are Taxable
If you let after-tax contributions sit and grow inside the 401(k) before converting, the earnings portion converts as taxable income. Same-day sweeps avoid this almost entirely.
Interaction with Side Income
If you run a side business and have a Solo 401(k), you cannot double up the $70,000 limit across unrelated employers if they are part of a controlled group. Unrelated W-2 + true side business = two separate $70,000 buckets. We break that down in the side hustle retirement guide below.