retirement · 2026-05-01
Compute how much you can move into Roth via the mega backdoor in your 401(k) — based on your salary, employer contributions, and the IRC 415(c) annual additions limit.
| Annual salary | $200,000 |
| Employee 401(k) contribution | $23,000 |
| Employer match received | $9,000 |
| Employer profit-share / nonelective | $0 |
| Current age | 35 |
| Total contributions so far | $32,000 |
| Annual additions limit | $69,000 |
| Limit as % of salary | 34.5% |
If your 401(k) plan allows after-tax contributions + in-service Roth conversions (also called "in-plan Roth rollovers" or "after-tax-to-Roth"), you can stuff up to $40,000+ extra into Roth annually beyond the standard $23k limit.
The IRC 415(c) annual additions limit (2024: $69,000) is the ceiling on ALL contributions to your 401(k) — yours + employer's. Most people fill it with $23k employee + $10-12k match. The gap between match-cap and $69k is mega backdoor capacity.
If your plan doesn't have all three, the mega backdoor isn't available. Ask HR — many plans added it 2020-2024.
No. Roth 401(k) and traditional 401(k) share the $23,000 elective deferral limit. After-tax contributions are a SEPARATE bucket above and beyond that — they fill the gap to the $69,000 annual additions limit. Roth 401(k) ≠ after-tax contributions.
If your plan fails non-discrimination testing (Highly Compensated Employees over-contributing), excess after-tax contributions get refunded. Most large-employer plans pass safe-harbor and don't limit HCEs. Smaller employers / startups: ask plan administrator before counting on full mega backdoor.
Math says: mega backdoor wins if (long-term equity return ~7%) > (after-tax debt rate). For a 30-year-old with a 6% mortgage, the mega backdoor wins by ~1%/yr compounded for 30 years = significant. For 20% credit card debt, kill the debt first. Mid-rate debt (4-7%): roughly a wash, do whichever feels better.