The 'mega backdoor Roth' strategy involves after-tax contributions to a 401(k). What is the total 2025 annual 401(k) contribution limit (including all sources) that makes this possible?

For 2025, the IRS allows total contributions to a defined contribution plan of up to $70,000 (for those under 50) under Section 415. The employee elective deferral limit is $23,500, with an additional $7,500 catch-up for those 50 and older. The gap between the employee deferral and the Section 415 limit can be filled with employer matching and after-tax (non-Roth) contributions. If your plan allows in-service withdrawals or in-plan Roth rollovers, you can convert those after-tax contributions to Roth status, effectively sheltering significantly more money in tax-free growth.