The 415(c) limit is the ceiling on total additions to a 401(k) in one year, from every source combined. For 2026 it is $72,000, or 100% of your pay if that is lower. It counts your own pre-tax and Roth contributions, employer matching and profit-sharing contributions, and any after-tax contributions. Catch-up contributions for people 50 and older sit on top of it and do not count.
It matters because the well-known limit, $24,500 in 2026, covers only your elective deferrals. The gap between $24,500 and $72,000 is where employer money and after-tax contributions live, and it is the space that makes a mega backdoor Roth possible.
Example: you defer the full $24,500, and your employer matches $10,500. That is $35,000, leaving $37,000 of room. If your plan allows after-tax contributions, you can put in up to $37,000 more and convert it to Roth. Without that plan feature, the remaining room simply goes unused.
The limit is per employer plan, not per person, in most cases. Someone with two unrelated employers can have a separate $72,000 limit at each, though the $24,500 elective limit is shared across all plans.
The common mistake is not knowing the limit exists, and so never asking whether the plan allows after-tax contributions. The second is exceeding it through a mid-year job change where both employers contribute heavily; the excess has to be pulled back out with its earnings.