One ceiling, four kinds of money
Section 415(c) caps everything that goes into a defined contribution plan in a year, from every source at once. Your elective deferrals, whether pre-tax or Roth. Your employer’s match. Any profit sharing or non-elective contribution. And any after-tax contributions you make on top. All four come out of the same number.
This is the part that surprises people, because the elective deferral limit is the one everyone knows and it is a different, lower figure. Hitting the deferral limit does not mean the plan is full. What fills the plan is the annual additions ceiling, and the deferral limit is only part of what counts against it.
Catch-up is the exception
Section 414(v)(3) puts catch-up contributions outside the annual additions limit rather than inside it. Someone aged 50 or over has the whole ceiling available and the catch-up, which is why a calculator that adds them together is understating the room.
It is worth being precise about what this means in practice: the catch-up is not extra after-tax room. It is extra elective deferral room, and it is used by deferring more, not by contributing after-tax. The two are separate ceilings for separate kinds of contribution.
Compensation is the other ceiling
Contributions from all sources cannot exceed your compensation, and section 401(a)(17) caps the compensation an allocation may be based on. For most people the dollar limit is what binds; for anyone earning less than it, compensation is. Our capacity calculator shows which of the two is in play, because the answer changes what to do about it.
What the statute does not decide
Whether your plan accepts after-tax contributions at all, which most do not. Whether it permits in-plan Roth conversion or in-service withdrawal, without which after-tax money sits in the plan and its earnings grow taxable. And whether non-discrimination testing forces a refund, which can happen in a plan where few people contribute after-tax. Those are all the plan document’s answers rather than the statute’s, and the ceiling tells you nothing about them.