Employer money does not add to the ceiling
A generous match feels like more going into the plan, and it is. What it is not is more room. Employer contributions count against the same annual additions limit as your own, so every dollar of match is a dollar of after-tax capacity you no longer have.
The arithmetic is unforgiving and it runs the wrong way round from the intuition. Two people on the same pay, deferring the same amount, with different employers: the one with the better match has less mega backdoor room, not more. The total going in is the same either way, because the ceiling is the same either way.
Which is not an argument against the match
It is worth saying plainly, because the arithmetic invites the wrong conclusion. Employer money is money you did not have to contribute. A dollar of match is better than a dollar of after-tax room by a wide margin, and nobody should turn down a match to preserve capacity.
What the arithmetic actually tells you is that the room is not fixed. It moves during the year as match accrues, and the figure you calculate in January is not the figure you will have in December.
Which is why the timing matters
If you contribute after-tax early in the year against a capacity figure that assumes no match, and then the match arrives, the total can exceed the ceiling. Plans handle that by refusing or refunding contributions, neither of which is pleasant.
The safer order is to work out the capacity net of the match you expect for the whole year, and to recheck it if anything changes: a bonus, a raise, a change of contribution rate. Our capacity calculator takes employer contributions as an input for exactly this reason, rather than assuming them away.
The same ceiling is what makes a 401(k) true-up worth understanding: a plan that trues up pays the match you missed, and that payment lands against this limit too.