Plans differ in when they let you make after-tax contributions, and the difference has real consequences. Some plans only accept after-tax money as a percentage of each paycheck; others let you make a lump-sum contribution. Getting this wrong can cost you room you can never get back.
Per-pay-period mechanic
Many large-employer plans require after-tax contributions to be elected as a percentage of each paycheck. The catch: there is usually no make-up contribution. If you under-elect early in the year, or miss paychecks, the room those periods represented is simply gone — you cannot true it up in December.
This makes the front-loading vs spreadingdecision important. Front-loading fills your room faster (good if you might leave mid-year) but can shrink an employer match that is applied per pay period rather than trued-up annually. Read your plan's match formula before front-loading.
Annual / lump-sum mechanic
Plans that accept a lump-sum after-tax contribution are more forgiving. You can wait until late in the year, total up your elective deferral and employer additions, and contribute exactly the remaining §415(c) room in one move — no risk of leaving room on the table from an early mis-election.
The mid-year job change trap
The §415(c) limit is per-employer-plan, but watch the §402(g) deferral limit, which is per person across all plans in the year. If you switch employers, your new plan does not know how much you already deferred at the old one — and over-deferring creates an excess that must be corrected. The after-tax room calculation also resets per plan, so a per-pay plan at a new employer gives you only the pay periods remaining after you start.
How the calculator helps
Set the calculator above to the per-pay mechanic and enter your remaining pay periods. It divides your remaining room into a per-paycheck figure, so you can translate “$27,500 of room” into the contribution percentage to elect for the rest of the year. Confirm your plan's exact mechanic and any match-true-up rule with HR.