401(k) Catch-Up Math: Fix Your Rate This Week

There is a specific cruelty to 401(k) contributions: you cannot fix them in January. Money only enters the plan through payroll, the year ends December 31, and whatever gap you carried through the fall is gone for good. That is why TheStreet's October roundup of Q4 retirement deadlines leads with catch-up contributions, and why I am writing this in the first week of October instead of the last week of December. If your rate is wrong, you still have roughly six pay periods to fix it. Barely, but enough.

Know your actual ceiling for 2026

The standard employee deferral limit for 2026 is $24,500. Workers 50 and older can add an $8,000 catch-up, for a combined ceiling of $32,500. Workers aged 60 to 63 get the larger SECURE 2.0 super catch-up of $11,250, for a total of $35,750. Those figures come from IRS Notice 2025-67, as summarized by Mercer Advisors in its September 2026 analysis and by the IRS's own limit summaries. The IRA limits are separate and smaller, $7,500 plus a $1,100 catch-up at 50 and older, and IRAs give you until tax day. The 401(k) gives you nothing past December 31. Prioritize accordingly.

Figure 1: Maximum 401(k) employee contributions for 2026 by age band. Source: IRS Notice 2025-67 via Mercer Advisors and IRS summaries, 2026.

The per-paycheck math, worked once

Find your year-to-date contributions on your last pay stub. Subtract from your ceiling. Divide by the paychecks left in the year. That is your required rate, and here is what it looks like in practice. Say you are 52, your ceiling is $32,500, and you have contributed $21,000 through September. Your gap is $11,500. With six biweekly paychecks left, you need about $1,917 per check going in. If that number made you flinch, good, it was supposed to. The flinch in October is information. The same flinch in December is just regret.

Two practical warnings from experience and from the reporting. First, a deferral change typically takes a full pay cycle to process, per TheStreet's summary, so a change filed today may first appear on the paycheck after next. Build that lag into your division: if only five checks will actually carry the new rate, divide by five. Second, watch the ceiling from the other side. If you front-loaded contributions early in the year and your employer matches per paycheck, maxing out in November can mean missing match dollars in December. Ask HR how your plan true-ups the match before you slam the rate to the top.

The new Roth rule for higher earners

This is the 2026 change that surprises people. If your FICA wages from your employer exceeded $150,000 in 2025, SECURE 2.0 requires all of your catch-up contributions to go into the Roth side of the plan, after tax, starting January 1, 2026, according to Mercer Advisors and IRS guidance summaries. Your base $24,500 can still be split pre-tax and Roth however you choose. Only the catch-up slice is forced Roth. Two consequences follow. Your take-home pay drops more per catch-up dollar than it did with pre-tax contributions, so the October rate increase and the Roth tax hit can land in the same paycheck. And if your plan does not offer a Roth option at all, you may be blocked from making catch-up contributions entirely. That is a question for HR this week, not a discovery for your December self.

Your move this week

Pull one pay stub. Do the three-line calculation. File the rate change the same day, because of the processing lag. Then write down whether the Roth mandate applies to you and whether your plan even has a Roth side. Fifteen minutes, three numbers, and the most time-sensitive money decision of your fourth quarter is handled.

FAQ

What is the 401(k) contribution limit for 2026? $24,500 for employees under 50, $32,500 for workers 50 and older including the $8,000 catch-up, and $35,750 for ages 60 to 63 including the $11,250 super catch-up, per IRS Notice 2025-67.

When is the deadline to contribute for 2026? December 31, 2026, and contributions only count when they come out of a paycheck dated in 2026. There is no January grace period like the one IRAs offer.

Who must make catch-up contributions as Roth in 2026? Workers 50 and older whose FICA wages from their employer exceeded $150,000 in 2025, under the SECURE 2.0 mandate that took effect January 1, 2026. The base contribution is unaffected.

How long does a contribution rate change take to show up? Typically one full pay cycle, according to TheStreet's Q4 deadlines reporting. File the change assuming one fewer paycheck than the calendar suggests.

What if my plan has no Roth option? If the Roth mandate applies to you and your plan offers no Roth side, you may not be able to make catch-up contributions at all. Ask HR now, while there is time for the answer to matter.