Most match formulas are not flat. Many step up or change shape at a service milestone, and the contribution rate you set on your first day does not move with them. The IRS confirms employers may make matching contributions based on your elective deferrals, which means the match follows what you defer and when you defer it. Both halves matter.
The rate you picked was right for the plan you had then
You already know what a match is. That is not the gap.
The gap is that a match formula is a plan document provision, and plan documents get amended. Three structures are common. A flat percentage, where the employer matches a set portion of what you defer up to a cap. A tiered structure, where the match percentage or the cap changes with years of service. And a structure that changes at a specific service milestone, which is the one that quietly costs people money.
Here is the mechanic, using illustrative numbers only. Suppose a plan matches 50 cents per dollar on the first 6% of pay you defer, and at ten years of service the cap moves from 6% to 8%. Someone who set 6% on their start date and never revisited it is still deferring 6% in year eleven. The plan is now willing to match on 8%. That extra 2% of match is available, and nothing in the payroll system will tell you it went unclaimed. There is no error message for a contribution rate that has merely gone stale.
Brad uses an aviation analogy for this that fits better than most. A pilot correcting course constantly is not fussing. A deviation of a single degree feels like nothing in the first hour and puts you in a different city by the end of a long flight. A contribution rate is the same kind of setting. It was correct when you entered it, nothing announced when it stopped being correct, and the cost is not visible in any single paycheck.
This is a different problem from paying too much inside the plan, which is worth understanding separately. We covered that in the real cost of a set-it-and-forget-it 401(k).
The true-up gap, which is the part almost nobody explains
This is the most valuable thing in this post, so it gets the most room.
The IRS is explicit that matching contributions are made for an employee "who contributes elective deferrals to the 401(k) plan," and it gives the standard example of a plan contributing 50 cents for each dollar an employee chooses to defer (IRS 401(k) Resource Guide, Plan Participants, page last reviewed August 3, 2026). Read that as a timing statement, not just an amount statement. The match attaches to a deferral. No deferral in a given pay period means nothing for the match to attach to in that pay period.
Now add the annual limit. For 2026 the elective deferral limit is $24,500 (IRS Notice 2025-67 and news release IR-2025-111, both November 13, 2025). Say someone earning well into six figures sets a high deferral percentage in January because they want the money invested early in the year. They hit $24,500 in September. Payroll correctly stops their deferrals, because continuing would breach the 402(g) limit.
From September through December there are no deferrals. In a plan that calculates the match per pay period and has no true-up provision, there is nothing to match for those months, and that match is simply not paid. The person contributed the full annual maximum and still collected less match than a colleague who spread the same $24,500 evenly across all twenty-six pay periods. Same total contribution, same salary, less employer money, purely because of when the dollars went in.
A true-up provision is what fixes this. It is a plan feature that looks back at the full plan year after it closes, calculates what the match would have been on an annual basis, and deposits the difference. Plans that have one make front-loading harmless. Plans that do not have one make front-loading expensive.
Two things worth being clear about. A true-up is not required, so its absence is not an error by your employer. And it is not something you can elect. It either exists in the plan document or it does not, which is exactly why it is worth finding out before you change your deferral pattern rather than after.
Per-paycheck caps versus annual caps
Underneath the true-up question sits a smaller one that decides how much the true-up matters: whether your plan's match cap is applied per pay period or across the year.
A per-paycheck cap means the match is computed and closed out every pay period. Defer nothing this period and that period's match is gone. Defer far above the cap this period and the excess above the cap earns no additional match, because the cap already bound.
An annual cap means the plan looks at the year as a whole. Uneven contributions matter much less, because the calculation is not being closed out fourteen days at a time.
That distinction is the whole reason front-loading carries risk. Front-loading into an annual-cap plan with a true-up changes the timing of contributions but may not be appropriate for all investor. Front-loading into a per-paycheck-cap plan with no true-up trades employer money for a few extra months of market exposure. Whether that trade is reasonable depends on numbers specific to you, and it is a genuine tradeoff rather than a settled answer. It is worth working through with your advisor and your CPA rather than assuming either direction.
Match dollars vest on their own schedule
Three vesting clocks can run at once, and people routinely collapse them into one.
Your own deferrals are always fully yours. The IRS puts it plainly: "You must be fully (100%) vested in your elective deferrals." Employer money is different. The same guidance notes a plan "may require completion of a specific number of years of service for vesting in other employer or matching contributions," and gives the example of two years of service producing a 20% vested interest, with additional years raising the percentage.
If a pension also exists alongside your 401(k), that is a third and completely separate vesting schedule with its own rules and its own service requirements. Being fully vested in one tells you nothing about the others.
Why this matters for the match specifically: capturing more match and keeping more match are two different questions. Someone considering a job change inside a vesting window is weighing a real number against a career decision, which is its own kind of trap and one we wrote about separately in the golden handcuffs dilemma. The answer sits in your plan document rather than in any general rule.
And if you do leave before you are fully vested, the vested portion does not disappear. It becomes a balance at a former employer, which is a separate decision with its own tradeoffs. That one is covered in what to actually do with an old 401(k).
How to actually check this, in about fifteen minutes
You need two documents and one screen.
First, the Summary Plan Description. This is the plain-language booklet describing how your plan works. The Department of Labor's guide What You Should Know About Your Retirement Plan confirms the SPD is the document to check for your plan's own rules, and that it must be provided to you automatically and without charge. If you cannot find yours, request it from HR or the plan administrator. The IRS notes you can also request the full plan document in writing, though the administrator may charge a reasonable fee for that copy.
In the SPD, search for these terms rather than reading front to back:
- "matching" or "employer contribution" to find the formula itself. Read for the match rate and the compensation cap it applies to.
- "years of service" or "service" near that formula. This is where a milestone tier hides. Note whether the cap or the rate changes, and at what service year.
- "true-up" or "true up." If the term appears, read the surrounding paragraph. If it does not appear anywhere, that is meaningful information and worth confirming directly with the plan administrator rather than assuming absence.
- "payroll period" or "each pay period" in the matching section. This is usually how you learn the cap is applied per paycheck rather than annually.
- "vesting" or "vested." Find the schedule that applies to employer and matching contributions specifically, not the sentence about your own deferrals.
Second, your own service date. Usually on your HR profile or an annual benefits statement. You need it to know which tier you are actually in, which is not always the tier you were in when you set your rate.
Third, the contribution-rate screen in your plan's portal. Compare the percentage you are currently deferring against the cap you just found in the SPD for your current service tier. If your rate is below that cap, the difference is match you are eligible for and not receiving.
Two questions worth asking the plan administrator directly, because the SPD does not always answer them cleanly: whether the match is calculated per pay period or annually, and whether the plan has a true-up. Both are quick answers for someone who administers the plan daily.
One shortcut before you start from scratch. For several large San Diego-area employers we have already worked through the plan specifics, including how the match is structured and where the service tiers fall. If you work at one of these, start there rather than with a blank SPD: Northrop Grumman, Intuit, Southern California Edison, SDG&E, or Qualcomm. If your employer is not on that list, everything above still applies. The list is a shortcut, not a prerequisite.
The match is one input, not the decision
Capturing the full match is usually a sensible thing to look at, because it is employer money with a knowable trigger. It is still only one input.
The same contribution decision also involves your marginal tax bracket now versus what you expect in retirement, whether some or all of the deferral should be Roth rather than traditional, what else is competing for the same cash flow, and whether a pension sits alongside the plan and changes the shape of your retirement income. If a pension election is part of your picture, that decision has its own mechanics, which we walked through in lump sum versus annuity.
The weighting shifts if you are close to the end. Inside roughly five years either side of the day you stop working, match capture stops being a thirty-year compounding question and becomes one of eight decisions landing more or less at once, which is the subject of the ten-year retirement window.
None of those can be answered from a match formula alone, and no general rule can tell you what your contribution rate should be. What the formula can tell you is whether you are currently leaving employer money on the table, which is a narrower and much more answerable question.
If it would help to have someone read the plan documents alongside you, that is what BAS Financial's complimentary 401(k) fee review covers, and what a plan costs is part of the same conversation as what it pays you. Any fees associated with working together are discussed during that review.
Do you know whether your plan has a true-up?
Questions people actually ask
What is a 401(k) true-up?
A plan provision that recalculates the employer match on an annual basis after the plan year closes and deposits any shortfall. It matters most for people whose contributions are uneven across the year, including anyone who hits the annual deferral limit before December. True-ups are not required, so a plan may or may not have one.
Why did my 401(k) match stop partway through the year?
The most common reason is that your own deferrals stopped, usually because you reached the annual elective deferral limit, which is $24,500 for 2026 per IRS Notice 2025-67. Matching contributions are made based on elective deferrals, so when the deferrals stop in a per-pay-period calculation, the match generally stops with them. A plan with a true-up would make up the difference after year end.
Does maxing out my 401(k) early cost me the match?
It can, and it depends on two plan features rather than on the total you contribute. If your plan calculates the match per pay period and has no true-up provision, contributing the annual maximum by September can produce less total match than spreading the same amount across the full year. If the plan has a true-up, or applies its match cap annually, front-loading generally does not cost you match.
Does the 401(k) match change with years of service?
In some plans, yes. Match formulas can be tiered by years of service, or can change at a specific service milestone. The formula that applies to you is in your Summary Plan Description. A contribution rate set before a milestone will not adjust itself after the milestone passes.
Is the employer match vested immediately?
Not always. Your own elective deferrals are always 100% vested. Employer and matching contributions may be subject to a vesting schedule requiring a number of years of service, and the IRS gives the example of two years producing a 20% vested interest with additional years increasing it. Safe harbor plans are an exception and provide for immediate full vesting of those contributions.
Where do I find my employer match formula?
In the Summary Plan Description, which your plan must provide automatically and without charge per Department of Labor guidance. Search it for "matching," "employer contribution," and "years of service." You can also request the full plan document in writing from the plan administrator, who may charge a reasonable fee for the copy.
This article is provided for educational purposes only and should not be construed as investment, tax, legal, or retirement plan advice. Employer-sponsored retirement plans vary significantly, and plan provisions, matching formulas, vesting schedules, and contribution limits differ by employer. Individuals should review their Summary Plan Description and consult their plan administrator, tax professional, and financial advisor regarding their specific circumstances before making any retirement plan decisions.