No — you cannot put your whole paycheck into a 401k. Federal payroll taxes must come out of every check before any retirement deduction is applied, the IRS caps employee deferrals at $24,500 for 2026 no matter how much you earn, and most employer plans layer their own percentage ceiling on top of that.1Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 The real ceiling on what you can defer is always lower than your gross pay.
The 2026 IRS Cap
The federal limit on elective deferrals is set by 26 U.S.C. § 402(g) and adjusts for inflation.2Office of the Law Revision Counsel. 26 US Code 402 – Taxability of Beneficiary of Employees Trust – Section: Limitation on Exclusion for Elective Deferrals For 2026, the standard employee deferral limit is $24,500. That figure covers the combined total of traditional pre-tax and Roth 401k contributions across every employer plan you participate in during the year.
If you’re 50 or older by year-end, you can add $8,000 in catch-up contributions, bringing your personal maximum to $32,500. Workers aged 60, 61, 62, or 63 get an enhanced catch-up under SECURE 2.0 of $11,250, for a total of $35,750.1Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500 Once you turn 64, you drop back to the standard $8,000 catch-up.
Payroll Taxes Come Out First
Federal law requires your employer to withhold payroll taxes from every paycheck before any voluntary deduction. Social Security tax takes 6.2% of your wages up to $184,500 in 2026, and Medicare tax takes 1.45% of all wages with no cap.3Office of the Law Revision Counsel. 26 USC 3101 – Rate of Tax4Social Security Administration. 2026 Cost-of-Living Adjustment (COLA) Fact Sheet That’s 7.65% of your gross pay that must come off the top regardless of your deferral election.
If you earn over $200,000 ($250,000 for married filing jointly), an Additional Medicare Tax of 0.9% applies to wages above the threshold.3Office of the Law Revision Counsel. 26 USC 3101 – Rate of Tax 401k contributions reduce your taxable income for federal and state income tax, but they do not reduce the wages used to calculate FICA. Payroll will always pull those taxes before it touches your deferral.
Several states also impose mandatory disability insurance or paid family leave withholding that works the same way, and court-ordered wage garnishments for child support or alimony take priority over retirement contributions in the deduction sequence.
Your Plan Sets Its Own Ceiling
Beyond the IRS number, your specific plan almost certainly caps your deferral as a percentage of gross pay. Many employers set that ceiling at 50%, 75%, or 80%. The reason is practical: your paycheck still needs to cover premiums for health insurance, disability, and life coverage. If a maxed-out deferral leaves nothing for those deductions, you could lose coverage partway through the year.
Your plan’s Summary Plan Description spells out the percentage cap along with vesting schedules and matching formulas. HR or the plan administrator must provide it on request. Confirm the plan’s cap before setting an aggressive rate; a percentage the payroll system rejects can take weeks to sort out.
Extra Rules for High Earners
If you earned $160,000 or more from your employer in the prior year, the IRS classifies you as a highly compensated employee.5Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions That triggers the Actual Deferral Percentage (ADP) test, which compares the average deferral rate of HCEs to everyone else in the plan. If HCEs contribute at much higher rates, the plan fails.6Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
When a plan fails, the employer has two and a half months after the plan year ends to refund the excess to HCEs or make additional contributions for non-HCEs. Refunded amounts are taxable in the year you receive them.6Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests Some employers avoid the problem by adopting a safe harbor plan, which is exempt from ADP testing. Ask your administrator which structure your plan uses; it directly affects how much you can keep in.
There’s a second rule aimed at high earners. Starting in 2026, SECURE 2.0 requires that if you earned more than $145,000 from your current employer in the prior year, any catch-up contributions must go into a Roth (after-tax) account rather than a traditional pre-tax account.7Federal Register. Catch-Up Contributions This doesn’t change how much you can contribute; it changes the tax treatment. If your plan doesn’t yet offer a Roth option and you’re above the threshold, you may temporarily lose access to catch-ups until your employer adds one.
Front-Loading Can Cost You the Match
Even if your plan lets you push a big share of each paycheck into the 401k, hitting the annual limit early in the year can cost you thousands in employer matching contributions. Most employers calculate the match per paycheck, for example 50% of the first 6% you contribute each pay period. Once your deferrals stop because you’ve hit the IRS ceiling, there’s nothing left for the employer to match.
The gap can be dramatic. An employee earning $345,000 who defers 25% per paycheck hits the annual cap around April and collects a match for only those early months. Spread evenly across all pay periods, the same employee would receive roughly three times more in matching dollars over the year.
Some plans include a “true-up” that reconciles the total match at year-end and pays the difference. Not all do. Before choosing an aggressive rate, find out whether yours has one. If it doesn’t, calibrate the percentage so contributions last through December.
What Happens If You Over-Contribute
Going over the IRS annual limit creates an excess deferral, and the consequences are steep if you don’t fix it fast. You have until April 15 of the following year to withdraw the excess along with any earnings it produced. That deadline does not move even if you extend your tax return.8Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
Miss the April 15 window and the excess is taxed twice: once in the year you contributed it and again when you eventually withdraw it from the plan.8Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan If you’re under 59½, the late corrective distribution can also trigger the 10% early withdrawal penalty.9Internal Revenue Service. 401(k) Plan Fix-It Guide – Elective Deferrals Werent Limited to the Amounts Under IRC Section 402(g) This most often happens when someone participates in two employer plans during the same year; each plan’s payroll system tracks only its own contributions, so neither knows you’ve crossed the combined ceiling. Keep a running total yourself and tell the administrator before you cross it.