Can You Make a Lump Sum 401(k) Contribution? Limits and Options

A true lump sum 401(k) contribution — writing a personal check from your savings account and handing it to your plan — isn’t allowed. Employee contributions have to move through your employer’s payroll system. What you can do is defer a large share of a single paycheck or bonus so the deposit lands in one shot, and employer profit-sharing contributions can arrive as a genuine one-time deposit. For 2026, the employee elective deferral limit is $24,500 and the combined cap from all sources is $72,000, with more room if you qualify for catch-up contributions.¹

Why a Personal Check Doesn’t Work

Your elective deferrals have to be withheld from compensation your employer would otherwise pay you. You sign a salary reduction agreement, your employer withholds the elected amount from each paycheck or bonus, and the plan administrator deposits it. That pipeline exists so the money gets reported correctly on your W-2 and deposited on time under Department of Labor rules.²

The practical consequence: every dollar you defer has to originate from wages, salary, commissions, or bonuses. You can’t move funds from a savings or brokerage account into your 401(k) as a contribution. This is the single biggest constraint people run into when they want to make one large deposit.

Deferring a Bonus as a Workaround

The closest an employee can get is electing to defer a high percentage of a single large payment. Many plans allow deferrals of up to 100% of a bonus. A $30,000 bonus with a 100% deferral election, for someone who hasn’t yet contributed anything else in 2026, would drop $24,500 into the plan in one payroll cycle, with the remaining $5,500 paid out as taxable wages.¹

Front-loading works, but watch the match. If your employer calculates the matching contribution on each paycheck, hitting the $24,500 ceiling early means you stop contributing and the match stops with you. Someone earning $150,000 with a 4% match who maxes out in February instead of December can leave thousands of employer dollars on the table.

Some plans include a true-up provision that reconciles matching contributions at year-end. The employer compares what you actually received in matching to what you would have received based on full-year compensation and deferrals, then makes up the difference. Before front-loading, check whether your plan has one. If it doesn’t, spreading deferrals across the year protects the match, and deferring a large year-end bonus is often the cleanest way to concentrate the contribution without cutting off matching on earlier paychecks.

One firm deadline: all employee elective deferrals for a given plan year have to be withheld from compensation paid by December 31 of that year. You can’t make a deferral election in January for the prior year.³

Employer Profit-Sharing as a Real Lump Sum

Genuine lump sum deposits usually come from the employer side. Discretionary profit-sharing contributions give the employer full flexibility over timing and amount, and a single large year-end deposit — or one made months into the following year — is entirely normal. The plan document has to authorize profit-sharing and specify how the money gets allocated, but the employer decides each year how much, if anything, to contribute.

The big timing difference: an employer can designate a contribution for the prior plan year as late as the due date of its federal income tax return, including extensions.⁴ For a C-corporation that means April 15 of the following year, extendable to October 15. For S-corporations and partnerships, March 15, extendable to September 15.⁵ The breathing room lets the employer’s tax advisor finalize profits and pick a contribution amount that produces the intended deduction. The contribution has to be formally designated in writing as an addition for the prior plan year.

Vesting Can Take a Bite

A lump sum profit-sharing deposit in your account isn’t automatically yours. Employer contributions are typically subject to a vesting schedule; your own elective deferrals are always 100% vested immediately.⁶

Cliff vesting means you own 0% until you hit three years of service, then jump to 100%. Graded vesting increases your ownership each year, typically 20% per year starting in year two, reaching 100% after six years.⁶ Leave the company before you’re vested and you forfeit the unvested portion. Ask your plan administrator for your schedule before you count a large profit-sharing deposit as part of your balance.

Solo 401(k): The Most Flexible Route

If you’re self-employed with a solo 401(k), you wear both hats. On the employee side you can elect to defer up to $24,500 for 2026, plus catch-up if eligible. On the employer side you can contribute up to 25% of your net self-employment compensation as profit-sharing.⁷

The timing advantage is real. You can make the deferral election by December 31 of the plan year and then deposit both the deferral and the employer profit-sharing contribution as late as your tax return filing deadline, including extensions. A sole proprietor on extension could fund the entire prior-year contribution as late as October 15 — a true lump sum for the previous year.⁷

Calculating the employer portion takes some algebra because the contribution reduces your earned income. The IRS publishes a specific formula that accounts for the deduction of one-half of self-employment tax and the plan contribution at the same time.⁸ The effective rate for a 25% plan works out to roughly 20% of net self-employment income before the plan deduction. Publication 560 has the worksheets; a tax professional will nail down the exact figure.

The Two Caps Any Lump Sum Has to Fit Inside

Every contribution — one-time or spread out — has to stay within two separate IRS ceilings.

Elective Deferral Limit

The 2026 limit on what you can defer from your own pay across all 401(k), 403(b), and governmental 457 plans combined is $24,500.¹ The cap is per person, not per plan. Contribute $15,000 at one employer and switch jobs mid-year and you can only defer $9,500 at the next one.³

Catch-up contributions raise the ceiling for older participants:

Age 50 and older: an additional $8,000, for a total of $32,500.
Ages 60 through 63: a higher catch-up under SECURE 2.0 of $11,250, for a total of $35,750.¹

Starting in 2026, participants who earned more than $150,000 in wages from the sponsoring employer during 2025 have to make their catch-up contributions on a Roth basis. Below that threshold, pre-tax or Roth remains your choice.

Total Annual Addition Limit

The second cap, under Section 415(c), covers everything landing in your account for the year: your deferrals, employer matching, and employer profit-sharing. For 2026, it’s the lesser of 100% of your compensation or $72,000.⁹ Catch-ups don’t count toward this cap, so with catch-ups the real ceiling is $80,000 at age 50 and older, or $83,250 at ages 60 through 63.¹

Defer the full $24,500 and the employer has $47,500 of room left for matching and profit-sharing before hitting the ceiling. Large lump sum profit-sharing deposits are where plans most often bump into this limit, so the employer’s tax advisor needs to run the numbers before writing the check.

After-Tax Contributions Open More Room

Some plan documents allow voluntary after-tax employee contributions — money that isn’t pre-tax or Roth, just additional savings deposited after you’ve already paid income tax on it. These count toward the $72,000 annual addition limit but not toward the $24,500 elective deferral limit.¹⁰

That creates a real opening. Defer $24,500, get $15,000 in matching, and you’ve used $39,500 of the $72,000 cap. If your plan allows after-tax contributions, you could add up to another $32,500 to reach the ceiling, all through payroll. Many participants then convert those after-tax dollars to a Roth account, a strategy commonly called the mega backdoor Roth. Not every plan permits after-tax contributions or in-plan Roth conversions, so confirm with your plan document or benefits administrator.

Rollovers Are Not Contributions

Worth flagging because it comes up: rolling money in from an old 401(k) or IRA doesn’t count as a contribution and doesn’t touch either limit.¹¹ A rollover can look like a lump sum arriving in your account, but legally it’s a continuation of existing retirement savings, not new money going in. If your goal is to consolidate old accounts, a rollover does that without eating into your 2026 contribution room.

If You Go Over the Limits

Push past either ceiling and the plan has to fix it. The correction depends on which limit was breached.

Excess Elective Deferrals

If your deferrals across all plans exceed $24,500 (or $32,500/$35,750 with catch-ups), the excess has to be distributed back to you by April 15 of the following year. That deadline is firm; extending your personal tax return doesn’t move it.¹²

Corrected on time, you include the excess in taxable income for the year you deferred it. Any earnings on the excess are taxable in the year they’re distributed.¹³ Miss the April 15 deadline and the excess gets taxed in the year you deferred it and taxed again when the plan eventually distributes it — double taxation on the same dollars. The excess may be stuck in the plan until a distribution is otherwise allowed under the plan terms, potentially years later.¹²

Excess Annual Additions

When total contributions from all sources exceed the $72,000 ceiling, the correction usually targets the employer’s contribution. The plan identifies which employer dollars caused the breach and either forfeits that amount to an unallocated suspense account (reducing future employer contributions) or distributes the excess if it came from after-tax employee contributions. Both corrective distributions and excess annual addition refunds are exempt from the 10% early withdrawal penalty.¹⁴

These problems show up most often when a large employer profit-sharing deposit lands late in the extended filing period without accounting for the deferrals and matching that already accumulated during the year. A conversation between the plan administrator and the employer’s tax advisor before the deposit is the cheapest fix.

1
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
2
Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026)
3
Office of the Law Revision Counsel. 26 U.S.C. 402 – Taxability of Beneficiary of Employees’ Trust
4
Internal Revenue Service. 401(k) Plan Fix-It Guide – You Haven’t Timely Deposited Employee Elective Deferrals – Section: Timing of Other Contributions
5
Internal Revenue Service. Starting or Ending a Business 3
6
Internal Revenue Service. Retirement Topics – Vesting
7
Internal Revenue Service. Publication 560 (2025), Retirement Plans for Small Business
8
Internal Revenue Service. Calculation of Plan Compensation for Sole Proprietorships
9
Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living Notice 2025-67
10
Office of the Law Revision Counsel. 26 U.S.C. 415 – Limitations on Benefits and Contribution Under Qualified Plans
11
Office of the Law Revision Counsel. 26 U.S. Code 415 – Limitations on Benefits and Contribution Under Qualified Plans
12
Internal Revenue Service. Consequences to a Participant Who Makes Excess Deferrals to a 401(k) Plan
13
Internal Revenue Service. Instructions for Forms 1099-R and 5498 (2025) – Section: Corrective Distributions
14
Internal Revenue Service. Retirement Topics – Exceptions to Tax on Early Distributions