How a 401(k) Actually Works

The 401(k) plan is the default retirement vehicle for roughly 70 million American workers, holding more than $8 trillion in assets. Despite its dominance, very few participants understand how the plan they rely on actually operates. Here is the mechanic.

The basic structure

A 401(k) is a profit-sharing plan qualified under IRC §401(a) that includes a "cash or deferred arrangement" under IRC §401(k). The CODA lets an employee choose between receiving cash wages or having that amount contributed to the plan on a pre-tax (or Roth) basis. Until the participant elects deferral, the money is part of taxable wages. Once deferred, it is excluded from current Form W-2 Box 1 income (or included for Roth) and grows tax-deferred until distribution.

The three contribution sources

• Employee elective deferrals. Capped at $23,500 for 2025 under IRC §402(g), plus a $7,500 age-50 catch-up under §414(v) and the SECURE 2.0 super catch-up of $11,250 for ages 60–63.
• Employer contributions. Match, non-elective, or profit-sharing. No statutory percentage minimum; the §415(c) ceiling is the only overall cap.
• After-tax employee contributions. Allowed if the plan document permits. These fill the gap between the elective deferral and the $70,000 §415(c) annual additions limit, and they are the basis of the mega backdoor Roth strategy.

The nondiscrimination tests

Without testing constraints, a 401(k) would tend to concentrate benefits in highly compensated employees, who can afford to defer more. IRC §401(k)(3) imposes the Actual Deferral Percentage (ADP) test and §401(m) imposes the parallel Actual Contribution Percentage (ACP) test on match. The HCE average deferral rate cannot exceed the NHCE average by more than (a) 1.25 times, or (b) the lesser of two percentage points or twice the NHCE rate.

If the plan fails ADP/ACP, excess HCE deferrals are returned (and taxed) by March 15 of the following year. For plans with low NHCE participation, this can mean executives get a partial refund of their deferral every year.

The safe harbor escape

IRC §401(k)(12) and §401(k)(13) provide a "safe harbor" exemption from ADP/ACP testing. The plan agrees to one of two prescribed contribution formulas — generally a 100% match on the first 3% plus 50% on the next 2% (4% total at full deferral), or a 3% non-elective contribution to all eligible employees — and the testing requirement disappears. Most well-run small and mid-size plans are safe harbor for this reason. SECURE 2.0 §103 expanded auto-enrollment safe harbor terms.

Vesting

Employee deferrals are 100% immediately vested at all times. Employer contributions vest on a schedule chosen by the plan — either 3-year cliff (0% before 3 years, 100% after) or 6-year graded (20% per year starting in year 2), as constrained by IRC §411 and ERISA §203. Safe harbor non-elective and matching contributions are immediately 100% vested by statute.

Distributions and the 10% penalty

Withdrawals before age 59½ trigger a 10% additional tax under IRC §72(t), with exceptions including separation from service in or after the year you turn 55 (the "Rule of 55"), substantially equal periodic payments, disability, and qualified domestic relations orders. Hardship withdrawals are not exempt from the 10% tax — only from the loan-or-hardship-required liquidity restriction.

Common mistakes

• Stopping at the match. The full $23,500 deferral (plus catch-ups) usually beats the after-tax alternatives even after the match cap. Contribute to the full §402(g) limit if cash flow permits.
• Forgetting the §402(g) cross-employer cap. The $23,500 limit applies per person, not per plan. Two W-2 jobs with two 401(k)s require you to coordinate.
• Ignoring the after-tax bucket. If the plan permits after-tax contributions and in-plan Roth conversions (or in-service rollovers), the mega backdoor Roth unlocks up to $40,000+ of additional Roth savings annually.
• Defaulting to the target-date fund without checking expense ratios. Some target-date funds cost 0.07% per year; others cost 0.85%. Over 30 years that difference compounds to roughly 22% of ending wealth.

Sources

• Internal Revenue Code §401(k), cash or deferred arrangements (Cornell LII): law.cornell.edu/uscode/text/26/401
• Internal Revenue Code §402(g), limit on elective deferrals: law.cornell.edu/uscode/text/26/402
• IRS, "401(k) Plan Overview": irs.gov/retirement-plans/401k-plan-overview
• U.S. Department of Labor, "Choosing a Retirement Solution for Your Small Business": dol.gov/ebsa/publications
• Treasury Regulation §1.401(k)-1, qualified cash or deferred arrangements: law.cornell.edu/cfr/text/26/1.401(k)-1
• SECURE 2.0 Act of 2022, §103 (auto-enrollment safe harbor), Pub. L. 117-328: congress.gov/bill/117th-congress/house-bill/2617

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