What Is a 401(k)? Guide to Retirement Savings

A traditional 401(k) is an employer-sponsored defined contribution plan that lets you set aside a portion of your paycheck before federal income taxes are calculated. Your contributions are deducted directly from your gross pay, reducing your taxable income for the year. If you earn $80,000 and contribute $10,000 to your 401(k), you are only taxed on $70,000 of income — an immediate tax benefit that can amount to thousands of dollars in savings depending on your marginal tax bracket.

Once inside the account, your money grows tax-deferred. Dividends, interest, and capital gains are not taxed as long as the funds remain in the plan. This tax-deferred compounding is one of the most powerful wealth-building mechanisms available to individual investors. Consider the math: a $10,000 annual contribution growing at 7% per year accumulates to approximately $1.01 million over 30 years. In a taxable brokerage account with a 22% marginal rate on annual gains, the same contributions would grow to roughly $780,000 — a difference of more than $230,000 purely from the tax deferral.

You pay ordinary income tax on withdrawals in retirement, when many workers are in a lower tax bracket than during their peak earning years. This "tax arbitrage" — contributing in high-earning years and withdrawing in lower-income retirement years — is the fundamental advantage of the traditional 401(k) structure. For more on this topic, visit our <a href="/retirement/">Retirement</a> hub.

It is worth noting that these limits apply only to employee deferrals. Employer contributions — matching and profit-sharing — can push the total annual addition to a 401(k) up to $70,000 for workers under 50 and $77,500 for those 50 and older. Highly compensated employees (those earning more than $155,000 in 2026) may face additional restrictions under non-discrimination testing rules.

Employer matching contributions are one of the most valuable benefits in any compensation package. A common match formula is 50 cents for every dollar you contribute, up to 6% of your salary — effectively a 3% bonus on top of your base pay. Some employers are more generous, matching dollar-for-dollar up to 4% or 6%. According to Vanguard's How America Saves 2025 report, the average employer match rate was 4.5% of pay.

Here is a concrete example: if you earn $75,000 and your employer matches 100% of contributions up to 4% of salary, contributing at least $3,000 (4% of $75,000) earns you an additional $3,000 from your employer. That is an immediate 100% return on your first $3,000 of contributions — a guaranteed return that no market investment can replicate. Failing to contribute enough to capture the full match is, in practical terms, leaving part of your compensation on the table.

Most employer matches are subject to a vesting schedule, which determines when you gain full ownership of the employer's contributions. A common structure is graded vesting over three to six years: you might own 20% after one year, 40% after two, and so on until you are fully vested. If you leave your job before becoming fully vested, you forfeit the unvested portion of employer contributions. Your own contributions, however, are always 100% vested immediately.

Under SECURE 2.0, employers can now also make matching contributions to employees' Roth 401(k) accounts — previously, all employer matches had to go into the pre-tax portion of the plan regardless of the employee's contribution election.

The 401(k) is designed for retirement, and the IRS enforces that purpose with strict withdrawal rules. Distributions taken before age 59½ are generally subject to a 10% early withdrawal penalty on top of ordinary income taxes. There are limited exceptions: a hardship withdrawal may be available for immediate and heavy financial needs such as medical expenses, prevention of eviction, or funeral costs — but even hardship withdrawals are taxed as income.

Required Minimum Distributions (RMDs) are another critical rule. Under SECURE 2.0, the RMD starting age has been pushed to 73 for those born between 1951 and 1959, and to 75 for those born in 1960 or later. This gives your money more time to grow tax-deferred, but you must begin withdrawing minimum amounts by April 1 of the year following the year you reach the applicable age. Failure to take RMDs triggers a steep penalty — 25% of the amount that should have been withdrawn (reduced from 50% under SECURE 2.0). Roth 401(k) accounts are now exempt from RMDs starting in 2024, aligning them with <a href="/posts/2026-02-28/deep-dive-irs-publication-590-explained-the-complete-guide-to-retirement-account-rules-for-2026">Roth IRA rules</a>.

When you leave an employer, you have several options for your 401(k) balance: leave it in the former employer's plan (if permitted), roll it over to your new employer's 401(k), roll it into an IRA, or take a cash distribution (which triggers taxes and potentially the 10% penalty). A direct rollover — where the funds transfer institution-to-institution without you receiving a check — avoids the mandatory 20% withholding that applies to indirect rollovers. For most people, rolling into an IRA provides the widest range of investment options and the most control over fees.