How a 401(k) Works

A 401(k) is a U.S. workplace retirement plan. You contribute from pay, your employer may add money, and the account is invested in the plan’s lineup, often mutual funds or similar vehicles. The structure is tax-advantaged saving, not a guaranteed account value. Balances can rise or fall with markets. Withdrawal rules are stricter than a taxable brokerage account. Plan documents control. Figures below use 2026 IRS limits and should be confirmed for later years.

What a 401(k) is

You elect a percentage of eligible pay, or sometimes a dollar amount per paycheck. Traditional (pre-tax) deferrals reduce current taxable wages for federal income tax; later withdrawals are generally ordinary income. Designated Roth deferrals are after-tax going in; qualified withdrawals of contributions and earnings can be tax-free if rules are met. Many plans offer both.

The plan is held in a trust. You choose among the investment lineup, or a default such as a target-date fund. Newer plans may auto-enroll and auto-escalate deferrals, with a right to opt out or change the rate. Auto-enrollment is a participation tool, not proof that the default rate is enough. A 401(k) is not a pension that promises a monthly check for life. Future value is not guaranteed by the employer or the IRS.

Contributions and annual limits

The IRS caps elective deferrals. For 2026 the limit for 401(k), 403(b), governmental 457(b), and the Thrift Savings Plan is $24,500, or 100% of compensation if lower. Age 50 or older catch-up, if the plan allows it, is $8,000, for $32,500 total. Employees who turn 60–63 in 2026 may use a higher $11,250 catch-up if the plan permits.

SECURE 2.0 changed catch-up treatment for higher earners. For 2026, if 2025 FICA wages were at or above $150,000 (the IRS threshold as adjusted), catch-up deferrals generally must be designated Roth if the plan offers Roth. Employer match, profit sharing, or nonelective amounts sit outside your elective-deferral cap, but a combined annual-additions limit applies: $72,000 for 2026, not counting catch-up. Deferrals to a prior employer’s plan in the same calendar year still count toward the elective limit.

Employer match and vesting

A match is extra compensation deposited when you contribute, according to a formula. A common illustrative formula is 50% of your deferrals up to 6% of pay, or 100% of deferrals up to 3% of pay. Those are examples of plan design, not averages you should assume. Some employers match Roth and pre-tax deferrals the same way; the match itself is often placed in a pre-tax source even if your deferral was Roth, depending on the plan.

Vesting determines when employer money becomes yours if you leave. Your own deferrals are always yours. Employer money may vest on a cliff (for example, 100% after three years of service) or a graded schedule (a rising percentage each year). If you leave before you are fully vested, you typically forfeit the unvested portion. Vesting is why “free money” is not always fully free on day one.

Missing a match you could have received by contributing a little more is a common leak. If you contribute 2% and the formula matches up to 6%, you left match on the table. Stretching to the match cap is often a first savings target after high-interest revolving debt is under control, because the match is an immediate addition to the account under the plan’s formula. It is still invested and can lose value.

How to read a match formula

Ask payroll or the plan site three questions: what percentage of your deferral is matched, up to what percent of pay, and whether compensation for the formula includes bonuses. Then compute the dollar match at your current deferral and at the cap. Use your actual pay, not a national anecdote.

Traditional vs Roth 401(k)

The traditional versus Roth choice is about tax timing. Pre-tax deferrals can lower this year’s taxable income. Roth deferrals do not. In retirement, qualified Roth distributions can be tax-free, while traditional distributions are generally taxable. Which is better depends on your current tax rate, your expected future tax rate, and how much room you have in each tax bracket—none of which can be known with certainty.

Many people split deferrals, especially if they are unsure. A split is a hedge, not a formula that maximizes anything. Required minimum distribution rules have also changed: designated Roth 401(k) accounts are no longer subject to lifetime RMDs for the original owner in the same way traditional 401(k) balances are, following SECURE 2.0. Traditional 401(k) balances still generally face RMDs after the applicable age unless rolled to a Roth IRA under conversion rules, which is a taxable event. Confirm current RMD ages and plan procedures; Congress has changed them more than once.

Investing inside the plan

A 401(k) is not itself an investment. The funds inside it are. Typical menus include target-date funds, stock and bond funds, and a capital-preservation option. Expense ratios are an ongoing cost, not a forecast of which fund will win.

Asset allocation usually matters more than last year’s winner. A target-date glide path is a design choice, not a promise that the date in the name is when you should retire or that the balance will suffice. Markets do not pay a fixed compound rate. A calculator return of 6% or 7% is an illustration. Run the same contributions at a lower assumed return to see sensitivity, not to pick a “correct” forecast.

• Contribute at least enough to capture the full match if you can do so without skipping rent, food, or required debt minimums.
• Know whether your deferrals are pre-tax, Roth, or split, and why you chose that mix.
• Read the fee column on the fund list; small percentage differences compound over decades as a cost.
• Revisit the deferral rate after a raise so the savings rate does not silently fall as a share of pay.
• Keep beneficiary designations current; they can override a will for plan assets.

Account growth in a 401(k) is not guaranteed. Matching contributions can be unvested. Tax rules and limits change. Use plan documents and current IRS figures, not a single assumed return, when you make decisions.

A labeled contribution example

This example is hypothetical and uses 2026 elective-deferral math. It is not a recommended savings rate.

Suppose Jordan earns $72,000 in eligible pay and the plan matches 100% of deferrals up to 4% of pay. A 4% deferral is $2,880 a year. The employer would add another $2,880 if Jordan defers at least 4% all year, for $5,760 from pay plus match before investment change. That formula is an example, not a typical national match.

If Jordan instead defers 10%, employee deferrals are $7,200, still under the $24,500 limit, and the match stays $2,880 because the cap was 4% of pay. None of these flows imply a future balance. A negative year can leave the account lower even while contributions continue. A retirement calculator can project a balance under an assumed return; try more than one rate, including a lower one.

Loans, hardship, and leaving a job

Some plans allow loans, often up to the lesser of $50,000 or 50% of the vested balance, repaid from payroll. A loan is not extra income. Money taken out is not invested during the loan, and leaving the job can make the balance due quickly, with unpaid amounts treated as a distribution. Hardship withdrawals, if allowed, are for specific IRS-defined needs and may be taxable plus a 10% additional tax before 59½, with exceptions.

Leaving a job typically means keep the account if allowed, roll to an IRA or a new plan, or cash out. Trustee-to-trustee rollovers help avoid withholding surprises. A cash-out can trigger taxes, possible additional tax, and lost time in the market—without any promised compounding rate. Age 59½ is the usual penalty-free line, with exceptions such as the “rule of 55” for certain separations from that employer’s plan. Qualified Roth distributions also need a five-year clock. Read the plan’s distribution booklet before you assume a withdrawal is tax-free.

Limits, taxes, and what calculators assume

This guide cannot tell you whether to choose Roth or pre-tax, how much you will need, or what funds will return. It cannot override eligibility, waiting periods, or the investment menu. After-tax “mega” features and in-plan Roth conversions are plan-specific.

The retirement and investment calculators on this site use constant-return assumptions you type in. They do not model sequence-of-returns risk or tax-bracket changes unless you adjust inputs. A 401(k) is a workplace tool because of payroll discipline, possible matching, and tax treatment. It is still a risky investment account in a legal wrapper. Treat participation as a savings process you control, and treat the balance as a variable you do not.