401(k), explained: free money, low fees, and the rules that bite

If you work for an American employer, the single most consequential investment account you will ever own was probably opened for you by payroll paperwork you barely read. The 401(k) — named after a section of the tax code, which is exactly as romantic as it sounds — is the backbone of US retirement saving, and its mechanics reward understanding with genuinely free money.

How it works

A 401(k) takes a slice of your salary before it reaches your bank account and invests it in funds you choose from the plan's menu. Because contributions come out pre-tax (in a traditional 401(k)), your taxable income drops today; the money then grows for decades without annual tax; you pay income tax on withdrawals in retirement. Annual contribution limits are set by the IRS and adjusted most years for inflation — check the current figure rather than trusting any article, including this one. Under the SECURE 2.0 legislation, mandatory catch-up provisions and Roth treatment of certain catch-ups have reshaped the rules for older savers; the direction of travel is more flexibility, more complexity.

The match is the whole game

Many employers match a portion of what you contribute — a common shape is 50 cents on the dollar up to 6% of salary. That is a 50% immediate return on the matched money, guaranteed, before any market movement. No investment on earth reliably beats it, which makes the first rule of the 401(k) absolute: contribute at least enough to capture the full match. Not doing so is declining a raise. If money is genuinely too tight this month, the emergency fund guide shows how to build the floor that makes the match reachable.

Choosing the investments inside

A 401(k) is a container, not an investment — the returns come from what you hold inside it. Plan menus typically run from target-date funds (which automatically shift from stocks to bonds as your retirement year approaches) to a shelf of index and active funds. For most savers the honest answer is the boring one: a low-cost broad index fund, or a target-date fund if you want one decision made for you. Fees compound in reverse — the index funds guide explains why a fraction of a percent matters more over thirty years than any fund-picker's promise. One menu-specific check: the expense ratios on your plan's options, because a plan full of 1% funds deserves a conversation with HR about adding cheaper ones.

Roth or traditional, and the withdrawal rules

Many plans offer a Roth 401(k) option: contributions are taxed now, qualified withdrawals are tax-free later. The choice is a bet on your tax bracket trajectory — traditional usually wins when you expect to be in a lower bracket in retirement; Roth wins when you expect the opposite (early-career earners often fit). Rules that keep people honest: withdrawals before 59½ generally face income tax plus a 10% penalty, with limited exceptions (hardship, first home, the "rule of 55" for separated service, substantially equal payments). Required minimum distributions eventually force taxable withdrawals from traditional accounts on the IRS's schedule. The marginal-tax guide explains the bracket logic that decides Roth versus traditional.

Outside the US the machinery differs — the UK's workplace pensions and the retirement basics guide cover the international map — but the three principles travel perfectly: capture every match, keep the fees near zero, and let the compounding do the decades of work.

Sources and further reading

Links were reviewed 2026-09-25. Regulatory permissions, firm status and product terms can change; use the current official register before acting.

1. SEC Investor.gov — 401(k) plan
2. SEC Investor.gov — individual retirement arrangement (IRA)

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