How a 401(k) Works

A 401(k) is the largest financial asset most Americans will ever own, and it is also the one people understand least — because it arrives by default, funded by a payroll deduction they did not choose, invested in a fund they did not select. It works anyway, which is part of the design. But a modest amount of attention to four decisions — how much, which type, which funds, and when to leave it alone — changes the outcome by an enormous margin over a career.

This guide covers what happens inside the account and where the money comes from. To project a balance from your own contributions, salary, and employer match, use the 401(k) calculator.

What a 401(k) is

A 401(k) is an employer-sponsored retirement account named after the section of the tax code that authorises it. You elect a percentage of your pay to be diverted into it before it reaches your bank account, your employer may add money of its own, and the combined balance is invested and grows without annual taxation.

Three parties are involved and it helps to keep them distinct. Your employer sponsors the plan and sets its rules — the match formula, the vesting schedule, whether loans are allowed. A recordkeeper administers the accounts. Fund managers run the investments you choose from a menu the plan selects. The money is held in trust for you, separate from your employer's assets, which is why a company failing does not put your balance at risk.

Where the money comes from

Your contributions are the foundation. You elect a percentage of pay, and it is deducted each period automatically — which is dollar cost averaging by construction, as covered in dollar cost averaging explained.

Employer contributions come in two shapes. A match is conditional on your own contribution: a common formula is 50% of what you contribute up to 6% of salary, meaning someone earning $100,000 who contributes 6% receives $3,000. A non-elective contribution is paid regardless of what you do, though it is less common.

Investment growth is where the balance eventually comes from. Over a long career the growth typically exceeds everything both you and your employer contributed — the mechanism explained in compound interest explained.

The employer match is the whole point

If there is one number to act on, it is the match threshold.

Consider someone earning $100,000 with a 50%-up-to-6% match. Contributing 6% means $6,000 of their own money and $3,000 from the employer — an immediate 50% return before any market movement. Contributing 3% means $1,500 of match, leaving $1,500 unclaimed. Contributing nothing forfeits $3,000 a year.

Over a 35-year career, that forfeited match plus its compounded growth typically runs into six figures. There is no investment available to an ordinary employee that reliably matches a guaranteed 50% return, which is why "contribute to the match" precedes almost every other piece of retirement advice — including paying down moderate-rate debt.

Contribution limits

The IRS caps how much you may defer each year, adjusts the figure for inflation, and allows larger amounts from age 50 and a higher amount still in a specific age band under SECURE 2.0. Employer contributions do not count against your own deferral limit, though a separate combined cap applies.

The 401(k) calculator applies the current limits automatically, reading them from the same dated registry that feeds every tax figure on the site — the effective year and source are recorded on the data sources page.

One practical consequence of the limit deserves attention. If you front-load contributions and hit the annual cap in, say, September, some plans stop matching for the remaining months because there is no contribution to match. Plans with "true-up" provisions correct this at year end; many do not. Spreading contributions evenly across the year avoids the problem entirely.

Traditional or Roth

Most plans now offer both.

Traditional contributions come out before income tax, reducing this year's taxable income at your marginal rate. The balance grows untaxed and withdrawals in retirement are taxed as ordinary income.

Roth contributions come out of already-taxed pay, so there is no deduction now. Qualified withdrawals in retirement are entirely tax-free.

The comparison mirrors the IRA version discussed in Roth IRA vs traditional IRA: traditional wins if your tax rate is higher now, Roth wins if it will be higher later. Since nobody knows future rates, holding some of each is a defensible hedge.

Note that neither choice affects FICA. Payroll tax applies to gross wages regardless of what you contribute, as explained in what is FICA.

Vesting: when the match becomes yours

Your own contributions are yours immediately and always. Employer money may not be.

Cliff vesting grants the full employer balance after a set period — nothing before, everything after. Graded vesting grants it in increments, perhaps 20% a year over five years.

Leaving before you are fully vested forfeits the unvested portion. This is worth checking before resigning, particularly if a vesting date is months away, because the sum involved can be substantial.

Fees, quietly

Two costs apply: plan administration charges and the expense ratios of the funds you hold. Both reduce your return every year, and because returns compound, so does the drag.

A fund charging one percentage point more than an alternative does not cost you 1% — it costs you 1% compounded across the entire holding period, which over decades can consume a meaningful share of the final balance. Reviewing the fund menu for lower-cost options is among the highest-value hours available to any participant.

Getting money out

From 59½, withdrawals are permitted, taxed as income in a traditional account and tax-free from a qualified Roth.

Before 59½, expect income tax plus a 10% penalty, with exceptions including separation from service in or after the year you turn 55.

Required minimum distributions eventually force withdrawals from traditional balances at the age set in current law, whether or not you need the money.

When changing jobs, four options exist: leave it, roll it to the new plan, roll it to an IRA, or cash out. The first three preserve tax deferral; the fourth triggers tax and penalty and permanently removes the balance from compounding. It is consistently the most expensive decision available.

Common mistakes

Contributing below the match threshold. Leaves guaranteed compensation unclaimed.

Never changing the auto-enrolment default. Default rates are frequently set at 3% or so — enough to start a habit, not enough to fund a retirement.

Cashing out when changing jobs. Tax, penalty, and the permanent loss of decades of compounding on that balance.

Front-loading into the annual cap without a true-up. Can silently stop the match for the rest of the year.

Ignoring the fund menu entirely. Default options are not always the lowest cost, and the difference compounds.

Timing the market inside the account. Moving to cash during a downturn converts a paper loss into a realised one and usually misses the recovery.

Overlooking vesting when resigning. Leaving weeks before a vesting date can cost thousands.

Practical tips

Set the contribution rate to at least the full match today, then raise it by one percentage point with every pay increase — the increase is never missed because it was never in your take-home.

Check whether your plan offers automatic escalation, which does this for you.

Review the fund expense ratios once. It is a one-off task with a permanent benefit.

Keep beneficiary designations current after marriage, divorce, or a birth. They override your will for this account.

If you leave an employer, decide deliberately rather than by default — small balances are sometimes cashed out automatically by plans if you take no action.

And check the vesting schedule before handing in notice.

Where to go next

Project your balance from contributions, salary, and match with the 401(k) calculator. See whether the total is enough with the retirement calculator, and compare an IRA alongside it with the Roth IRA calculator.

To see how a pre-tax contribution changes your paycheck, use the paycheck calculator and read gross vs net pay. For the account-type decision, see Roth IRA vs traditional IRA. More tools sit in the retirement hub.

This guide is educational and does not constitute financial or tax advice. Consult a qualified professional about your own situation.