US guide A 401(k) is a United States retirement account, and the rules below reflect the US tax system. If you live elsewhere, your country almost certainly has its own workplace pension or retirement accounts with different names and rules.

For a lot of people, a 401(k) is the first serious retirement account they ever have — and also the one nobody quite explains. You sign some onboarding paperwork on your first day, tick a box, and then the money quietly disappears from each paycheck. Understanding what actually happens to it makes the whole thing far less mysterious, and helps you avoid leaving real money on the table.

This guide walks through how a 401(k) works from the ground up: what it is, the two main flavours, the employer match, how much you can put in, how the money is invested, and what happens when you eventually change jobs or retire.

What a 401(k) actually is

A 401(k) is a retirement savings account offered through an employer. The odd name comes from the section of the US tax code that created it. The key idea is simple: you agree to have a portion of each paycheck sent straight into the account before it ever reaches your bank, and that money is then invested so it can grow over the decades until you retire.

Because the contribution happens automatically through payroll, saving becomes the default rather than something you have to remember to do. That automation is quietly one of the most powerful features of the whole system — you are far more likely to keep saving when it happens without any monthly decision on your part.

A 401(k) is a "defined contribution" plan, which means what you end up with depends on how much goes in and how the investments perform, not on a fixed pension promise. That places more responsibility on you, but it also means the account is genuinely yours to carry from job to job.

Traditional vs. Roth 401(k)

Many employers let you choose between two versions, and the difference comes down to when you pay income tax.

Traditional 401(k) Roth 401(k)
When you contribute Money goes in before income tax, lowering your taxable income this year. Money goes in after tax, so there is no tax break today.
While it grows No tax on growth along the way. No tax on growth along the way.
When you withdraw in retirement Withdrawals are taxed as income. Qualified withdrawals are generally tax-free.

The rough logic: a traditional 401(k) helps most if you expect to be in a lower tax bracket in retirement than you are now, while a Roth can help if you expect the opposite, or simply value the certainty of a tax-free pot later. Plenty of people split the difference and use some of each. There is no universally correct answer, and it is a reasonable question to raise with a tax professional if your situation is complicated.

The employer match: don't leave it behind

Here is the part worth reading twice. Many employers will match a portion of what you contribute — effectively adding free money to your account as part of your pay package. A common arrangement might be something like "50% of your contributions up to 6% of your salary," though the exact formula varies from one employer to the next.

Suppose you earn $50,000 and your employer matches 50% of what you put in, up to 6% of pay. If you contribute 6% ($3,000 over the year), your employer adds $1,500 on top. That is an immediate boost to your savings you rarely find anywhere else, though matched money is often subject to a vesting schedule before it is fully yours. Contributing less than the amount needed to earn the full match means turning down part of your compensation.

Quick tip

If money is tight and you can only do one thing, aim to contribute at least enough to capture your full employer match. It is one of the closest things to free money that ordinary personal finance offers.

How much you can contribute

The IRS sets a yearly cap on how much you can put into a 401(k) from your own paycheck, and it usually rises a little most years to keep pace with inflation. For the 2025 tax year, that employee contribution limit was $23,500. People aged 50 and over could add an extra "catch-up" contribution — $7,500 for 2025 — and recent law added a higher catch-up for a narrow older age band. Employer contributions sit on top of your own, under a separate, larger overall limit.

Because these figures change from year to year, treat the numbers above as a snapshot of 2025 rather than a permanent rule. Always confirm the current year's limits directly on IRS.gov before you rely on them — it is the authoritative source and it is updated when the limits change. The broader point is more durable than any single figure: a 401(k) lets you shelter a substantial amount each year, far more than most other retirement accounts allow.

How the money is invested

A common misunderstanding is that a 401(k) is itself an investment. It is not — it is a container. Inside it, you choose from a menu of investment options your plan offers, usually a selection of mutual funds. Your contributions buy into those funds, and your balance rises and falls with them.

Most plans include target-date funds, which are built around an approximate retirement year (for example, a "2055 fund"). These automatically hold a mix of investments that gradually becomes more conservative as the target date approaches, which is why they are a popular default for people who would rather not manage the details themselves.

One thing genuinely worth checking is fees. Funds charge an annual expense ratio, and even small differences compound into meaningful amounts over decades. If you want to understand why small percentages matter so much over time, our guide on compound interest shows the maths. And if the idea of choosing funds feels daunting, the difference between broad funds and picking individual companies is covered in index funds vs. individual stocks.

Vesting: when the match is truly yours

Your own contributions are always 100% yours from day one. Employer contributions can be different: some companies apply a vesting schedule, meaning you have to stay employed for a certain period before their contributions fully belong to you. A schedule might grant ownership gradually over several years, or all at once after a set point.

This matters if you are thinking about changing jobs. Leaving shortly before you are fully vested could mean forfeiting some of the employer money. It is worth knowing your plan's schedule — it is spelled out in the plan documents your employer provides.

What happens when you change jobs

Changing employers does not mean losing your 401(k). You generally have a few options for the vested balance:

  • Leave it in your former employer's plan, if the plan allows it.
  • Roll it into your new employer's 401(k), if that plan accepts rollovers.
  • Roll it into an Individual Retirement Account (IRA), which often opens up a wider range of investment choices.
  • Cash it out — usually the least favourable route, because of taxes and a possible early-withdrawal penalty.

A direct rollover, where the money moves straight from one account to the other without passing through your hands, avoids triggering taxes and penalties. If you are weighing an IRA, our companion guide on Roth vs. traditional IRAs explains how those accounts differ.

Getting the money out

A 401(k) is built for retirement, and the rules gently steer you toward leaving the money alone until then. In general, withdrawing before age 59½ triggers income tax plus an additional early-withdrawal penalty, though there are specific exceptions. Later in life, rules known as required minimum distributions eventually oblige you to start drawing certain accounts down.

These rules are detailed and occasionally change, so this is very much a "check the current guidance" area rather than something to act on from memory. The practical takeaway for most people saving today is simpler: money you put in a 401(k) is money you are committing to your future self, so contribute what you can comfortably leave invested for the long haul.

How to make the most of it

A few steps cover most of what matters:

  • Enrol and contribute at least up to the match. This is the highest-priority move.
  • Pick an investment option you understand — a target-date fund is a reasonable default if you are unsure.
  • Check the fees on the funds you choose, and favour lower-cost options where the choice is otherwise similar.
  • Increase your contribution over time, for example whenever you get a raise, so saving grows alongside your income.
  • Leave it invested through the ups and downs rather than reacting to short-term market swings.

None of this requires being a market expert. A steady contribution, captured match, and sensible low-cost fund, left alone for years, does the heavy lifting.

Common mistakes to avoid

  • Contributing too little to earn the full employer match.
  • Cashing out a balance when changing jobs instead of rolling it over.
  • Ignoring the investment choice and leaving contributions sitting in cash by default.
  • Panic-selling during a market dip and locking in a loss.

The bottom line

A 401(k) turns retirement saving into something that happens automatically, with two powerful tailwinds: the tax treatment and, for many people, an employer match that adds free money. You do not need to understand every rule to benefit — enrolling, capturing the full match, choosing a sensible low-cost investment, and leaving it to grow will put most people well ahead.

If you are still building the habits underneath all this, it helps to start investing with small amounts and to understand the compound growth that makes decades of steady contributions add up.

Frequently asked questions

How much should I contribute to my 401(k)?

A sensible starting point is to contribute at least enough to receive your employer's full match, because that match is part of your compensation. Beyond that, many people aim to work toward saving a larger share of income over time as their budget allows. The right number depends on your income, expenses, and other goals, so treat any rule of thumb as a starting point rather than a fixed target.

What is the difference between a traditional and a Roth 401(k)?

With a traditional 401(k), contributions are made before income tax, which lowers your taxable income now, and you pay tax when you withdraw the money in retirement. With a Roth 401(k), contributions are made from money that has already been taxed, and qualified withdrawals in retirement are generally tax-free. Which is better depends partly on whether you expect your tax rate to be higher now or later.

What happens to my 401(k) if I leave my job?

Your own contributions are always yours. When you leave, you can usually keep the money in the old plan, roll it into your new employer's plan, or roll it into an IRA. A direct rollover moves the money without triggering taxes or penalties. Cashing it out early is usually the least favourable option because of taxes and possible penalties.

Can I lose money in a 401(k)?

Yes. A 401(k) is an account that holds investments such as stock and bond funds, and those investments rise and fall in value. Over short periods the balance can drop. The reason many long-term savers stay invested is that markets have historically trended upward over long periods, though past performance never guarantees future results.

When can I take money out of a 401(k) without a penalty?

In general, withdrawals are intended for retirement, and taking money out before age 59 and a half often triggers income tax plus an additional early-withdrawal penalty, with some exceptions. Because the rules and exceptions are detailed and can change, check the current IRS guidance or speak with a qualified professional before making an early withdrawal.

Sources & further reading

The explanations and examples here are our own. To keep the details accurate, and to give you authoritative places to confirm the current figures for yourself, we drew on official US government resources. These are the right places to check the latest contribution limits and rules: