How a 401(k) works

A 401(k) is a retirement savings plan offered through your employer. It's named after the section of the US tax code that created it — not exactly a memorable brand, but four decades later it's the primary retirement vehicle for most American workers. Each pay period, a portion of your paycheck flows into the account before it ever hits your bank. The money is invested, grows over decades, and you draw it down in retirement. Simple in outline; surprisingly easy to misuse in practice.

Where the money comes from

Your contributions are deducted automatically from your paycheck. You choose a percentage or a flat dollar amount, and your payroll system handles the rest. You can usually change your contribution rate at any time through your employer's benefits portal — you are not locked into whatever you set when you were hired and still reading the onboarding handbook without absorbing any of it.

One important note: annual contribution limits are set by the IRS and adjusted periodically for inflation. The IRS publishes current limits at irs.gov. There are also higher "catch-up" limits for workers aged 50 and over. Do not rely on any website — including this one — for a specific dollar figure; check the IRS directly for the current year.

Traditional vs Roth 401(k): the tax question

Many employers now offer both a traditional 401(k) and a Roth 401(k). The difference is entirely about when you pay income tax:

• Traditional 401(k): contributions come out of your paycheck before income tax. Your taxable income this year goes down by whatever you contribute, which reduces your current tax bill. In retirement, every dollar you withdraw is taxed as ordinary income at whatever rate applies then.
• Roth 401(k): contributions are made after tax — you pay income tax on that money now, just as you would on the rest of your paycheck. The payoff comes later: qualified withdrawals in retirement, including all the growth the account has accumulated, are generally tax-free.

Which is better? It depends on one question: do you expect your tax rate to be higher now, or in retirement? If you're early in your career and currently in a low tax bracket, paying tax now (Roth) can make a lot of sense. If you're in a high-earning peak decade and expect lower income in retirement, the traditional approach — deferring tax until withdrawal — may serve you better. When genuinely unsure, splitting contributions between both is a reasonable hedge.

The employer match: the part you really shouldn't leave on the table

Many employers sweeten the deal by matching a portion of what you contribute. A common structure works something like this: the employer matches 50 cents for every dollar you put in, up to a cap of, say, 6% of your salary. Under this arrangement, if you contribute 6% of your pay, you effectively receive an additional 3% deposited into your account by your employer at no extra cost to you.

This is, without exaggeration, the best guaranteed return available to most people. Contribute 6%, instantly receive a 3% bonus before a single investment does anything. Not contributing enough to capture the full match is the equivalent of declining part of your salary.

Match structures vary widely by employer — some match dollar-for-dollar, some only up to a tiny percentage, some offer no match at all. Check your plan documents or ask HR exactly what your employer offers and what contribution rate triggers the full match.

Tax-deferred growth and compounding

Inside a 401(k), your investments grow tax-deferred: you don't pay capital gains tax on dividends reinvested or on growth within the account each year. That means the full balance compounds — not a balance reduced annually by a tax bill. Over long timeframes, the difference this makes is substantial.

To illustrate with a hypothetical: imagine two people both invest $500 a month for 30 years and both earn the same average return. The one investing in a taxable account pays tax on gains along the way; the one inside a 401(k) doesn't. By the end, the 401(k) balance will typically be meaningfully larger, even though the same amount was contributed. The Roth version adds another layer: qualified withdrawals face no tax at all.

What your money is actually invested in

A 401(k) is not itself an investment — it's a tax-advantaged wrapper that holds investments. Your employer's plan will offer a menu of options, which typically includes:

• Target-date funds — the simplest option for most people. You pick a fund named for roughly the year you plan to retire (e.g., a "2055 fund"), and it automatically shifts from aggressive to conservative as that date approaches. One choice, done.
• Index funds — low-cost funds that track a market index like the S&P 500. Broad diversification, minimal fees.
• Actively managed funds — fund managers trying to beat the market. They charge higher fees and, on average over long periods, most don't justify those fees after costs. Worth scrutinising before choosing.
• Bond funds, stable value funds — lower risk, lower expected return; useful as you approach retirement and need to reduce volatility.

Pay attention to expense ratios (the annual fee expressed as a percentage). A 1% fee versus a 0.05% fee sounds trivial but compounds into a meaningful difference over decades.

Early withdrawal: the expensive option

Taking money out of a traditional 401(k) before age 59½ triggers two hits: you owe income tax on the withdrawal at your current rate, plus a 10% early withdrawal penalty. On a hypothetical $10,000 withdrawal, you might walk away with $6,000 after tax and penalty, depending on your bracket. Exceptions exist — certain hardship withdrawals, disability, separation from service at age 55 or older, and a few others — but the general rule is that early withdrawal is expensive and should be a last resort.

Loans from a 401(k) are also possible in many plans. You avoid the penalty and tax as long as you repay on schedule, but there are risks: if you leave your job, the loan often becomes due quickly, and money pulled out of the market isn't compounding.

What happens when you change jobs

Your 401(k) balance doesn't disappear when you leave an employer. You typically have three options:

• Roll it over to your new employer's plan — if the new plan accepts rollovers and has decent investment options, this keeps things consolidated.
• Roll it over to an IRA — individual retirement accounts often offer a wider range of investment options and lower costs than employer plans.
• Leave it in the old plan — allowed if the balance is above a minimum threshold. Fine as a temporary measure, but easy to lose track of over time.

Request a direct rollover (institution-to-institution) rather than taking a check. If the money touches your hands, you have 60 days to redeposit it or the IRS treats it as a withdrawal — with all the tax and penalty implications that follow.

What to actually do

If your employer offers a 401(k) with a match, the single most important step is to contribute at least enough to capture the full employer match. Everything after that — Roth vs traditional, investment selection, contribution rate beyond the match — involves trade-offs worth thinking through carefully. But leaving matching dollars unclaimed is the one clear mistake with no upside.

Once the match is secured, the next logical question is whether to contribute more to the 401(k) or route additional savings elsewhere (such as a Roth IRA). That depends on your plan's investment options, fees, and your overall financial picture. For current contribution limits and eligibility rules, go directly to the source: irs.gov.

Sources & further reading

• IRS — 401(k) Plans (irs.gov)
• US Department of Labor — 401(k) Plans (dol.gov)
• Investopedia — 401(k) Plan

Related

• 📚 What is a Roth IRA? — the individual retirement account that pairs well with a 401(k).
• 📚 What is an index fund? — the investment type that dominates most good 401(k) menus.
• 📚 Compound interest explained — why tax-deferred growth matters so much over time.
• 📚 More explainers in the Learn hub