How does a 401(k) work? A plain-English guide

A 401(k) is an employer-sponsored retirement account that lets you set aside part of your paycheck before taxes are taken out. Your money grows tax-deferred until you withdraw it in retirement, and many employers add matching contributions. Here’s how the whole system works, step by step.

What is a 401(k)?

A 401(k) is a type of defined contribution plan. That means you and your employer put money into an individual account, and the final balance depends on how much you contribute and how your investments perform. Unlike a pension, which promises a set payout, a 401(k) puts the responsibility on you to save and invest.

Most plans offer two tax treatments: Traditional and Roth. With a Traditional 401(k), your contributions come out of your paycheck before income tax is calculated, lowering your taxable income now. You pay ordinary income tax when you withdraw the money in retirement. With a Roth 401(k), you contribute after-tax dollars, so withdrawals in retirement are tax-free (if you meet certain conditions).

Employers often match a portion of your contributions, which is essentially free money. For example, a common match is 50% of your contributions up to 6% of your salary. If you earn $60,000 and contribute 6% ($3,600), your employer adds $1,800.

How a 401(k) works step by step

Here’s the typical process from enrollment to retirement:

1. Enroll: You sign up through your employer’s plan administrator. Some companies automatically enroll you at a default rate (often 3% of pay) unless you opt out.
2. Choose your contribution rate: You decide what percentage of your paycheck goes into the plan, up to the IRS annual limit. For 2026, the employee contribution limit is about $24,500, plus a catch-up amount if you’re 50 or older. (Source: IRS 401(k) contribution limits)
3. Pick investments: Your money is invested in funds you choose from the plan’s menu. Common options include target-date funds, index funds, and bond funds.
4. Payroll deductions: Each paycheck, your contribution is taken out and deposited into your 401(k) account.
5. Employer match (if offered): Your employer adds their matching contribution, usually on a schedule.
6. Growth: Your investments grow over time. Because it’s tax-deferred (or tax-free in a Roth), you don’t pay taxes on gains each year.
7. Withdrawals: You can start taking money out penalty-free at age 59½. Withdrawals from a Traditional 401(k) are taxed as ordinary income. If you withdraw before 59½, you generally owe a 10% penalty plus taxes.

Key terms you’ll see on your statements

Understanding these terms helps you read your 401(k) statements and disclosures:

• Vesting: This refers to your ownership of employer contributions. You are always 100% vested in your own contributions. Employer matches may vest over time, meaning you must work a certain number of years before you can keep them if you leave.
• Contribution limit: The maximum amount you can contribute in a year. For 2026, it’s about $24,500 for employees under 50. (Source: IRS)
• Catch-up contribution: An extra amount allowed for people aged 50 and older. The IRS sets this annually.
• Target-date fund: A fund that automatically shifts your asset allocation from aggressive (more stocks) to conservative (more bonds) as you approach a target retirement year.
• Expense ratio: The annual fee charged by a mutual fund or ETF, expressed as a percentage of your investment. Lower is generally better.
• Rollover: Moving your 401(k) money to another retirement account, such as an IRA or a new employer’s plan, without paying taxes or penalties.

Worked example: How contributions and matches add up

Let’s follow a realistic scenario. Maria, age 35, earns $70,000 a year. Her employer offers a 50% match on the first 6% of her salary. Maria decides to contribute 6% of her pay, which is $4,200 a year. Her employer adds $2,100 (50% of $4,200). That’s a total of $6,300 going into her 401(k) each year.

Now let’s assume Maria’s investments earn an average annual return of 6% (this is a hypothetical rate, not a guarantee). We’ll calculate her balance after 10 years, ignoring fees and inflation, using the future value of an annuity formula: FV = P × [((1 + r)^n – 1) / r], where P = $6,300, r = 0.06, n = 10.

First, (1.06)^10 = 1.7908. Subtract 1 to get 0.7908. Divide by 0.06 to get 13.180. Multiply by $6,300 to get $83,034. So after 10 years, Maria’s account would be worth about $83,034.

Here’s the year-by-year breakdown:

Wait, the table shows $87,701, but my formula gave $83,034. Why the difference? Because the table assumes contributions are made at the beginning of each year, while the formula assumes end-of-year contributions. In reality, contributions are made each pay period, so the actual balance would fall somewhere in between. The key point is that consistent contributions plus compound growth can build a significant nest egg over time. (Learn more about how compound interest works.)

The risks

Investing involves risk, including the possible loss of principal. Your 401(k) balance is not guaranteed; it depends on the performance of the funds you choose. Stocks can be volatile, and bond funds can lose value when interest rates rise. Even target-date funds, which automatically become more conservative as you age, carry risk.

Another risk is outliving your savings. Because you control the investment choices, poor decisions or high fees can reduce your retirement income. Also, if you change jobs, you need to decide what to do with your 401(k) – leaving it, rolling it over, or cashing out. Cashing out triggers taxes and a 10% penalty if you’re under 59½.

What people get wrong about 401(k)s

Here are four common mistakes beginners make:

• Thinking the employer match is optional to take. Some people don’t contribute enough to get the full match, leaving free money on the table. If your employer matches 50% up to 6%, you need to contribute at least 6% to get the full match. Not doing so is like turning down a raise.
• Confusing “tax-deferred” with “tax-free.” Traditional 401(k) contributions lower your taxes now, but you pay income tax on every dollar you withdraw in retirement. If you expect to be in a higher tax bracket later, a Roth 401(k) might be more suitable, but that’s a personal decision.
• Ignoring fees. A fund with a 1% expense ratio might not sound like much, but over decades it can eat a huge portion of your returns. Compare the expense ratios of your plan’s options. Lower-fee index funds often outperform higher-fee actively managed funds over the long run.
• Borrowing from your 401(k) without understanding the consequences. Loans from your 401(k) must be repaid with interest, and if you leave your job, the outstanding balance may be due immediately. If you can’t repay, it’s treated as a distribution, subject to taxes and a 10% penalty if you’re under 59½.

Putting it together

A 401(k) is a powerful savings tool because of the tax advantages and the potential for employer matches. The mechanics are straightforward: you contribute a percentage of your pay, choose investments, and let time and compound growth work. The main things to watch are contribution limits, fees, and vesting rules. Start early, contribute enough to get any match, and review your investment choices periodically.