401(k) Explained — Complete Guide

A 401(k) is the single most powerful wealth-building tool available to most American workers. It offers tax advantages that no regular brokerage account can match, and if your employer offers a match, it's literally free money. Yet millions of workers either don't participate, don't contribute enough, or invest their 401(k) poorly. Understanding how this account works can be worth hundreds of thousands of dollars over your career.

How It Works

A 401(k) is an employer-sponsored retirement plan. You contribute money from your paycheck before taxes are taken out (for a traditional 401(k)) or after taxes (for a Roth 401(k)). The money grows tax-deferred — you pay no taxes on gains, dividends, or interest while the money is in the account.

For 2025, the contribution limit is $23,500 per year ($31,000 if you're 50 or older). These limits typically increase annually with inflation. Your contributions are invested in a menu of options chosen by your employer, usually including index funds, target-date funds, bond funds, and sometimes company stock.

When you retire and start withdrawing money (after age 59 1/2), you pay income tax on the withdrawals from a traditional 401(k). If you have a Roth 401(k), your withdrawals are tax-free because you already paid taxes on the contributions.

The Employer Match

This is the most important feature and the reason you should prioritize your 401(k) above almost everything else. Many employers match a percentage of your contributions — for example, 50% of the first 6% of your salary, or dollar-for-dollar up to 3%.

If your employer matches 50% of the first 6%, and you earn $80,000, contributing 6% ($4,800) gets you an additional $2,400 from your employer. That's a 50% instant return on your money. There is no investment in the world that offers a guaranteed 50% return. Not contributing enough to get the full match is leaving free money on the table.

Traditional vs Roth 401(k)

The choice between traditional and Roth depends on your tax situation.

Traditional 401(k): Contributions reduce your taxable income now. If you're in the 24% tax bracket and contribute $23,500, you save $5,640 in taxes this year. But you'll pay income tax when you withdraw in retirement.

Roth 401(k): Contributions come from after-tax dollars — no immediate tax break. But all growth and withdrawals in retirement are completely tax-free. If your $23,500 grows to $200,000 over 20 years, you pay zero tax on that $176,500 in gains.

General rule: If you expect your tax rate to be higher in retirement than it is now (because you're early in your career, or tax rates rise), choose Roth. If you're in your peak earning years and expect lower income in retirement, choose traditional. Many people split contributions between both.

Common Mistakes

Not enrolling at all. About 20% of eligible workers don't participate. If you're not in your 401(k), you're turning down free money and tax advantages.

Only contributing up to the match. The match is the minimum. If you can afford more, contribute more — ideally the full $23,500. The tax-deferred growth compounds significantly over decades.

Investing too conservatively. Many young workers put their 401(k) in money market funds or stable value funds. If you're 25 and won't touch this money for 40 years, you should be almost entirely in stock index funds. A target-date fund is an excellent default choice.

Not rolling over old 401(k)s. When you leave a job, don't leave your 401(k) behind. Roll it into an IRA or your new employer's plan. Orphaned accounts often get neglected and invested poorly.

Borrowing from your 401(k). Many plans allow loans, but this is almost always a bad idea. You miss out on market gains while the money is out, and if you leave your job, the loan often becomes due immediately.

The Power of Time

A 25-year-old who contributes $500 per month to a 401(k) earning 9% annually will have about $2.3 million by age 65. Start at 35 and the same contribution produces about $920,000. Start at 45 and it's roughly $340,000. Time is the most powerful variable. The earlier you start, the more compound growth does the work for you.