What Is a 401(k) and How Does It Work?

It's the most common way Americans build retirement wealth — and one of the most valuable benefits your job offers. Here's how it works.

The basics

A 401(k) is a retirement savings account offered through your employer. You choose a percentage of each paycheck to contribute, and it's invested (usually in funds you pick) so it can grow over decades. Because contributions come straight out of your pay, saving happens automatically — before you can spend the money.

The employer match — free money

Many employers match part of what you contribute, e.g. 50% of your contributions up to 6% of your salary. That's an instant return you can't get anywhere else. Always contribute at least enough to get the full match — see exactly what it's worth over a career on the 401(k) calculator.

Traditional vs Roth 401(k)

• Traditional: contributions are pre-tax (lowering your taxable income now); you pay tax when you withdraw in retirement.
• Roth: contributions are after-tax now; qualified withdrawals in retirement are tax-free.

Younger savers often favor Roth for decades of tax-free growth — the same logic as Roth vs Traditional IRA.

Contribution limits & vesting

The IRS sets an annual contribution limit (it rises most years), with higher "catch-up" limits once you're 50+. Some employer matches "vest" over time, meaning you earn full ownership after a few years — check your plan.

How to start

1. Enroll through your employer's HR or benefits portal.
2. Set your contribution to at least the full match.
3. Pick low-cost index funds inside the plan.
4. Increase your percentage 1% a year — you'll barely feel it.

Recommended reading 📚

Learn the fundamentals of retirement investing (affiliate links — see our disclosure):

• The Simple Path to Wealth — a clear guide to investing for retirement.
• The Bogleheads' Guide to Investing — practical, low-cost investing wisdom.

Plan your retirement