If your employer offers a 401(k), it’s probably one of the best financial tools available to you. Yet a lot of people contribute the bare minimum — or nothing at all — because they don’t fully understand how it works.
This guide changes that.
What Is a 401(k)?
A 401(k) is a retirement savings account offered through your employer. You contribute money from your paycheck — before taxes — and it grows tax-deferred until you withdraw it in retirement.
The name comes from the section of the IRS tax code that created it (section 401(k)). Not a catchy name, but a powerful concept.
How It Works
You contribute pre-tax dollars. If you earn $60,000/year and contribute 10% ($6,000) to your 401(k), you only pay income tax on $54,000. Your contribution reduces your taxable income today.
The money grows tax-deferred. Inside the 401(k), your investments grow without being taxed each year. No annual capital gains tax. No tax on dividends. Everything compounds uninterrupted.
You pay taxes when you withdraw. In retirement (typically after age 59½), withdrawals are taxed as ordinary income. The assumption is that you’ll be in a lower tax bracket then than during your peak earning years.
The Employer Match: Free Money
Many employers match a portion of your 401(k) contributions. A common structure:
“We match 100% of contributions up to 3% of your salary, and 50% of contributions on the next 2%.”
Translation: if you earn $60,000 and contribute 5% ($3,000), your employer adds another $2,400. Your account gets $5,400 — you only put in $3,000.
That’s an immediate 80% return on your $3,000 contribution. Nothing in the financial world beats that.
Always contribute at least enough to get the full employer match. Always. Even if you’re paying off debt. Even if you’re broke. Leaving employer match on the table is one of the most expensive financial mistakes you can make.
401(k) Contribution Limits
For 2025, the IRS limits:
- Employee contribution limit: $23,500/year
- Catch-up contribution (age 50+): Additional $7,500/year (total $31,000)
- Total limit (employee + employer): $70,000/year
Most people don’t max it out — and that’s fine. The goal is to contribute as much as you reasonably can, starting with at least enough to get the full match.
Traditional 401(k) vs. Roth 401(k)
Many employers now offer both options:
Traditional 401(k):
- Contributions: pre-tax (reduces your taxable income now)
- Growth: tax-deferred
- Withdrawals: taxed as ordinary income
- Best if: you expect to be in a lower tax bracket in retirement than now
Roth 401(k):
- Contributions: after-tax (no immediate tax deduction)
- Growth: tax-free
- Withdrawals: completely tax-free in retirement
- Best if: you expect to be in the same or higher tax bracket in retirement, or you’re young with decades of growth ahead
If you’re early in your career or in a low tax bracket, Roth is often the better choice. The tax-free growth over 30–40 years is extraordinarily valuable.
When in doubt, a mix of traditional and Roth is a reasonable hedge.
What Can You Invest In?
Unlike an IRA (where you can invest in almost anything), a 401(k) limits you to the investment options your employer has chosen. These usually include:
- Target-date funds (e.g., “Target Date 2055”)
- Index funds (S&P 500, total market)
- Actively managed mutual funds
- Bond funds
- Sometimes company stock
Look for the lowest expense ratios. This is where employer plans vary wildly. Some plans offer excellent low-cost Vanguard or Fidelity index funds. Others offer only expensive actively managed funds with 1%+ expense ratios.
Your best move is usually to find the total stock market or S&P 500 index fund with the lowest expense ratio available in your plan.
If your plan only offers expensive options, still contribute enough to get the full match — the match overcomes the fee disadvantage. After that, consider using a Roth IRA for additional retirement savings.
What Happens If You Leave Your Job?
You don’t lose your 401(k) when you change jobs. You have options:
1. Roll it into your new employer’s 401(k). Usually the simplest option if the new plan has good investment options.
2. Roll it into an IRA. Open a traditional IRA at Fidelity or Vanguard and do a direct rollover. This gives you access to the full universe of investment options.
3. Leave it where it is. If your old employer allows it and the plan has good options, you can leave the account in place.
4. Cash it out. Do not do this. You’ll owe income taxes on the full amount plus a 10% early withdrawal penalty. For a $20,000 account, you might walk away with $12,000–13,000 after taxes and penalties. Let it keep growing.
401(k) Withdrawal Rules
Age 59½: You can withdraw without the 10% early withdrawal penalty. You still owe income tax on the amount.
Age 73: Required minimum distributions (RMDs) begin. The IRS requires you to start withdrawing a calculated percentage each year.
Early withdrawal: Before 59½, you owe income tax plus 10% penalty. Exceptions exist for disability, certain medical expenses, and a few other cases.
Loans Against Your 401(k)
Some plans allow you to borrow against your 401(k) balance. Generally, this is a bad idea:
- The borrowed money isn’t growing while it’s out of the account
- If you leave your job, the loan may become due immediately
- You pay it back with after-tax dollars (then pay taxes again on withdrawal — double taxation)
Use a 401(k) loan only as a genuine last resort.
How Much Should You Contribute?
A simple contribution ladder:
- Step 1: Contribute enough to get the full employer match (free money)
- Step 2: Max out a Roth IRA ($7,000/year if eligible)
- Step 3: Go back and increase your 401(k) contributions up to the limit
If you can only do step 1, that’s still a great start. The match alone can add up to significant wealth over a career.
Common advice: aim for 15% of your income toward retirement savings total (including employer contributions). If you’re starting late, aim higher.
The Bottom Line
A 401(k) is one of the most powerful wealth-building tools available to employees. The tax advantages, employer match, and decades of tax-deferred compounding make it irreplaceable.
If you have access to one:
- Enroll if you haven’t
- Contribute at least enough to get the full match
- Pick low-cost index funds
- Increase your contribution by 1% every year until you’re at 15%+
Most of building retirement wealth isn’t complicated. It’s consistent, boring, and automatic. Set it up, then mostly ignore it.
For investing outside your 401(k), Acorns is a simple way to start building a taxable account — it rounds up your purchases and invests the difference automatically, so you’re building wealth in the background even when you’re not thinking about it.
## Related Reads- Roth IRA Explained — after you get the 401(k) match, a Roth IRA is usually the next best move
- Roth IRA vs Traditional IRA — understanding the tax trade-off that applies to both IRAs and 401(k)s
- Index Funds for Beginners — most 401(k) plans offer index funds; here’s how to pick the right ones
This article contains affiliate links. If you sign up through our links, we may earn a commission at no extra cost to you.