A 401(k) is the largest single tax-advantaged container in U.S. retirement law, and the only one that comes with another party (your employer) actively putting money in alongside you. Used correctly, it's the most powerful tool the average wage-earner controls. Used carelessly — through under-contributing, missing the match, or cashing out at job changes — it becomes the most expensive opportunity cost in personal finance.
What a 401(k) is
A 401(k) is an employer-sponsored, defined-contribution retirement plan named after the section of the Internal Revenue Code that authorizes it. You elect a percentage of your pay to be deducted pre-tax (Traditional) or after-tax (Roth) and deposited into the plan, where it's invested in the plan's menu of mutual funds or target-date funds. Earnings grow tax-deferred (Traditional) or tax-free at qualified withdrawal (Roth).
2026 contribution limits
Employee deferral: $23,500 (under age 50)
Catch-up (age 50+): additional $7,500
Super catch-up (ages 60-63 under SECURE 2.0): up to $11,250 instead of $7,500
Combined cap (employee + employer + after-tax): $70,000 (under 50) or $77,500 (50+)
Highly-compensated employee threshold: $160,000 in prior-year compensation
The employer match — never leave it on the table
The match is the highest-return investment in the legal U.S. financial market. A 50% match on the first 6% of salary is a guaranteed 50% return on every contributed dollar within that range, before any market gains. On a $75,000 salary contributing 6% ($4,500), you receive $2,250 of free money annually. Over a 35-year career with average salary growth, the cumulative match plus its compound growth often exceeds $400,000 — pure addition to your nest egg with no out-of-pocket cost.
Walkthrough: 30-year-old at $75,000 salary
Starting balance: $50,000
Annual contribution: 10% of salary = $7,500
Employer match: 50% on first 6% = $2,250
Total annual deposit: $9,750
Salary growth: 3% per year (matches inflation expectations)
Expected return: 7% nominal (about 4% real after 3% inflation)
Years to age 65: 35
Projected balance at 65 (nominal): ~$1.55 million
Of that, ~$420K is contributions, ~$165K is employer match, ~$965K is compound growth
4% rule monthly income: ~$5,170/month (about $1,900 in today's purchasing power assuming 3% inflation)
Traditional vs Roth 401(k) — which to choose
Traditional reduces your tax bill now (contributions are deductible from current income) and tax-defers growth. You pay ordinary income tax on the entire withdrawal in retirement. Best when your current marginal bracket is higher than your expected retirement bracket.
Roth uses after-tax contributions but produces tax-free withdrawals at qualified retirement age. Best for young workers in lower brackets or anyone concerned about future tax-rate increases.
Both is often the practical answer — split contributions to diversify tax exposure across retirement years.
Note: the employer match always goes into the Traditional (pre-tax) bucket, even if your contributions are Roth.
Five most expensive 401(k) mistakes
Cashing out at job change. The average cash-out at separation costs the saver about $1.4 million in lifetime nest-egg value by retirement, plus a 10% penalty and full ordinary-income tax in the year of withdrawal.
Failing to capture the full match. Contributing 3% to a plan that matches 50% up to 6% leaves three percentage points of "free money" unclaimed.
Sitting in cash or money-market by default. A small fraction of plans default new contributions to a stable-value fund instead of a target-date fund. Check your allocation; the difference over 30 years is often 5x or more.
Taking a 401(k) loan and not paying it back. A defaulted loan triggers taxes, penalties, and permanent removal of the loaned amount from compound growth.
Over-concentrating in employer stock. The Enron lesson never goes out of date — concentration in any single stock above 10% of your portfolio is poor diversification, and employer stock adds career risk to the same position.
FAQ
Can I contribute to both a 401(k) and an IRA?
Yes — they're separate buckets with separate limits. Combined annual contributions can hit $23,500 (401k) + $7,000 (IRA) = $30,500 for someone under 50.
When can I withdraw without penalty?
Age 59½ for normal distributions. Exceptions include separation from service at age 55+, qualifying disability, substantially equal periodic payments (Rule 72(t)), and certain hardship withdrawals.
Are 401(k) withdrawals taxed?
Traditional withdrawals are ordinary income. Qualified Roth withdrawals are tax-free after age 59½ and a 5-year holding period.
What about RMDs?
Required minimum distributions begin at age 73 (rising to 75 in 2033). Roth 401(k) accounts no longer have lifetime RMDs starting in 2024.
Can my employer take back the match?
Only the unvested portion. Once vested, the match is yours regardless of departure date.
What's the difference between 401(k) and 403(b)?
403(b) is the public-sector and nonprofit equivalent — same limits, slightly different investment menu rules. See our 403(b) Calculator.