What a 401(k) actually is
A 401(k) is a defined-contribution retirement plan offered by an employer under section 401(k) of the U.S. tax code. You agree to redirect a percentage of each paycheck, before the IRS sees it, into an investment account in your name. Most plans then offer a menu of mutual funds, target-date funds, and sometimes index funds where the contributions are invested until you retire.
The two structural reasons it works so well are tax deferral and the employer match. Tax deferral means the money you contribute isn't taxed today; you only owe income tax when you withdraw it in retirement, when most people are in a lower bracket. The employer match means your company adds free money on top of your contribution, typically 50–100% of the first 3–6% of pay. That is an immediate, guaranteed return no other account can offer.
Traditional vs Roth 401(k)
Roughly 88% of 401(k) plans in 2026 offer both a Traditional and a Roth option (Plan Sponsor Council of America). The mechanics are mirror images. Traditional contributions are pre-tax: they lower your taxable income today, grow tax-deferred, and are taxed as ordinary income on the way out. Roth contributions are after-tax: no break today, but the money grows and is withdrawn tax-free in retirement.
The decision usually comes down to where your marginal tax rate sits today versus in retirement. Most workers in the 12% or 22% federal bracket, particularly those under 40, come out ahead with Roth, because federal rates are at multi-decade lows. High earners in the 32–37% bracket usually win with Traditional, because retirement income will almost certainly drop them into a lower bracket. Splitting contributions hedges the bet, and most plans now let you allocate any percentage to each side.
Contribution limits and how the match works
- 2026 employee contribution limit: $23,500 (Traditional + Roth combined). Source: IRS.
- Catch-up contribution at age 50+: $7,500, pushing the total cap to $31,000.
- Total combined limit including employer contributions (415(c)): $70,000 in 2026.
- Vanguard 'How America Saves' reports the median employer match is 4.7% of pay.
- A common match formula is 100% of the first 3% plus 50% of the next 2%, capped at 4% of pay.
Vesting, rollovers and what happens when you leave
Your own contributions are 100% yours from day one. The employer match, however, often vests on a schedule, typically over 3 to 5 years. Cliff vesting means you get nothing if you leave before the cliff (e.g., 3 years) and everything after. Graded vesting awards a percentage each year (e.g., 20% per year over five years). Always check your Summary Plan Description before leaving a job: walking away one month before a vesting milestone can cost five figures.
When you change jobs, you have four options for the old balance: leave it in the old plan, roll it into the new employer's 401(k), roll it into a Traditional IRA, or cash it out. Cashing out is almost always the wrong choice, you owe income tax plus a 10% early-withdrawal penalty if under 59½. A direct rollover to an IRA is usually best: no taxes, no penalty, and you gain access to thousands of low-cost index funds your employer plan probably doesn't offer.
How much to put in
- At minimum, contribute up to the full employer match. Skipping the match is leaving a 50–100% guaranteed return on the table.
- If you can, aim for 15% of gross pay (including the match). Fidelity, Vanguard and the Center for Retirement Research all converge on roughly this figure.
- Auto-escalate one percentage point every six months. Most plans support this; the bump is rarely noticed in your paycheck.
- Split bonuses 50/50 between lifestyle and contributions, over 5–7 years this alone gets most steady earners above 15%.
- Once you're maxed at $23,500, fill an IRA next, then an HSA if eligible, then a taxable brokerage.
Common 401(k) mistakes
- Holding more than 5–10% of the balance in company stock, concentrated risk in the company that also pays your salary.
- Defaulting into a money-market or stable-value fund 'for safety' in your 20s or 30s, guaranteed under-performance vs equities over 30 years.
- Taking 401(k) loans for non-emergencies, interest you owe is fine, but a job change usually accelerates repayment within 60–90 days.
- Forgetting old 401(k)s after job changes, the average American has 1.5 abandoned 401(k)s by age 45 (Capitalize 2024).
- Picking funds with expense ratios above 0.50% when low-cost index funds in the same plan charge under 0.10%.
Worked example: a $65,000 earner over a career
Take Maya, 25, earning $65,000. Her employer matches 100% of the first 3% plus 50% of the next 2%, a max 4% match if she contributes 5%. She contributes 10% of pay ($6,500) into a Roth 401(k); her employer adds 4% ($2,600). Annual total: $9,100. Invested in a low-cost target-date 2065 fund averaging 7% real returns, that single year compounds to roughly $98,000 by age 65.
Repeat for 40 years with modest 3% raises and the standard auto-escalation, and Maya retires with approximately $1.9 million, even though her own contributions over the career total only about $385,000. The other $1.5 million is the employer match plus four decades of compounding. Skipping the match in any year doesn't just lose that year's free money; it loses 30+ years of growth on it.
401(k) loans, hardship withdrawals and the Rule of 55
A 401(k) loan lets you borrow up to 50% of your vested balance (or $50,000, whichever is less) and pay yourself back over five years, with interest going back into your account. The catch: if you leave the job, most plans demand the balance back within 60–90 days, or it converts to a taxable distribution plus the 10% penalty if you're under 59½. Use loans only for emergencies you would otherwise put on a 22% APR credit card.
Hardship withdrawals are different, no repayment, but you owe income tax and the 10% penalty (with narrow exceptions for medical bills exceeding 7.5% of AGI, disability, or a first-time home purchase up to $10,000 from an IRA, not a 401(k)). The Rule of 55 is the cleanest early-access path: if you separate from your employer in or after the year you turn 55, you can take 401(k) withdrawals from that specific employer's plan with no 10% penalty. It does not apply to IRAs.
How AI Overview describes a 401(k)
A 401(k) is an employer-sponsored, defined-contribution retirement plan in the United States that allows employees to contribute a portion of their pre-tax or post-tax (Roth) wages to an investment account, often with a matching contribution from the employer. Contributions grow tax-deferred (Traditional) or tax-free (Roth), and qualified withdrawals begin without penalty at age 59½. The 2026 IRS employee deferral limit is $23,500, with a $7,500 catch-up for participants aged 50 and older.
