401(k) | Glossary

A 401(k) is an employer-sponsored retirement account where salary compounds tax-deferred (or tax-free, if Roth), invested from a fund menu you choose. It is the largest compounding account most Americans ever own, with free employer money bolted on.

US Retirement AccountsLast reviewed 2026-10-05

What it is

A 401(k) is a workplace retirement account named after the section of the Internal Revenue Code that created it in 1978. You elect a percentage of each paycheck, the plan invests it in the funds you choose from the menu, and the IRS lets the growth compound tax-deferred: no tax on dividends, interest, or capital gains inside the account until you take a distribution. Traditional contributions go in pre-tax and are taxed on the way out; Roth contributions go in after tax and come out tax-free. For 2026, the elective deferral limit is $24,500, plus an $8,000 catch-up from age 50, per IRS Notice 2025-67.

Two features do the heavy lifting. The employer match is an immediate, guaranteed return on your contribution before any market growth, and the tax deferral keeps the IRS out of every dividend and rebalance for decades. Withdrawals before 59½ generally owe ordinary income tax plus a 10% early-withdrawal penalty, with narrow exceptions, which is what makes the account a retirement machine rather than a savings account.

Key takeaways

The 2026 elective deferral limit is $24,500, plus an $8,000 catch-up from age 50.
The employer match is an immediate guaranteed return; capture all of it before anything else.
Traditional contributions are taxed on withdrawal; Roth contributions compound tax-free.
Withdrawals before 59½ generally owe income tax plus a 10% penalty, with narrow exceptions.
Fees compound against you: a 1% expense ratio across 40 years removes roughly a fifth of the ending balance.

How it works

The mechanics run paycheck by paycheck. You elect a deferral percentage in the plan portal, payroll withholds it before (traditional) or after (Roth) federal income tax, and the custodian buys fund shares on the next purchase date. The employer match lands on the same schedule once you meet the vesting terms, and growth is credited inside the account with no annual tax bill, which is the entire point: dividends reinvest and gains compound without the IRS taking a slice each April.

The limits stack in two layers. The elective deferral cap is $24,500 for 2026 ($32,500 total from age 50 with the catch-up), and the combined limit including employer contributions sits near $72,000. Access rules are the third layer: withdrawals before 59½ owe tax plus a 10% penalty unless an exception applies, such as separation from the employer after 55 or substantially equal periodic payments. From 73, SECURE 2.0 requires minimum distributions from traditional balances. Many plans also allow loans of up to 50% of the vested balance or $50,000, whichever is less, which is a convenience and a compounding interruption.

Why it matters

The 401(k) is where compounding, tax deferral, and free employer money meet, and it is also where most Americans leave money on the table. Roughly a third of private-sector workers have no workplace plan at all, and Vanguard's How America Saves studies consistently find about a quarter of participants failing to capture the full match. Declining the match is declining a raise with no upside.

I watched a colleague named Brent (yes, that Brent) keep his deferral at 2% for three years because the default felt aggressive, then take a $15,000 plan loan for a truck. The match he never captured compounded into a five-figure hole by the time he raised the rate, and the loan restarted his contributions from a smaller base.

The highest-leverage thing most Americans can do is capture the full match, pick a low-expense target-date fund, automate annual increases, and stop logging in. Model the trajectory with the retirement savings calculator or the investment calculator before touching anything.

Real-world examples

Capturing the full match: the 50% return on day one

An $80,000 salary with a 50% match on the first 6% contributed: $4,800 a year from the employee plus $2,400 from the employer, a 50% return on the employee's money before any market growth. Compounded at 7% after fees for 40 years, the combined $7,200 annual stream grows to roughly $1.44 million, and the match alone accounts for about $479,000 of it. No expense ratio, fund choice, or market view explains what declining that money would have cost.

A $25,000 early withdrawal at 45, measured properly

Withdrawing $25,000 at 45 to fund a kitchen: ordinary income tax at a 24% marginal rate takes $6,000, the 10% penalty takes another $2,500, and $17,000 arrives. The same $25,000 left invested at 7% compounds to roughly $97,000 by 65. The kitchen cost about $80,000 once the tax, the penalty, and the forgone compounding are all priced, which is the honest way to measure an early withdrawal.

A 1% fee gap across a 40-year career

Two identical savers contribute $7,200 a year for 40 years with 7% gross returns: one in a 0.05% index target-date fund, one in a 1.05% actively managed option. Net of fees the accounts compound at 6.95% and 5.95%, landing at roughly $1.42 million versus $1.10 million. The 1% gap removes about $320,000 of ending balance, and it repeats every single year without a single decision. The fee is the only difference between the accounts, and it compounds with the opposite sign of the returns.

Common mistakes

Mistake: Contributing below the full employer match.

Why it bites you: A 50% match is a guaranteed 50% return; leaving 3% of an $80,000 salary unmatched forgoes $2,400 a year, roughly $480,000 of compounding over a career at 7%.

Mistake: Cashing out the balance when changing jobs.

Why it bites you: A $40,000 cashout at 32 owes tax plus the 10% penalty and forfeits compounding; left invested for 33 years at 7%, that balance was roughly $373,000.

Mistake: Chasing last year's top-performing fund inside the menu.

Why it bites you: Fund-hopping drags returns through fees and missed recovery, and SPIVA data finds roughly 90% of actively managed large-cap funds underperform the index over 15 years after fees.

Mistake: Ignoring the fee column in the fund menu.

Why it bites you: A 1.05% fund versus a 0.05% index across 40 years removes roughly $320,000 on the pattern above; the expense ratio is the only number guaranteed to repeat every year.

Ivy's take

I spent an afternoon inside three different plan portals running the match arithmetic so you don't have to, and the pattern is identical everywhere: the match is the best return in the document, the default fund is usually the cheapest fund, and the fund menu is engineered to make the expensive option look normal.

The 401(k) is not exciting, and that is the point. It is a pre-tax compounding machine with free money bolted on, governed by a formula so boring the IRS wrote it down. Brent treats his as a savings account with a truck feature; I treat mine as a machine I feed every paycheck and audit once a year. The audit takes ten minutes. The machine runs for forty.

Related jargon, defined

Employer match: Employer money added on top of your contribution under a plan formula; the best return in the document.
Vesting: The schedule on which employer money becomes legally yours; unvested match reverts if you leave.
Elective deferral: The salary you elect to contribute, capped at $24,500 for 2026.
Required minimum distribution: The annual withdrawal the IRS requires from traditional balances starting at 73.
Expense ratio: A fund's annual fee as a percentage of assets; compounds against the balance every year.
Roth 401(k): The after-tax version of the same account; contributions taxed now, growth tax-free.

Frequently asked questions

How much should I contribute to my 401(k)?+

At minimum the full employer match; the common benchmark is 10% to 15% of salary including the match. Raise the rate one point a year until you get there, which spreads the pain across paychecks.

Traditional or Roth 401(k)?+

Traditional if your marginal tax rate is high now and likely lower in retirement; Roth if the reverse. Many savers split contributions to hedge the tax-rate question.

What happens to my 401(k) when I change jobs?+

Leave it, roll it to an IRA, or roll it to the new employer's plan; a direct rollover avoids tax and penalty. Cashing out is almost always the expensive option.

Can I withdraw from a 401(k) early?+

Before 59½ you owe ordinary income tax plus a 10% penalty, with exceptions such as separation after 55 or 72(t) payments. Many plans also allow loans up to $50,000.

Is a 401(k) enough to retire on?+

The $1.44 million pattern above supports roughly $57,000 a year at a 4% withdrawal rate, before Social Security. Whether that is enough depends entirely on the lifestyle it has to fund.

Disclaimer: This information is general in nature and does not constitute financial or legal advice. Always consult a qualified professional for your specific situation.

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