401(k) Explained: How Employer Retirement Plans Work

How 401(k) explained works in practice

A 401(k) is a US defined-contribution retirement plan offered by employers. Employees contribute pre-tax (traditional) or after-tax (Roth) dollars, invested in mutual funds or target-date funds. Withdrawals in retirement are taxed (traditional) or tax-free (Roth) per plan rules.

The goal is not to memorize every term — it is to know which inputs matter and what outcome you are aiming for.

So what: When you can explain this in your own words, you are far less likely to accept a bad quote, fee, or assumption.

A real scenario worth running

$75,000 salary, employer matches 100% of first 4% contributed. Step by step: Contribute 4% = $3,000/year → employer adds $3,000 match → Total annual contribution $6,000 → At 7% return for 30 years ≈ $567,000 (contributions + growth). Bottom line: Missing the match leaves $3,000/year on the table — $90,000 over 30 years before investment growth.

So what: Plug your own numbers into the same logic before you decide.

What a 401(k) is

A 401(k) is an employer-sponsored retirement plan. You contribute from your paycheck — pre-tax (traditional) or after-tax (Roth) — and invest in mutual funds or target-date funds. Many employers add a match — free money tied to your contribution rate. The 2026 employee deferral limit is $23,500 ($31,000 if age 50+ with catch-up).

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

How employer matching works

Common match structures:

Always contribute enough to capture the full match — skipping it leaves guaranteed return on the table.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Traditional vs Roth 401(k)

Many plans let you split contributions between both — tax diversification hedges uncertainty.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Worked example: the cost of missing the match

$75,000 salary, employer matches 100% of first 4%, 7% average return, 30 years:

Missing the match leaves $3,000/year on the table — $90,000 in contributions alone over 30 years, before investment growth.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Vesting and job changes

When changing jobs: roll over to new employer plan or IRA. Cashing out triggers taxes and often a 10% early withdrawal penalty before age 59½.

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

401(k) vs IRA: how they fit together

Priority order for most people:

1. Contribute to 401(k) through full employer match
2. Max Roth or Traditional IRA if eligible
3. Return to 401(k) and max remaining limit
4. Taxable brokerage after tax-advantaged accounts

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Early access rules

So what: Run your own inputs before you commit — small changes in assumptions can shift the outcome sharply.

Common mistakes

1. 2026 employee deferral limit: $23,500 (+ $7,500 catch-up if 50+) — this quietly costs you over time.
2. Employer match varies — common 50% up to 6% of salary..
3. Traditional 401(k) reduces taxable income now; Roth uses after-tax dollars — this quietly costs you over time.
4. Early withdrawal before 59½ usually incurs 10% penalty plus tax — this quietly costs you over time.

What to do next

Use our 401(k) Calculator to model your situation — change one input at a time to see what moves the result most.