How 401(k)s Work

If you have a 401(k), there is a reasonable chance you do not fully understand what is happening to the money leaving your paycheck each month. You are not alone. Most employees who have access to a 401(k) contribute too little, miss their employer match, or do not realize that the tax treatment which makes these accounts attractive is also the feature that can trip them up.

The 401(k) was not designed as a retirement savings vehicle. It was a tax break buried in a 1978 tax bill that nobody noticed at the time. The name comes from the section of the Internal Revenue Code that created it. Within a few years, companies realized they could replace expensive pension plans with a cheaper alternative: give employees a tax-advantaged account and let them figure it out. The shift from pension to 401(k) moved the risk of outliving your savings from your employer to you. Most people did not notice the swap.

The short answer

A 401(k) is a retirement savings account sponsored by your employer with special tax treatment. You contribute a portion of each paycheck, often with your employer adding matching funds up to a certain percentage. The money grows tax-free while invested, and you pay income tax when you withdraw after age 59 and a half. The tax advantages are real but the fees are real too, and the investment choices you make inside the account matter more than almost any other financial decision you will make.

The full picture

How contributions work

Every paycheck, a percentage of your salary goes into your 401(k) account before income taxes are calculated. If you earn $100,000 and contribute 10%, you have $90,000 subject to income tax, not $100,000. This is the core tax advantage: you defer the tax bill until retirement, when you may be in a lower tax bracket.

For 2026, the maximum you can contribute is $23,500 if you are under 50, or $31,000 if you are 50 or older with catch-up contributions (IRS contribution limits). These are annual limits that apply across all 401(k) plans you might have.

Your contributions are automatically deducted from your paycheck and deposited into your account. Most plans let you choose what percentage to contribute, change it at any time, and stop contributing without penalty.

The employer match

This is the part most likely to be misunderstood, and the part that matters most financially.

Many employers offer a matching contribution, meaning they add money to your 401(k) based on how much you contribute. The most common format is a dollar-for-dollar match on the first 3% of your salary, which means if you earn $80,000 and contribute at least $2,400 per year, your employer adds another $2,400. Some employers match 50 cents on the dollar up to a certain percentage.

The employer match is free money. It is that simple. If your employer offers a match and you are not contributing enough to get the full match, the immediate financial priority is to contribute enough to capture it. There are very few investment returns in life that are as reliable and guaranteed as an employer match.

The question of how much of your salary to contribute is personal and depends on your cash flow, other debts, and financial goals. Financial advisors often suggest 10-15% of your salary including the match. But the absolute minimum should always be enough to get the full match.

Where the money actually goes

The money in your 401(k) does not sit in cash. It is invested in a menu of options your employer selects, typically a list of mutual funds and sometimes ETFs, index funds, and target-date funds.

Target-date funds are the option most people default to without understanding. You pick a fund with a year close to when you plan to retire, such as 2055, and the fund automatically shifts from higher-risk stock-heavy investments to lower-risk bond-heavy investments as that year approaches. This is called rebalancing and it is designed to happen gradually without you having to think about it.

The fees inside a 401(k) are paid from the investment returns and are expressed as an expense ratio, the percentage of your investment that goes to the fund manager each year. Index funds typically charge 0.03-0.10% per year. Actively managed funds can charge 0.50-1.50% or more. Over 30 years, a 1% higher fee can cost you roughly 25% of your final account balance (SEC explanation of 401(k) fees).

This is why the cheapest funds in your 401(k) are almost always the best choice unless you have a specific reason to believe an actively managed fund will outperform its index over your time horizon. Most do not.

Traditional versus Roth

Some plans offer a Roth 401(k) option alongside the traditional 401(k). The difference is when you pay tax.

With a traditional 401(k), your contributions reduce your taxable income now. You pay ordinary income tax when you withdraw in retirement. With a Roth 401(k), you pay ordinary income tax on your contributions now, but all qualified withdrawals in retirement, including decades of growth, come out completely tax-free.

The right choice depends on whether you expect to be in a higher or lower tax bracket in retirement. A young person earning $60,000 who expects their income to grow substantially may well be in a higher bracket in retirement, making the Roth attractive. A high earner expecting a similar lifestyle and tax bracket in retirement may prefer the traditional deduction now.

Most people with access to both should consider splitting contributions between the two to hedge their tax bet.

The vesting question

You are always 100% vested in your own contributions from day one. That money is yours regardless of how long you stay with the employer.

Employer matching contributions are a different story. Most plans impose a vesting schedule on the match, meaning you only keep the employer contributions if you stay long enough.

The most common schedule is graded vesting over five or six years, where you earn 20% of the match per year. Under this schedule, if you leave after two years, you keep 40% of the employer match. A minority of plans have cliff vesting, where you get nothing until three years of service, then you are 100% vested immediately.

If you are job hunting, the vesting schedule is one of the most financially significant details in the benefits package. Leaving a job just before you are fully vested can cost you thousands of dollars in employer contributions.

What happens when you change jobs

When you leave a job, you have several options for your 401(k). You can leave it where it is if your plan allows small balances to remain. You can roll it into your new employer’s 401(k) if the new plan accepts rollovers. You can roll it into an IRA, which gives you more investment choices but removes the option to borrow against it. Or you can cash it out, which triggers immediate income tax plus a 10% penalty if you are under 59 and a half.

Rolling over into an IRA at a brokerage like Vanguard or Fidelity is often the best choice for people who have left a job, because IRA fees are typically lower than 401(k) fees and the investment menu is wider.

Why it matters

The 401(k) is the primary retirement savings vehicle for most working Americans, yet the median 401(k) balance for workers in their 60s is under $200,000 (Federal Reserve Survey of Consumer Finances). At a 4% withdrawal rate, that generates roughly $8,000 per year, not enough to supplement Social Security comfortably.

The problem is not that 401(k)s are bad. The problem is that contribution rates are too low, fees are too high, and the match is left on the table too often. A worker earning $70,000 who contributes 6% with a 3% employer match is saving $6,300 per year. Over 35 years at 7% average returns, that becomes roughly $900,000. The math is compelling. The behavioral problem is that 6% feels like a lot to take out of a monthly paycheck.

Starting early matters enormously. A 25-year-old who saves $5,000 per year until 65 at 7% returns accumulates roughly $1.1 million. A 35-year-old who starts saving the same amount accumulates roughly $505,000. The ten-year delay more than halves the outcome. Time is the dominant variable in retirement savings, not contribution size, not fund selection.

Common misconceptions

“I should maximize my 401(k) before paying off debt.” Not always. If you have high-interest debt like credit cards at 20%+, the guaranteed return from paying that off exceeds most investment returns. A reasonable strategy is to contribute enough to get the full employer match, then attack high-interest debt, then return to maximizing the 401(k).

“My employer’s plan is good because it has many fund options.” More options are not better if the cheapest options are still expensive and the index funds are underperforming. Evaluate the expense ratios and the index fund lineup, not the total number of funds.

“I should stop contributing to avoid paying higher taxes this year.” This is almost always wrong. Deferring income to a lower tax bracket in retirement is only possible if you actually have the money to contribute. The tax benefit of a 401(k) is a bonus, not the reason to contribute. The reason to contribute is the compound growth over decades.

“I will figure out my 401(k) when I am closer to retirement.” By then, the cost of starting late is already locked in. The best time to set a contribution rate and investment strategy is the first day you have access to the plan.

Key terms

Employer match — Money your employer adds to your 401(k) based on how much you contribute. Always capture the full match before other financial priorities.

Expense ratio — The annual fee charged by a fund, expressed as a percentage of your investment. Lower is better. Index funds almost always have the lowest expense ratios.

Roth 401(k) — A 401(k) variant where you pay income tax on contributions now but withdraw tax-free in retirement. Compare against traditional 401(k) based on expected future tax rates.

Target-date fund — A fund that automatically shifts from stocks to bonds as you approach a target retirement year. The simplest 401(k) investment option for people who do not want to manage their own allocation.

Vesting — The schedule that determines when you own employer contributions. You are always 100% vested in your own contributions immediately. Employer matching typically vests over three to six years.

Roll over — Moving a 401(k) from a former employer plan to a new employer plan or IRA without triggering a taxable distribution.