What happens when you put money into a 401(k)

When you contribute to a 401(k), your employer or plan administrator holds the money in an account registered with your name and Social Security number. You do not own the cash itself — you own shares or units in the investment funds you choose. The money grows or shrinks based on how those funds perform in the market. Your employer may add matching contributions (usually a percentage of what you put in), which is immediate gain on top of your own money.

The account is tax-deferred, meaning you do not pay federal income tax on the money or its growth until you withdraw it in retirement. Some plans offer a Roth 401(k) option, where you pay tax now but withdrawals in retirement are tax-free. The trade-off is immediate: standard 401(k) contributions lower your taxable income this year; Roth contributions do not.

You cannot touch the money without penalty until age 59½, with narrow exceptions for hardship or separation from your employer. The account stays in your name even if you change jobs — you can leave it where it is, roll it to your new employer's plan, or move it to an individual retirement account (IRA).

Key Takeaways

  • Your 401(k) contributions go into mutual funds or target-date funds you select, not into a savings account, and their value changes daily with the market.
  • Most employers match a percentage of your contribution — typically 3 to 6 percent — which is money you forfeit if you do not contribute enough to capture it.
  • You choose how to split your money among the funds your plan offers, and you can change that split once or twice a year, or when you change jobs.
  • Withdrawals before age 59½ trigger a 10 percent penalty plus income tax, with limited exceptions for hardship, loans, or separation from your employer.
  • If you leave your job, you can roll your 401(k) into a new employer plan or an IRA without tax consequences, but you must complete the rollover within 60 days if you take the money yourself.

How to choose which funds to invest in

Your plan administrator sends you a document called the Summary Plan Description or a fund menu listing every investment option available. Most plans offer 10 to 25 funds. Read the fund names and descriptions carefully — they tell you what the fund invests in and how risky it is.

Common fund types are: stock funds (higher risk, higher potential growth), bond funds (lower risk, lower returns), money market funds (very safe, very low returns), and target-date funds (a mix that automatically shifts from stocks to bonds as you approach retirement). A target-date fund labeled "2050" is designed for someone retiring around 2050 and handles the rebalancing for you — this is the simplest choice if you do not want to think about it.

Each fund has a fee, usually listed as an expense ratio (a percentage of your balance charged annually). Lower is better. A fund charging 0.05 percent costs far less than one charging 1.0 percent over decades. Your plan statement shows the expense ratio for each fund. If your plan offers a low-cost index fund (often tracking the S&P 500 or total stock market), that is usually a solid default choice.

You do not have to pick one fund. Most people split their money across several — for example, 60 percent in a stock index fund and 40 percent in a bond fund. This is called diversification and reduces the damage if one type of investment drops sharply.

How much to contribute and when

You decide what percentage of your paycheck goes into the 401(k), up to an annual limit set by the IRS. For 2024, that limit is $23,500 (or $31,000 if you are 50 or older). Your employer deducts your contribution before calculating your paycheck, so the money never hits your bank account.

The most common advice is to contribute enough to capture your full employer match. If your employer matches 100 percent of contributions up to 6 percent of your salary, you should contribute at least 6 percent — that is assistance programs. Leaving it on the table is the same as refusing a raise.

After capturing the match, contribute what you can afford. Many people start with 3 to 5 percent and increase it by 1 percent each year, or when they get a raise. The more you contribute now, the more time your money has to grow before retirement. A 25-year-old who contributes $500 a month will have far more at 65 than a 45-year-old who contributes the same amount.

You can change your contribution amount whenever your plan allows — usually once or twice a year, or whenever you have a life event like a marriage, birth, or job change. Check your plan's rules or ask your HR department.

Rebalancing and adjusting your investments over time

Once you choose your funds, the market will move them out of balance. If stocks rise faster than bonds, your stock funds will grow to a larger share of your total balance than you intended. Rebalancing means selling some of the funds that have grown and buying more of the ones that have fallen, to return to your original split.

Most people rebalance once a year, usually in December or when they review their statement. Some plans offer automatic rebalancing — you set it once and the plan does it for you. This forces you to buy low and sell high, which is the opposite of what emotions tell you to do and is why it works.

As you get closer to retirement, shift gradually from stocks to bonds. A common rule is to hold your age in bonds — if you are 50, hold 50 percent bonds and 50 percent stocks. This reduces the damage if the market crashes the year before you retire. If your plan offers a target-date fund matching your retirement year, it does this automatically.

What happens to your 401(k) if you change jobs

Your 401(k) belongs to you, not your employer. When you leave the job, the money stays in your account. You have four choices: leave it in your old employer's plan, roll it into your new employer's plan, roll it into an individual retirement account (IRA), or withdraw it.

Rolling it over (moving it without cashing it out) avoids taxes and penalties. If your old employer sends you a check, you have 60 days to deposit it into a new retirement account or you owe income tax and a 10 percent penalty on the full amount. The safest method is a direct rollover, where the old plan sends the money straight to the new plan or IRA — you never touch it and no clock starts.

An IRA gives you more fund choices than most employer plans and usually lower fees. A 401(k) offers higher contribution limits and sometimes a loan option (borrow from your own account and repay it). If your new employer's plan has high fees or few fund options, rolling to an IRA is often smarter. If the new plan is good, rolling into it keeps everything in one place.

Fees and how they affect your money

Every 401(k) has costs. The plan itself may charge an annual administration fee (often $50 to $300 per year). Each fund charges an expense ratio, usually between 0.05 and 1.5 percent annually. Some plans also charge per-transaction fees if you trade frequently.

These fees compound over decades. A fund charging 1.0 percent instead of 0.10 percent will cost you roughly 10 percent of your final balance by retirement — that is real money. Your plan statement lists all fees. If your plan's fees are high and you cannot change funds, rolling to an IRA when you leave the job can save thousands.

Your employer may subsidize some fees, especially if the plan is large. Ask your HR department or plan administrator for a breakdown of all costs. Many plans now offer low-cost index funds specifically to keep overall expenses down.

Withdrawals before retirement and penalties

Taking money out before age 59½ triggers a 10 percent early withdrawal penalty plus income tax on the amount withdrawn. A $10,000 withdrawal might net you $7,000 after a 20 percent tax rate and the 10 percent penalty.

Exceptions exist: you can withdraw without penalty if you are permanently disabled, face a severe financial hardship (medical bills, eviction, foreclosure), or separate from your employer after age 55. Some plans allow loans — you borrow from your own account and repay it with interest, which goes back into your account. If you leave the job, you can take a distribution, but it is still subject to tax and penalty unless you roll it over.

Hardship withdrawals require documentation and plan approval. The rules vary by plan, so ask your administrator what qualifies. Even if you meet the hardship test, you may have to exhaust other options (like a loan) first.

Frequently Asked Questions

Can I invest in individual stocks through my 401(k)?

Most employer 401(k) plans do not offer individual stocks — only mutual funds, index funds, and target-date funds. Some large plans include a brokerage window that lets you buy stocks, but this is rare and usually charges extra fees. IRAs are more flexible and allow individual stock purchases through most brokers.

What if I do not choose any funds — what happens to my money?

Many plans have a default fund, usually a target-date fund or money market fund. If you do not make a choice, your contributions go there automatically. Check your plan documents to see what the default is. It is better to choose intentionally than to accept the default by accident.

Should I invest more aggressively when I am young?

Younger workers have more time to recover from market downturns, so a higher stock allocation (70 to 90 percent stocks) is common. As you approach retirement, shift to a more conservative mix. A target-date fund does this automatically. Your comfort with risk matters too — if a market drop keeps you awake at night, a more conservative mix is right for you even if you are young.

Can I borrow from my 401(k)?

Many plans allow loans, usually up to 50 percent of your balance or $50,000, whichever is less. You repay it with interest (the rate is set by your plan, usually prime rate plus 1 percent). The interest goes back into your account. If you leave your job, the loan is usually due within 60 days or it becomes a taxable withdrawal. Loans are a last resort because you lose growth on the borrowed money.

What is the difference between a 401(k) and a 403(b)?

A 403(b) is the nonprofit and public school version of a 401(k). The rules are similar — tax-deferred contributions, employer match, investment in funds, withdrawal penalties before 59½. The fund options and fees often differ. If your employer offers one, the same principles apply: choose low-cost funds, capture the full match, and rebalance annually.