Yes, most 401(k) contributions are pre-tax, which means they reduce your taxable income in the year you make them

When you contribute to a traditional 401(k), the money comes out of your paycheck before federal income tax is calculated. Your employer deducts the contribution amount, and that money does not show up on your W-2 as wages. This lowers the total income the IRS taxes you on for that year.

A $10,000 contribution to a traditional 401(k) means you pay federal income tax on $10,000 less of your salary. If you earn $60,000 and contribute $10,000 to a traditional 401(k), you pay federal income tax only on $50,000. You still pay Social Security and Medicare taxes (FICA) on the full $60,000, but not income tax.

This is different from a Roth 401(k), which is post-tax. Roth contributions do not reduce your current taxable income. You pay income tax on the money before it goes into the account, but withdrawals in retirement are tax-free. Most 401(k) plans offer only the traditional pre-tax version, though some employers offer both.

Key Takeaways

  • Traditional 401(k) contributions lower your taxable income for the year you make them, reducing the federal income tax you owe.
  • You still pay Social Security and Medicare taxes on 401(k) contributions, even though you do not pay income tax on that money.
  • Roth 401(k) contributions are post-tax, meaning they do not reduce your current taxable income but withdrawals in retirement are tax-free.
  • The tax benefit of a traditional 401(k) is deferred — you pay income tax on the money when you withdraw it in retirement, not when you contribute it.
  • Your employer may match your contributions, and those matching dollars are also pre-tax and do not count toward your taxable income.

How the tax deduction works on your paycheck and tax return

Your payroll department handles the pre-tax deduction automatically. When you enroll in a 401(k) and choose a contribution amount, that amount is subtracted from your gross pay before taxes are withheld. Your employer then sends the contribution directly to the 401(k) plan administrator.

On your W-2 form at the end of the year, the "Wages, tips, other compensation" box (Box 1) shows your income after the 401(k) deduction has already been taken out. You do not have to claim the deduction again on your tax return — it is already reflected in the W-2 amount. This is why a pre-tax 401(k) reduces your tax bill automatically, without any extra paperwork on your part.

If you contributed $8,500 to a traditional 401(k) during the year and your gross salary was $55,000, your W-2 will show $46,500 in Box 1. You file your tax return based on that $46,500 figure, not the original $55,000.

The difference between pre-tax and post-tax 401(k) contributions

A traditional 401(k) is pre-tax. You reduce your taxable income now and pay income tax later when you withdraw the money in retirement. A Roth 401(k) is post-tax. You pay income tax on the contribution now, but withdrawals in retirement are tax-free.

The choice between them depends on whether you expect your tax rate to be higher or lower in retirement than it is now. If you think you will be in a lower tax bracket in retirement, a traditional 401(k) makes sense — you get the deduction at a higher rate now and pay tax at a lower rate later. If you think your tax rate will be the same or higher in retirement, a Roth 401(k) may be better — you lock in today's tax rate and avoid tax on growth.

Not all employers offer a Roth 401(k) option. If your plan offers only a traditional 401(k), pre-tax is your only choice through that plan. Some people contribute to both a traditional 401(k) at work and a Roth IRA separately, but the 401(k) itself is either one or the other.

Annual contribution limits and how they interact with taxes

The IRS sets an annual limit on how much you can contribute to a 401(k). For 2024, the limit is $23,500 for people under 50 and $31,000 for people 50 and older (the extra $7,500 is called a catch-up contribution). These limits apply to the total of your own contributions; employer matching does not count toward your limit.

The full amount you contribute, up to the limit, reduces your taxable income if you use a traditional 401(k). If you contribute $15,000 to a traditional 401(k) in a year when you earn $70,000, you pay federal income tax on $55,000. If you contribute $23,500 (the maximum), you pay federal income tax on $46,500.

Limits change each year based on inflation. Check your plan documents or the IRS website for the current year's limit. Your employer's payroll system will not let you contribute more than the legal limit, so you do not have to track this yourself — the system enforces it.

When you withdraw the money and pay tax on it

With a traditional 401(k), you defer paying income tax until you withdraw the money. Once you reach age 59½, you can withdraw money from your 401(k) without a penalty. When you do, that withdrawal counts as income for that year, and you pay federal income tax on it at your ordinary income tax rate.

If you withdraw $40,000 from your traditional 401(k) in a year when you have no other income, you pay federal income tax on that $40,000. The amount of tax depends on your tax bracket for that year. You also pay state income tax on the withdrawal in most states.

If you withdraw money before age 59½, you generally owe a 10% early withdrawal penalty on top of the income tax, unless an exception applies (such as disability, medical expenses, or a hardship withdrawal under specific rules). This is why a 401(k) is meant to be long-term savings — the tax deferral works best if you leave the money alone until retirement.

Employer matching and how it affects your taxes

Many employers offer to match your 401(k) contributions — for example, matching 50% of what you contribute, up to 6% of your salary. This matching money is also pre-tax. It does not show up on your W-2 as taxable wages, and it does not count toward your annual contribution limit.

If your employer matches $5,000 of your contributions in a year, that $5,000 is pre-tax just like your own contributions. It reduces your taxable income and grows tax-deferred inside the account. When you withdraw it in retirement, you pay income tax on it then, just like your own contributions.

Employer matching is one of the main reasons to contribute to a 401(k) — it is essentially assistance programs, and the tax deferral makes it even more valuable. If your employer offers a match, contributing at least enough to get the full match is usually a smart financial move.

State income tax and 401(k) contributions

Most states treat 401(k) contributions the same way the federal government does — as pre-tax deductions that reduce your state taxable income. If you live in a state with income tax and contribute $12,000 to a traditional 401(k), you reduce both your federal and state taxable income by $12,000.

A few states do not have income tax (such as Texas, Florida, and Wyoming), so the state tax question does not apply. If you live in one of those states, you only benefit from the federal tax deduction. If you move to a different state in retirement, the tax treatment of your 401(k) withdrawals may change depending on that state's rules.

Frequently Asked Questions

Do I pay Social Security and Medicare taxes on 401(k) contributions?

Yes. Even though 401(k) contributions are pre-tax for federal income tax purposes, you still pay Social Security tax (6.2%) and Medicare tax (1.45%) on the full amount of your salary, including the part that goes into the 401(k). Only federal income tax is avoided.

Can I change from traditional to Roth 401(k) mid-year?

Yes, most plans allow you to change your contribution type during the year or during the annual open enrollment period. If you switch from traditional to Roth, future contributions will be post-tax, but past contributions for that year remain as they were made. Check with your plan administrator about the timing and process.

What happens to my tax deduction if I leave my job?

The tax deduction is already locked in on your W-2 for the year you made the contributions. If you leave your job mid-year, your W-2 will show only the contributions you made up to that point. You can roll the 401(k) balance to an IRA or your new employer's plan without triggering taxes, as long as you do a direct rollover.

Do I have to pay taxes on 401(k) growth and investment gains?

No, not while the money is in the account. All growth, dividends, and investment gains inside a traditional 401(k) are tax-deferred. You pay income tax only when you withdraw the money in retirement. This tax-deferred growth is one of the main advantages of a 401(k).

What if my income is too high for a Roth IRA but I want post-tax retirement savings?

If your employer offers a Roth 401(k), you can use that regardless of income. There are no income limits for Roth 401(k) contributions, unlike Roth IRAs. This makes a Roth 401(k) a valuable option for higher earners who want tax-free retirement withdrawals.