Does a 401k Reduce Taxable Income? What a 1% Contribution Costs Your Paycheck

Does a 401k reduce taxable income? Yes: a traditional contribution comes out of your pay before federal income tax is worked out, so your taxable income drops by the amount you defer. On a $60,000 salary paid every two weeks, $2,307.69 a check, moving 1% of pay into the plan sends $23.08 to your 401(k) and lowers your net check by only about $20.31. The missing $2.77 comes off your federal withholding instead. That is the point of running the numbers with pre-tax deductions in them: the cost is never the full contribution.

Pre-tax money leaves before income tax is worked out

Payroll happens in an order, and that order is what saves you money. Gross pay comes first. Then pre-tax deductions are subtracted. Whatever is left is the wage figure your employer runs through the federal withholding tables. A traditional 401(k) deferral sits in that pre-tax group, so the week your contribution goes up, your federal withholding goes down.

The yes or no answer is in the FAQ on whether a 401(k) reduces taxable income, and the 401(k) glossary entry covers what the account itself is. For the full order of operations on a stub, read how to calculate take-home pay.

Start from the plain version of this salary: a single filer, standard W-4, no pre-tax deductions. Federal withholding is $193.08 a check. FICA is $176.54, made up of $143.08 of Social Security and $33.46 of Medicare. Before state tax, that leaves $1,938.07 in the bank.

What a one percent bump costs per paycheck

One percent of $60,000 is $600 a year, which is $23.08 on a biweekly check. A $60,000 salary with the standard deduction puts a single filer in the 12% federal bracket, so each pre-tax dollar cuts federal withholding by about 12 cents.

Contribution rate	Into the 401(k)	Federal withholding	Take-home	Drop in take-home
0%	$0	$193.08	$1,938.07	none
1%	$23.08	$190.31	$1,917.76	$20.31
3%	$69.23	$184.77	$1,877.15	$60.92
5%	$115.38	$179.23	$1,836.54	$101.53
6%	$138.46	$176.46	$1,816.23	$121.84
10%	$230.77	$165.39	$1,734.99	$203.08

So going from nothing to 6% of pay, which is a common level, puts $138.46 a check into the account for $121.84 of lost spending money. The higher your bracket, the better that trade gets. In the 22% bracket the same $23.08 contribution costs about $18 of take-home pay.

Those figures cover federal withholding and FICA only. In almost every state the contribution lowers state withholding as well, which makes the real cost smaller than the drop shown above. The California paycheck calculator shows that state line, and every other state has its own page.

Traditional and Roth contributions compared on the same salary

Take 5% of the same $60,000. That is $115.38 a check, $3,000 a year, and both kinds of 401(k) put the same $3,000 into the account. They do not cost the same today.

With a traditional contribution, the wages used for withholding fall to $2,192.31. Federal withholding becomes $179.23, FICA stays $176.54, and take-home pay is $1,836.54.

With a Roth contribution, the money is taken after income tax is figured. Withholding stays at $193.08 and take-home pay is $1,822.69.

The gap is $13.85 a check, about $360 a year. That $13.85 is tax you are choosing to pay now instead of later. Roth money then comes out tax free in retirement under the plan’s rules. Traditional is the reverse deal: cheaper per check now, taxed when you withdraw.

The taxes a 401(k) does not reduce

Social Security and Medicare are figured before the 401(k) line is taken. At every rate in the table above, Social Security stays $143.08 and Medicare stays $33.46, because both still apply to the full $2,307.69. No contribution rate and no W-4 change will move them.

Some other pre-tax items do cut FICA:

Deduction	Cuts income tax wages?	Cuts FICA wages?
Traditional 401(k)	Yes	No
Roth 401(k)	No	No
HSA through payroll	Yes	Yes
Health or dependent care FSA	Yes	Yes

Put $100 a check into an HSA and FICA wages fall to $2,207.69, so Social Security drops to $136.88 and Medicare to $32.01. That is $7.65 a check a 401(k) never saves you.

One state exception is worth knowing. Pennsylvania taxes elective contributions as you earn them, so a traditional 401(k) does not lower Pennsylvania state withholding the way it lowers federal.

Employer match, and why it never shows in net pay

A match is not pay to you. It goes straight into your retirement account, so it is not in your gross pay, it is not taxed in the year it lands, and it changes nothing on your stub except maybe an informational line.

Match formulas vary a lot by employer, so read your plan document rather than guessing. If a plan matched 50 cents per dollar up to 6% of pay, then at 6% on this salary you would put in $3,600 for the year and the employer would add $1,800. Your paycheck still only feels the $121.84 a check from the table.

Picking a rate you can live with this year

The safest way to find your number is one point at a time. Raise the rate, wait for the next stub, and decide from the real figure rather than the one in your head. If there is a match, getting the full match is usually the first target, since that part is money you are leaving behind otherwise.

There is also a yearly cap on what you can defer, with a higher cap once you are 50 or older. The IRS contribution limit page has the current figures. A rate change mid-year only affects the checks that are left, so a rate set late in the year moves your yearly total far less than you might expect.

Run it on your own pay

Enter your salary, pay frequency, filing status and state in the paycheck calculator on the home page, then add your contribution as a pre-tax deduction. You will see the federal, Social Security, Medicare and state lines change together, and the net figure at the bottom is the one that tells you whether the rate is livable.