How 401(k) Contributions Change Your Paycheck: The $400 That Costs $312

The mechanics: what "pre-tax" actually does

A traditional 401(k) contribution is deducted from your pay before federal income tax is calculated. Contribute $400 and the IRS taxes you as if you earned $400 less that month. In the 22% bracket, that's $88 of federal tax you don't pay — so your take-home drops by $400 − $88 = $312.

Two things pre-tax does not do:

• It doesn't reduce FICA. Social Security (6.2%) and Medicare (1.45%) are computed on your gross wages including 401(k) contributions. That $400 still carries its $30.60 of FICA either way.
• It doesn't eliminate the tax. Traditional contributions are tax-deferred, not tax-free — you'll owe income tax when you withdraw in retirement.

Worked example: $80,000 salary, 6% contribution

6% of $80,000 is $4,800 a year — $400 a month (about $184.62 per biweekly paycheck). For someone in the 22% federal bracket:

• Pre-tax contribution: $400.00/month
• Federal income tax saved: $88.00/month ($400 × 22%)
• FICA saved: $0.00 — still owed on the full amount
• Paycheck reduction: about $312/month (plus any state-tax savings, which vary)

In other words, you're buying $400 of retirement savings for $312 of spending power — a 22% instant discount courtesy of the tax code. Bump it to $800 a month and the paycheck cost is roughly $624. The math scales linearly: paycheck cost ≈ contribution × (1 − your marginal rate).

Most states follow the federal pre-tax treatment, which sweetens the deal further — but a few don't, so the state-tax piece of the discount varies by where you live.

Roth 401(k): the full-price version

Roth contributions are after-tax: the $400 comes out of pay that's already been taxed. So a $400 Roth contribution cuts your paycheck by the full $400 — no $88 discount today.

Why would anyone choose that? Because Roth withdrawals in retirement (contributions and growth, with the usual rules) are tax-free. Traditional gives you the discount now; Roth gives it to you later. Which wins depends on your tax rate today versus your tax rate in retirement — unknowable in advance, which is why many savers split the difference. The mechanics of each are compared in our Roth vs. traditional guide.

The move nobody does the math on

Here's the aggressive truth: most people anchor on the contribution number, not the paycheck number. Someone hesitating over "can I afford $400 a month?" is really asking "can I afford $312 a month?" — and at a 24% bracket it's $304, at 32% it's $272. The higher your bracket, the cheaper each dollar of savings is. The people for whom saving is cheapest are the ones who need the nudge least.

The other underused move: raising your rate by 1% a year. On $80,000, each 1% is $66.67 a month in contributions but only about $52 a month out of the paycheck at 22%. You barely feel it; compounding does the rest.

Run your own numbers

Your bracket, state, and pay frequency change the exact paycheck effect. Model it:

• Take-home pay calculator — see how retirement contributions change your actual paycheck
• What a pay raise really adds to your paycheck — the same marginal-rate math, in reverse
• Roth vs. traditional 401(k) and IRA — which account type fits your situation
• Is overtime taxed more? — another place marginal rates surprise people