US paycheck explained: federal income tax vs FICA

Your first US paycheck almost always comes with a surprise: the number that hits your account is noticeably smaller than the salary you agreed to. That gap is the difference between gross pay (what you earn) and net pay (what you take home). Understanding the deductions in between — chiefly federal income tax and FICA — turns a confusing pay stub into something you can actually plan around.

Gross vs net

Gross pay is your salary or hourly wage before anything is removed. Net pay is what remains after taxes and any voluntary deductions like health insurance or 401(k) contributions. The mandatory taxes generally fall into three buckets:

• Federal income tax
• FICA (Social Security and Medicare)
• State (and sometimes local) income tax

Each works differently, so it’s worth taking them one at a time.

Federal income tax

Federal income tax is progressive, meaning it’s charged in brackets. You don’t pay your top rate on every dollar — you pay each rate only on the income that falls within that bracket. So a higher salary pushing you into a new bracket doesn’t suddenly tax all your income at the higher rate; only the portion above the threshold is.

Your employer doesn’t wait until tax season to collect this. They withhold an estimated amount from each paycheck and send it to the IRS on your behalf. How much they withhold depends on the Form W-4 you filled out when you started — it tells your employer about your filing status, dependents and any extra withholding you want. Get your W-4 right and your withholding roughly matches your real bill, so you neither owe a big amount nor lend the government money interest-free all year.

A couple of practical points:

• If you consistently get a large refund, you’re over-withholding — adjusting your W-4 puts more in each paycheck instead.
• Life changes (marriage, a second job, a new child) are good moments to revisit your W-4.

FICA: Social Security and Medicare

FICA is separate from income tax and funds two specific programs:

• Social Security, which supports retirement, disability and survivor benefits. It’s charged at a flat percentage of your wages up to an annual wage cap — earnings above that cap aren’t subject to the Social Security portion.
• Medicare, which helps fund health coverage for people 65 and older. It’s a flat percentage with no wage cap, and high earners pay a small additional Medicare amount above a certain income level.

The key differences from income tax: FICA rates are flat (not bracketed), and they apply from your very first dollar of wages — there’s no tax-free allowance. Your employer also matches your Social Security and Medicare contributions, though you only see your own half on the stub.

State tax varies — a lot

This is where two people on identical salaries can take home very different amounts. State income tax is set by each state, and the rules differ widely:

• Some states have no income tax at all.
• Others use flat rates.
• Many use their own progressive brackets, layered on top of federal tax.

A handful of cities and counties add local income taxes too. Because of this, “after-tax” pay is genuinely location-dependent, so always check the rules where you actually live and work.

Reading your pay stub with confidence

Once you know the categories, a pay stub stops being intimidating. Look for:

• Gross pay for the period.
• Federal withholding, driven by your W-4.
• Social Security and Medicare lines (that’s your FICA).
• State and any local withholding.
• Pre-tax deductions like 401(k) or health premiums, which lower your taxable income.

The most useful habit is checking that your year-to-date withholding is tracking sensibly against what you expect to owe. If it’s drifting, a quick W-4 update is far less painful than a surprise in April. Knowing which deduction does what means you can finally answer the only question that matters: where did the rest of my paycheck go?

Estimates and general information only — not financial or tax advice. Verify figures with an official source before acting on them.