What payroll taxes are and where they come from

Payroll taxes are amounts withheld from your paycheck by your employer and sent to federal and state tax agencies. They fund Social Security, Medicare, and income tax. When you see your pay stub, the gross amount (what you earned) minus these withholdings equals your net pay (what you take home).

Your employer calculates these withholdings based on information you provided on Form W-4 when you were hired. The W-4 tells your employer how many allowances you claim, which affects how much federal income tax comes out. The employer then uses IRS tax tables and your state's tax rates to figure the exact dollar amounts.

You do not calculate payroll taxes yourself — your employer does. But understanding how the math works helps you read your pay stub, spot errors, and know what to expect.

Key Takeaways

  • Federal income tax withholding depends on your W-4 allowances, your pay frequency, and current IRS tax tables that change yearly.
  • Social Security tax is 6.2% of your gross pay up to an annual cap (the cap changes each year), and Medicare tax is 1.45% with no cap.
  • State and local income tax rates vary by where you live and work, and some states have no income tax at all.
  • Your pay stub shows gross pay, each withholding amount, and net pay; comparing these numbers to the formulas below reveals calculation errors.
  • If too much or too little is withheld, you adjust your W-4 and your employer recalculates starting with your next paycheck.

Federal income tax withholding calculation

Federal income tax withholding uses three pieces of information: your W-4 form, your pay frequency (weekly, biweekly, monthly), and the IRS withholding tables published each year. Your employer looks up your gross pay in the table that matches your filing status and pay frequency, then subtracts an amount for each allowance you claimed on your W-4.

The IRS publishes new withholding tables every January. If you earned $1,200 biweekly, claimed single with one allowance, and live in a state with no income tax, the 2024 table would tell your employer to withhold a specific amount — for example, $137. If you claimed zero allowances instead, the withholding would be higher because you have no allowances to subtract.

The exact dollar amount depends on the year's table, so withholding changes annually even if your pay and W-4 stay the same. Your employer receives these tables and applies them automatically; you see the result on your pay stub as "Federal Income Tax" or "FIT".

Social Security and Medicare tax calculation

Social Security tax is straightforward: 6.2% of your gross pay, but only up to a wage cap that the IRS sets each year. In 2024, that cap is $168,600 — meaning once you earn that much in a calendar year, no more Social Security tax comes out for the rest of the year. Your employer tracks your year-to-date earnings and stops withholding once you hit the cap.

Medicare tax is 1.45% of your gross pay with no cap — it comes out on every dollar you earn, all year. If you earn over $200,000 as a single filer (or $250,000 married filing jointly), an additional 0.9% Medicare tax applies to the amount above that threshold. Most employees do not hit this threshold, so they see only the standard 1.45%.

Together, Social Security and Medicare are often called FICA taxes (Federal Insurance Contributions Act). On a $2,000 biweekly paycheck, Social Security would be $124 (6.2% of $2,000) and Medicare would be $29 (1.45% of $2,000), assuming you have not yet hit the Social Security wage cap for the year.

State and local income tax calculation

State income tax rates and rules vary widely. Some states have no income tax at all (Alaska, Florida, Nevada, South Dakota, Tennessee, Texas, Washington, Wyoming). Others tax income at a flat rate — Colorado and Illinois, for example, use a single percentage applied to all income. Many states use tax brackets similar to federal tax, where different portions of your income are taxed at different rates.

Your employer withholds state tax based on your state's tax tables and your W-4 equivalent form (some states call it a state withholding certificate). If you work in one state but live in another, you typically pay tax to the state where you work, though some states have reciprocal agreements that change this rule.

Local income taxes exist in some cities and counties — Ohio, Pennsylvania, and Kentucky have local taxes in certain areas. Your employer withholds these too if you work in a locality that has them. The rate depends on your local jurisdiction and is added to your state withholding.

Reading your pay stub to verify the math

Your pay stub lists gross pay at the top, then shows each withholding as a separate line: Federal Income Tax, Social Security, Medicare, State Income Tax, and any local taxes. It also shows year-to-date totals for each category. Use these numbers to spot-check your employer's math.

For Social Security, multiply your gross pay by 0.062 (6.2%). The result should match the "Social Security" line on your stub, unless you have already hit the annual wage cap. For Medicare, multiply gross pay by 0.0145 (1.45%). For federal income tax, the math is harder to verify without the IRS tables, but you can compare your withholding to previous paychecks — if nothing changed and the withholding jumped, ask your payroll department why.

If you spot a discrepancy, contact your payroll or HR department with your pay stub in hand. Common errors include wrong W-4 information in the system, incorrect pay frequency entered, or a failure to stop withholding Social Security tax after hitting the wage cap.

What to do if withholding is wrong

If you are having too much withheld, you will owe less at tax time or receive a refund. If too little is withheld, you will owe money when you file. To adjust, fill out a new Form W-4 and give it to your HR or payroll department. Changes take effect on your next paycheck.

Increasing your allowances on the W-4 reduces federal income tax withholding. Decreasing allowances increases it. You can also claim dependents or other credits on the W-4 to fine-tune the amount. The IRS provides a withholding calculator on its website (irs.gov) to help you figure out what to claim.

State withholding adjustments work the same way — fill out a new state W-4 equivalent and submit it to payroll. If you move to a different state, notify payroll when ready so they can update your state tax withholding.

Why your withholding might change year to year

Even if your pay and W-4 stay the same, your withholding can change because the IRS updates tax tables every January. Inflation and tax law changes mean the tables shift, which affects how much comes out. Your employer automatically uses the new tables, so you may see a small change in your take-home pay without doing anything.

If you have a major life change — marriage, divorce, a second job, or a child — your withholding should change too. These events affect your tax situation and your W-4. The IRS recommends reviewing your W-4 whenever your life changes or at least once a year.

Frequently Asked Questions

Why is my Social Security tax withholding different in December than it was in January?

You likely hit the annual Social Security wage cap. Once your year-to-date earnings reach the cap (set by the IRS each year), your employer stops withholding Social Security tax for the rest of the calendar year. Medicare tax continues all year with no cap.

Can I claim zero allowances on my W-4 to have more withheld?

Yes. Claiming zero allowances increases federal income tax withholding. You can also claim negative allowances (the W-4 form allows this) to withhold even more if you expect to owe a large amount at tax time.

What happens if my employer withholds the wrong amount?

You will either owe money or receive a refund when you file your tax return. If the error is large or ongoing, contact your payroll department with your pay stub to have them correct the W-4 information in their system. Changes take effect on your next paycheck.

Do I have to pay state income tax if I work in a state with no income tax?

No state income tax is withheld if you work in one of the nine states with no income tax. However, if you live in a state that has income tax and work in a no-tax state, you may still owe tax to your home state depending on that state's rules.

Is the Social Security wage cap the same every year?

No. The IRS adjusts the Social Security wage cap each January based on wage growth. It has been between $160,000 and $170,000 in recent years, but the exact amount changes annually.