Reason 1: federal income tax withholding
Your employer calculates federal withholding from your W-4 using IRS Publication 15-T. If you marked a higher filing status, claimed no dependents, or wrote in extra withholding on Step 4(c), your federal tax line gets bigger. Run the IRS Tax Withholding Estimator to see if your settings match your actual situation.
Reason 2: FICA
6.2 percent Social Security plus 1.45 percent Medicare equals 7.65 percent off the top of every dollar (no deduction reduces FICA wages except a Section 125 cafeteria plan). For someone earning $200,000, FICA is roughly $15,300 per year.
Reason 3: state income tax
If you live or work in a state with income tax, state withholding stacks on top of federal. California, New York, New Jersey, Oregon, Hawaii, Minnesota use progressive brackets. Pennsylvania, Illinois, North Carolina use a flat rate. Nine states have no state income tax at all.
Reason 4: local tax
NYC residents pay NYC income tax. Yonkers residents pay a Yonkers surcharge. Philadelphia residents pay city wage tax (non-residents working in Philly pay a slightly lower rate). Detroit, several Ohio cities, KCMO, St. Louis and others charge local tax.
Reason 5: supplemental wage withholding
If your paycheck included a bonus, commission, or back pay, that portion was likely withheld at the federal supplemental rate (22 percent up to $1M; 37 percent above). FICA also applies. The 22 percent is withholding, not your actual tax rate, your real bill is settled when you file.
Reason 6: pre-tax deductions
401(k), HSA and Section 125 health insurance reduce taxable wages. If you upped your 401(k) contribution mid-year, your federal tax withholding fell, but your net might also have fallen because the 401(k) deduction itself comes off your check.
What to do
Run the paycheck calculator with your gross, state and filing status. Compare the estimated federal tax to what is actually being withheld. If they differ wildly, check your W-4 with the IRS estimator.