You run your salary through a paycheck calculator, then look at your actual paystub from ADP, Gusto, Paychex, or Workday — and the numbers don’t match. Sometimes by $20, sometimes by $200. Neither number is “wrong.” Here’s what accounts for the gap.
1. Your W-4 elections
Online calculators usually assume a default W-4: single (or your selected status), no dependents, no extra withholding, Step 2 unchecked. Your real W-4 might claim dependent credits, a working spouse, or a flat extra amount per check. Each of these changes federal withholding — often by $50–$150 per paycheck.
2. Pre-tax benefits come out first
Health insurance premiums, dental, vision, 401(k), HSA, FSA, and commuter benefits are deducted before taxes are computed. A $300/month health premium reduces your taxable wages by $3,600 a year — so your real withholding is lower than a calculator that doesn’t know about it. Most calculators (including ours) let you enter annual pre-tax deductions; use that field and the gap shrinks dramatically.
3. Withholding tables vs. annual math
Payroll providers withhold using IRS Publication 15-T percentage tables applied per pay period; calculators typically compute your full-year liability and divide by pay periods. Over a full year these converge, but an individual check can differ slightly — especially after a raise, bonus, or mid-year start.
4. Bonuses and supplemental wages
Bonuses, commissions, and RSU vests are usually withheld at a flat 22% federal supplemental rate, not your W-4 rate. A paycheck containing any supplemental pay won’t match a regular-salary estimate.
5. Local and city taxes
State-level calculators can miss city income taxes: Detroit’s 2.4%, Philadelphia’s wage tax, Ohio municipal taxes, Kentucky occupational taxes, and dozens more. If your paystub shows a line your calculator doesn’t, this is often it.
6. State-specific payroll deductions
Some states withhold small extra amounts most calculators skip: California SDI (1.3%, no cap), New Jersey’s unemployment/disability/family-leave employee contributions, New York’s PFL premium, Washington’s PFML and Cares Fund. Individually small, together they can move a biweekly check $15–$40.
How to Get Within a Few Dollars
- Use your gross per-period pay from your paystub, not your offer-letter salary.
- Enter your actual pre-tax deductions (sum the pre-tax lines on your stub).
- Match your filing status and pay frequency exactly.
- Compare annual totals, not single checks, if you started mid-year or got a raise.
Try it with the federal paycheck calculator or pick your state — for example New Jersey, Michigan, Florida, or South Carolina — and enter your real numbers. If it’s still off by more than a few dollars per check, your W-4 likely has an election you’ve forgotten about — pull it up in your payroll portal and check Step 3 and Step 4.