W-4

Employee's Withholding Certificate

US form employees complete so employers know how much federal income tax to withhold.

When you'd see it: Onboarding and anytime an employee's tax situation changes. The W-4 tells payroll how much federal tax to withhold.

Why it matters: The W-4 sets an employee's federal income-tax withholding. Get it wrong and the employee either overpays all year or owes a surprise bill at tax time.

Common mistakes: Confusing the W-4 (employee fills out, sets withholding) with the W-2 (employer issues, reports annual earnings).