PAYROLL GLOSSARY
W-4
Definition: Form W-4 is the IRS Employee's Withholding Certificate used by employees to tell employers how much federal income tax to withhold.
Employees complete Form W-4 and give it to their employer so payroll can withhold the correct federal income tax from each paycheck. The current form focuses on filing status, multiple jobs, dependents, other income, deductions, and any extra withholding the employee wants. It is not a tax return and it does not change Social Security or standard Medicare percentages.
The reason W-4 matters is that it controls the federal withholding inputs used with IRS Publication 15-T payroll tables. A stale W-4 can cause over-withholding or under-withholding even when salary has not changed. The IRS recommends revisiting the form when your personal or financial situation changes and using the Tax Withholding Estimator if you are not sure what to enter.
In practical terms, W-4 is one of the highest-leverage paycheck inputs because small changes there can meaningfully move take-home pay while gross wages stay the same.