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Gross pay, pay frequency, filing status, state
These four set the shape of the estimate. Gross pay can be an annual salary or an hourly rate; if you're paid hourly, add your expected weekly hours so the calculator can multiply out a realistic week. Filing status chooses the withholding tables, and the state applies the income tax rules that sit on top of federal withholding.
Nothing here is sent anywhere or filed. It's an estimate you use to plan, then reconcile against your first real paycheck after any change.