Internal Revenue Service United States Department of the Treasury

When Will My New Form W-4 Go Into Effect? If the change is for the current year, your employer must put your new Form W-4 into effect no later than the start of the first payroll period ending on or after the 30th day after the day on which you give your employer your revised Form W-4.

If the change is for next year, your new Form W-4 will not take effect until next year.