The Tax Cuts and Jobs Act lowered tax rates and made other changes that impact withholding on wages and other taxable compensation. Employees do not have to complete a revised Form W-4; employers will compute withholding based on existing W-4s on file.
However, you may want to complete a new Form W-4 to change your withholding allowances. This may be needed to increase or reduce withholding to better approximate what you’ll owe in taxes for the year. To help you determine whether a new form is necessary, you can use an IRS online withholding calculator (https://apps.irs.gov/app/withholdingcalculator/). It’s anonymous; you don’t have to put in your name or Social Security number to use it.