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.
Test for determining deductibility of IRA deductions. Active participants in employer retirement plans are subject to IRA deduction phase-out rules if adjusted gross income exceeds certain threshold.