The IRS provides a 2018 federal tax withholding calculator
The IRS provides a 2018 federal tax withholding calculator to help you make sure your withholdings are correct. Because of the: reductions to tax rates; elimination of dependent deductions; and increase in the Standard Deduction and other changes, it is wise to review your W-4 and revise it if necessary.
Even though your employer should be using the new tax tables, it is still a smart thing to do up to you to revise your W-4 so you don’t get a rude surprise when your taxes are prepared next year!
The IRS has provided a Withholding Calculator and link to the new W-4 form for 2018 and this IRS page also has a federal W-4 link.
Please also remember what you need for your federal W-4 may be different from what you need for Michigan (MI-W4) and what you need for your local withholdings if it applies to you.