If your W-4 form's out of date, it may be worthwhile to adjust it. This handy online calculator from the IRS will help you determine the necessary information to check your withholding form and, if necessary, learn how to fill out a new W-4 form.

Who can benefit from using the calculator?
Employees who would like to change their withholding, whose situations are only approximated by the worksheets, or those with non-wage income who would prefer to have tax on that income withheld are all good candidates for the withholding calculator.

What do I need to use the calculator?
Be sure to have your most recent pay stubs, income tax return, and estimates that are as accurate as possible on hand before using the calculator.

If, after using the calculator, you find that you need to change your withholding, simply use the results to help you complete a new W-4 form and give it to your employer.

IRS Withholding Calculator [IRS]