IRS Withholding Calculator
"The purpose of this application is to help employees to ensure that they do not have too much or too little income tax withheld from their pay. It is not a replacement for Form W-4, but most people will find it more accurate and easier to use than the worksheets that accompany Form W-4. You may use the results of this program to help you complete a new Form W-4, which you will submit to your employer." If you can parse this paragraph, you may find this tool useful.