For many taxpayers the number of allowances to enter on the form will be the same as the number of exemptions claimed on your tax return. See Line 6d of your last Form 1040 or Form 1040A. If you work more than one job or your spouse works or you itemize your deductions, you should use the work sheet that comes with the W-4 to calculate the number of allowances. If you are working more than one job, you should claim zero allowances on the job where you make the least amount of money. The IRS also has an online calculator that you can use to make the computation.