If you need to calculate your allowance, you can use the Personal Allowance Worksheet that comes with the form. Here are some instructions on it:
Start with the Personal Allowances Worksheet. Go to lines A through H. The questions asked on these worksheets relate to the current tax year. Also, married couples who anticipate filing jointly should calculate their withholding allowances jointly, and then divide those allowances between themselves. "If both you and your spouse are employed and expect to file a joint return, figure your withholding allowances using your combined income, adjustments, deductions, exemptions, and credits. Use only one set of worksheets. You can divide your total allowances any way, but you cannot claim an allowance that your spouse also claims" (from Publication 505).
If you will be taking the standard deduction and personal exemptions only and no other deductions (such as adjustments to income or itemized deductions), then skip ahead to number 7.
If you will be having a significant amount of income other than wages, and/or adjustments to income or think you itemizing on your tax return, instead of using the standard deduction, then continue on to the Deductions and Adjustments Worksheet. Work through lines 1 through 10 of that worksheet.
If you do not have more than one job, skip this step to number 8. Do you have two or more jobs as an employee? Continue on to the Two-Earners/Multiple Jobs Worksheet. This worksheet is used for unmarried persons with more than one job and for married couples who plan to file jointly and together they have two or more jobs. Work through lines 1 through 9 of this worksheet. You may need to refer to the tables below it also.
You will then come to a number that represents your withholding allowances.
Hope this helps. Have a wonderful day.