If you itemize your taxes, and thus use a Form 1040, the deductions come after your adjustable gross income (AGI) is determined. Even if you don't itemize, and use a 1040A, you can take standard deductions for yourself and potentially others in your household. Once these are figured in, you are left with your taxable income. That's the figure to check against the tax tables to find what you will pay.
Get your AGI from the appropriate line on your tax form. On the 1040, it's line 37; on the 1040A, it's line 21. On the 1040EZ, AGI can be found on line 4.
Figure out your post-adjustable gross income deductions. For the 1040, use the total deductions from Schedule A if you itemize. For the 1040A and 1040EZ, and if you choose not to itemize for the 1040, take the standard deduction for yourself -- and spouse, if applicable. The standard deduction is listed in the form instructions for each year.
Deduct the figure in Step 2 from your AGI. For example, if your AGI is $56,000, and you have a standard deduction of $9,350, the result is $46,650. This is your taxable income on the 1040EZ.
Deduct your exemptions if you file a 1040 or 1040EZ. Multiply the number of exemptions from the "Exemptions" area of the front of the form by the figure listed on the form. For example, with two exemptions times $3,650, the total is $7,300.
Subtract the result in Step 4 from the amount in Step 3 to find your taxable income. In this example, $46,650 minus $7,300 leaves $39,350 in taxable income.
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