How to Calculate Federal Tax

While you can calculate your federal taxes due by completing a Form 1040 in its entirety with all of the supporting documents, it is a good practice to estimate your tax liability at some point throughout the year as well. This may allow you to make changes to your situation to lower your tax liability in time for year-end. Estimating the taxes due with a blank Form 1040 and some estimated numbers is a good exercise and will help you understand the tax system better.

Calculate Gross Income

Gather all your documents pertaining to your income for the year. Have all your Form W-2s from each of your jobs and from your spouse's job. Complete Schedule C if you are self-employed to figure your business income. Include any IRA disbursements, alimony and unemployment income. Enter each of these amounts separately on the appropriate lines of Form 1040 in the income section.

Calculate Adjustments to Income

Prepare to calculate your adjustments to income. Common adjustments include deductions for qualified retirement plans, self-employed health insurance premiums, health care savings account contributions, moving expenses and student loan deductions. Enter the totals from each of these categories and any others that apply in the adjustments to income section on Form 1040. Subtract the total adjustments from the total income and enter your adjusted gross income on line 37 of Form 1040.

Exemptions and Deductions

Complete Schedule A if you are going to itemize your deductions. Multiply the exemption amount by the total number of exemptions and enter that onto line 42. Subtract all your available exemptions and deductions from your adjusted gross income to arrive at your taxable income.

Progressive Tax System

The U.S. government uses a progressive tax system, meaning that your taxable income is divided into portions, and each portion is taxed at a different rate. For example, if you are married filing jointly, the first $17,000 of your income is taxed at 10 percent, but the portion of your income from $17,001 to $69,000 is taxed at 15 percent. The highest percentage is 35 percent, and covers any taxable income over $379,150 for married people filing a joint return.

Calculate the Taxes Due

The easiest way to calculate how much your federal income taxes will be is to use the tax tables found in the instructions for Form 1040 or a computer software system that calculates it automatically. Manually calculate the tax due by using a table of tax brackets and calculate the taxes due on each portion of your income. Add these amounts together to arrive at the total federal income taxes you are liable for.