How to Calculate Income Tax. You can make a very rough estimate of your income tax on the basis of your salary, but that doesn't take other income or deductions into account. Here's a quick, easy way to estimate the income tax for all of your income.
Obtain an instruction booklet for Form 1040. You can obtain a paper copy of Form 1040 and instructions from your local library, post office or IRS office. This booklet includes a list of income tax amounts for specific incomes. You also can access the booklet online at the IRS website.
Fill out the sample Form 1040 included with the instructions in order to calculate your taxable income. You will need a general idea of your deductions and income amounts.
Refer to the instructions for Form 1040, pages 67-78. Locate your taxable income on the list. Find your income tax in the column under your marital and filing status.
Consult your state tax agency for information on how to calculate your state income tax. It varies from state to state. If you are unsure of how to find out your estimated tax, work through the income tax forms available on your state's website.
Visit the IRS website if you have any questions or need help (see Resources below). You can also contact your local IRS office for more information.