In order to calculate your estimated taxes you can use your prior year adjusted gross income. Suppose it was less than $125,000 - then you can pay either 90 percent of this year's or 100 percent of last year's income tax liability. After you estimate your taxes for the term, make sure you divide this amount into fourths and pay appropriately. Note: If you are a farmer, change the 90 percent shown above to 66.67 percent. There are many different rules that apply to farmers so you might want to refer to IRS Publication 225: Farmer's Tax Guide.