The Facts About Income Tax Millions faithfully file their 1040 forms each April. But some things about federal income taxes may surprise you. Share | Related Content Federal Income Tax Use this calculator to estimate your income tax liability along with average and marginal tax rates. Tax Deductions You Won't Believe Here are some examples of deductions from the IRS that were permitted and some that were, uh, too creative. Social Security Taxes Estimate how much of your Social Security benefit may be considered taxable.