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. How to Make the Tax Code Work for You When you take the time to learn more about how it works, you may be able to put the tax code to work for you. Capital Gains Tax Estimator Use this calculator to estimate your capital gains tax.