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. Understanding Marginal Income Tax Brackets An inside look at how marginal income tax brackets work. Comparing Investments This calculator compares the net gain of a taxable investment versus a tax-favored one.