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. What If You Get Audited? The chances of an IRS audit aren't that high. And being audited does not necessarily imply that the IRS suspects wrongdoing. Social Security Taxes Estimate how much of your Social Security benefit may be considered taxable.