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 Home Mortgage Deduction Use this calculator to assess the potential benefits of a home mortgage deduction. Red Flags for Tax Auditors Here are six flags that may make your tax return prime for an IRS audit. Federal Income Tax Use this calculator to estimate your income tax liability along with average and marginal tax rates.