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 Making Your Tax Bracket Work Have you explored all your options when it comes to managing your taxable income? 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.