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 Capital Gains Tax Estimator Use this calculator to estimate your capital gains tax. You May Need to Make Estimated Tax Payments If… Have income that isn’t subject to tax withholding? Or insufficient withholdings? You may have to pay estimated taxes. Social Security Taxes Estimate how much of your Social Security benefit may be considered taxable.