If your taxable income is $100,000 or less, then use the tax table and read the entry on the row corresponding to your taxable income and the column corresponding to your filing status, notes the IRS. If your taxable income exceeds $100,000, then use the tax computation worksheet.
After you have filled out form 1040 to line 43, or form 1040A to line 27, use the tax table or tax computation worksheet to fill the next line, says the IRS in the instructions for these forms. The table contains rows for ranges of taxable income and columns for filing status. Use the column corresponding to the filing status you selected in lines one through three of form 1040 or 1040A. Select the row that brackets your taxable income with the numbers in the first two columns. The intersection of this column and row gives you the tax amount.
If your taxable income is more than $100,000, the table does not contain entries, and you need to use the tax computation worksheet. Choose the section of the worksheet based on your filing status. Then choose the row that contains your taxable income within the range given in the first column. Enter your taxable income in that row in column (a). Multiply that amount by the ratio in column (b) and subtract the amount in column (c). This is your tax amount.