The taxes on the Form 1040 can be calculated in various ways based on the type of income being reported on a tax return.
If your income is from wages, taxable interest, taxable dividends then your taxes on your taxable income is based on the Tax Tables in the instructions for Form 1040 starting on page 64 - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#page=64
