To calculate sales tax you must:
1. Know the sales tax rate
2. Use the sales tax formula which is
Sales Tax = Total Sales X Sales Tax Rate
Example:
You have a business and you must collect a sales tax at a rate of 8.00%. Your customer bill before adding tax is $399.
Sales tax = $399 times 0.08
Sales tax would be $31.92.
$399 + $31.92 = $430.92 would be the total including sales tax.