Identify the taxable amount
Convert the tax rate to a decimal
Multiply the taxable amount by the tax rate decimal
Add the tax amount to the original amount if you want the total
Subtract the tax amount from the total if you want the pre-tax amount
Formula: Tax = Taxable Amount × Tax Rate
Formula: Total = Taxable Amount + Tax
Formula: Pre-tax Amount = Total ÷ (1 + Tax Rate)