Look up the sales tax rate for your area by contacting your state department of revenue and local department of revenue.
Divide the total sales tax rate by 100 to convert it from a percentage to a decimal. For example, if the total sales tax equals 5.6 percent, you would divide 5.6 by 100 to get 0.056.
Add 1 to the percentage. Continuing the example, you would add 1 to 0.056 to get 1.056.
Divide the total price by the result from above to calculate the net sale price before retail tax. Finishing the example, if your total cost was $126.72, you would divide 126.72 by 1.056 to find that the net sale price before retail tax would be $120.