Calculating Total Cost. Multiply the cost of an item or service by the sales tax in order to find out the total cost. The equation looks like this: Item or service cost x sales tax (in decimal form) = total sales tax. Add the total sales tax to the Item or service cost to get your total cost. Multiply retail price by tax rate
Let's say you're buying a $100 item with a sales tax of 5%. Your math would be simply: [cost of the item] x [percentage as a decimal] = [sales tax]. That's $100 x .
