How to Calculate Sales Over Last Year

From year to year, a company's sales vary for different factors, including new product launches, changes in business strategy and economic conditions. The annual change in sales from can be measured in raw numbers, rates of change or percentages. As an investor, knowing the change in sales helps you decide whether you want to invest in a company. As a business, knowing the change in sales can help you figure out whether your new business strategies are helping your sales.

Subtract the prior year's sales from the present year's sales to calculate the change in sales. For example, last year the company had $88 million in sales and this year it had $82 million in sales, subtract $88 million from $82 million to find the change in sales is a decrease of $6 million.

Divide the increase or decrease in sales over the prior year to calculate the rate of change in sales over the year. In this example, divide the decrease of $6 million by $88 million to get a declining rate of approximately 0.0682 dollars per year.

Multiply the rate of increase or decrease by 100 to find the percentage change in sales form last year to the current year. In this example, multiply 0.0682 by 100 to find the sales declined by 6.82 percent.