To calculate CPI, or Consumer Price Index, add together a sampling of product prices from a previous year. Then, add together the current prices of the same products. Divide the total of current prices by the old prices, then multiply the result by 100. Finally, to find the percent change in CPI, subtract 100. As an example of a direct stock index calculation, a stock index might consist of twenty-five underlying individual stocks, whose prices could simply be added together (e.g., price of stock # 1 + price of stock # 2 + = price of stock index) to calculate the price of the stock index. An index in which the prices are weighted based on current year quantities is called the Paasche index and its formula is: Where q0 is the quantity in the base period, qt is the quantity in current period, pt is the current price of the product and p0 is the price of the product in the base year,