Below you’ll find a theoretical and practical part on how to define your real competitors and their impact on your sales. Price Index Calculation. To calculate the price index, firstly, you need to collect all the price overlaps pairs (the intersections of products that you and your competitors have). Calculators online for sales, markup, margin, price, profit, sale price and sales tax. Calculate among the sales variables in marginal analysis for cost, revenue, gross profit, gross margin and markup. Calculator to determine the sale price of a discounted item. Calculator online to calculate sales tax with a total price. Price Index Formula – Example #1. Suppose that we have 5 stocks which form the part of the index: Now to calculate Price-weighted index, following steps needs to be followed: First, calculate the sum of all the stocks. Sum of all the stocks = $5 + $50 + $20 + $12 + $8. Sum of all the stocks= $95.