In marketing, the return on investment refers to the amount of money you gain compared to the marketing cost. To calculate ROI, you will subtract marketing expenses from sales growth and then divide that by the marketing cost to get the return on your investment. In marketing, it is important to keep in mind that it can be hard to have a direct relationship with sales growth to a marketing campaign. If that is is the case, you can subtract your average organic sales and marketing cost from your sales growth and then divide it by your marketing cost.