The marketing ROI formula for calculating return on investment is dependent on you earn on your portfolio or bank account, it's calculated as a percentage. Return on investment, or ROI, is the most common profitability ratio. There are several ways to determine ROI, but the most frequently used method is to divide. An investor cannot evaluate any investment, whether it's a stock, bond, rental Because it is a percentage, ROI can clear up some of the confusion caused by just looking at dollar value returns. Imagine two of your friends.