WHAT IS ROI ANYWAYS?
The Return on Investment calculation uses actually the same concept as Percent Change calculation. The theory it's the same, it is the calculation of the percent change from one period to another. In this case expressed as ROI and most frequently used to measure the monetary return on an investment. The result is also measure in percentage (%). It measures the percent of return from two numbers: amount invested (old value) and amount gained (revenue or "new value")
* See further explanation from Entrepreneur