The Ultimate Guide to RSI (Relative Strength Index): A Detailed PDF Guide The Relative Strength Index (RSI) is a widely used technical indicator used by traders and investors to evaluate the strength of a stock, commodity, or currency. Developed by J. Welles Wilder Jr. in 1978, the RSI is a momentum oscillator that measures the size of recent price changes to determine overbought or oversold conditions. In this full guide, we will explore the ins and outs of the RSI, including its calculation, interpretation, and application in trading. What is RSI? The Relative Strength Index (RSI) is a technical indicator that measures the strength of a security by comparing its recent gains and losses. It is calculated by dividing the average gain of up days over a specified period by the average loss of down days over the same period. The RSI is then plotted on a scale from 0 to 100. How is RSI Calculated? The RSI is calculated using the provided formula: RSI=100−1+RS100 Where: