The Williams %R is a momentum indicator that compares a stock’s current price to its Highest High and Lowest Low price over a specified period of time (typically 14 days). It is calculated using the following formula:

Williams %R = (Period Highest High - Current Price) / (Period Highest High - Period Lowest Low) x -100

Larry Williams - who first developed this indicator - considered values below -80 as oversold and above -20 as overbought.