Relative Strength Index (RSI)
Relative Strength Index (RSI) is a technical momentum indicator that compares the magnitude of recent gains to recent losses in an attempt to determine overbought and oversold conditions of an asset. It is calculated using the following formula:
| RSI = 100 – | 100 ———- 1 + RS |
RS = Average of x days’ up closes / Average of x days’ down closes
Source: http://www.investopedia.com/terms/r/rsi.asp