RSI - Relative Strength Index
The popular Relative Strength Index momentum oscillator was developed by J. Welles Wilder and detailed in his book New Concepts in Technical Trading Systems. It analyses Average Gains and Average Losses to measure the speed and magnitude of price movements.
The RSI is always between 0 and 100, with stocks above 70 considered overbought and stocks below 30 oversold. Divergence between the price and RSI can also be analysed for potential reversals.
RS = Average Gain in the Period / Average Loss in the Period
RSI = 100 - (100 / (1 + RS))
Average Gain is calculated as (Previous Average Gain * (Period - 1) + Current Gain) / Period except for the first day which is just an SMA. The Average Loss is similarly calculated using Losses.
Sufficient Data for Accuracy
RSI is one of several indicators that include an element of prior data. As such a 14 day RSI based on 50 days of underlying data will be significantly different to a 14 day RSI based on 500 days of data. This site will always include enough data to ensure 'accuracy'.