RSI Value Range
RSI (Relative Strength Index) can reach values from 0 to 100.
RSI Values Interpretation
High RSI means that the second part of the RSI formula (behind the minus sign) is very small and RS is very large, which means that recent up moves have been much greater than recent down moves. In other words, high RSI values are a sign of bullish market (which may be overbought ? depending on your particular view and trading style).
Conversely, low RSI means that RS is also low and recent down moves have been much greater than recent up moves ? in other words, bearish (and possibly oversold) market.
When there are no down moves and AvgD is zero, RS can't be calculated (you would be dividing by zero). In such case, you can consider RS close to infinite and RSI is 100.
Overbought / Oversold Condition on RSI
RSI value over 80 or 90 is considered overbought.
RSI value under 20 or 10 is considered oversold.
The tighter condition you choose (higher RSI value for overbought condition, lower RSI value for oversold condition), the fewer times RSI will get to the overbought or oversold territory and the fewer signals or trading situations you get.