Technicals
RSI (Relative Strength Index)
RSI is a momentum oscillator that scores recent gains against recent losses on a 0–100 scale.
The relative strength index, usually 14 days, compares the size of up closes with the size of down closes. Readings above 70 are traditionally called overbought. Readings below 30 are called oversold. In a strong trend those labels lie: a great stock can live above 70 while it trends, and a broken one can live below 30 while it falls.
Use RSI as a description of how one-sided the recent trading has been, not as an automatic reversal call. Divergence — price making a new high while RSI does not — is the more interesting signal, and even that is a prompt to check the fundamentals, not a trade ticket. Stocklytics is built around fundamentals and scores; RSI belongs next to the price chart, not instead of the earnings.
Formula
RSI = 100 − 100 ÷ (1 + average gain ÷ average loss), usually over 14 periods
Live example
Apple Inc. (AAPL) last printed at $345.33 with a market cap of $5.04T. Apply the formula on this page to that live price. Where the quote is missing an input, the stock page's financials are the next place to read it.
Related
MACD · Moving Averages · Volatility
See it on a list: best stocks · screener
Questions
What period should I use?
14 is the common default. Shorter periods are jumpier. Longer periods are slower. Pick one and keep it so your eye learns the scale.
Does RSI work on ETFs?
Yes. It is a price statistic. It does not know or care what the underlying business is.