What RSI actually is
RSI, short for Relative Strength Index, is a momentum oscillator created by J. Welles Wilder. It moves between 0 and 100 and measures the speed and size of recent price changes over a lookback period, 14 by default. A high reading means the recent up moves have been large relative to the down moves. A low reading means the opposite.
The word “strength” here is easy to misunderstand. RSI does not compare one stock against another; it compares a stock against its own recent behavior. It is asking a single question: over the last stretch of bars, how lopsided has the fight between buyers and sellers been? That is momentum, not direction, and the difference is where most of the misuse comes from.
How RSI is calculated
You never compute RSI by hand, but seeing the shape of the math makes the reading click. The point of the formula is simple: it compares the average size of up moves to the average size of down moves over the lookback period.
- 1. RS (relative strength) = average gain ÷ average loss over the lookback. The ratio of up-move size to down-move size.
- 2. RSI = 100 − 100 ÷ (1 + RS). This squeezes that ratio onto a 0 to 100 scale.
- 3. The takeaway = it is a balance meter. All up moves pushes RSI toward 100; all down moves pushes it toward 0.
That is the whole idea. When gains dwarf losses, RS is large and RSI climbs toward 100. When losses dominate, RS is small and RSI sinks toward 0. Everything you read into RSI is really a read on how one-sided the recent tape has been.
The 30/70 bands, and why they mislead
The classic reading is the one everyone learns first: above 70 is called overbought, below 30 is called oversold. That is a fine description of a stretched condition. It is a terrible trading signal on its own. In a strong trend RSI can stay above 70 for a long time, or below 30 all the way down, and price keeps going the same direction the entire time.
So say it clearly: overbought does not mean short, and oversold does not mean buy. A high RSI is not a ceiling; it is evidence the trend is strong. The bands are useful for spotting when a move is extended, but extension is a heads-up to manage risk, not a trigger to bet against the trend.
The 50 centerline and divergence
Two readings tend to matter more than the 30/70 extremes. The first is the 50 centerline. When RSI holds above 50, up moves are winning on balance, which favors the bulls; when it stays below 50, the sellers have the edge. Used this way RSI becomes a trend filter, not an overbought alarm. Many traders will only take long setups while RSI is above 50 and short setups while it is below.
The second is divergence. Divergence is when price and RSI disagree: price makes a higher high, but RSI makes a lower high. That gap is a warning the move is tiring, because the new price high was made on weaker momentum than the last one. It works in reverse too, a lower low in price with a higher low in RSI. Divergence is a caution flag, not a countdown timer, which is exactly where traders get into trouble with it.
The overbought-is-not-a-short trap
The most expensive mistake with RSI is a specific one: shorting a strong uptrend just because RSI crossed 70. It feels smart. It feels like you are selling into greed. In momentum names it is the fast way to get run over, because those are exactly the stocks where RSI stays pinned high while price keeps grinding up and stopping out every early short.
This is the overextension mistake WeTradePro is built to catch. The tool is not there to tell you an RSI reading; your chart already does that. It is there to flag when you are about to fight a trend because a number looks high, and to keep you from turning a healthy indicator into a reason to lose money.