The calculation averages price increases and price decreases over a look-back window and turns the ratio into a single bounded number. Because that window is fixed, every reading summarises exactly that stretch and nothing else. Change the window and the same chart produces a very different line, since a shorter setting reacts faster and reaches extremes constantly, while a longer one smooths them almost out of existence.
The words attached to the extremes cause most of the trouble. A high reading is conventionally called overbought and a low one oversold, but those labels describe the indicator, not the asset. Strong trends routinely pin the RSI near an extreme for long stretches while price keeps travelling, which is exactly when the label tempts people to trade against a move with plenty of life left in it. A high reading is not a sell instruction and a low one is not a buy instruction.
Divergence, where price makes a new extreme and the indicator does not, is the other common talking point. It is a real and observable feature of the data, and it is also a frequent false alarm, because divergences resolve in favour of the existing trend often enough that treating each one as a turning point is expensive. As with candle patterns, the shape only becomes visible after the bars that formed it have closed.
Used honestly, RSI is a compact way to describe momentum, much as a <a href="/glossary/moving-average/">moving average</a> compactly describes trend. It is derived from price, so it cannot know anything price does not already show, and it will always confirm rather than anticipate. If you want to understand what actually moves a market, the material in our <a href="/learn/">Learn section</a> is a better use of an afternoon. Nothing here is a recommendation to trade.
Technical Analysis in Practice
Key takeaways
- RSI is computed from past closing prices, so it can only ever confirm a move that has already happened.
- The look-back setting changes readings substantially, which makes the phrase the RSI says meaningless without stating the window.
- Overbought and oversold describe the indicator's position on its own scale, not a judgement about value or an instruction to act.
RSI (Relative Strength Index) — frequently asked questions
Does a high RSI mean I should sell?
No, and treating it that way is one of the most common beginner mistakes. A high reading means recent gains have outweighed recent losses within the look-back window, which is what a strong uptrend looks like from the inside. Indicators can sit near an extreme for a long time while price continues in the same direction. RSI describes momentum; it does not issue instructions.
Which RSI settings are best?
There is no correct answer, only trade-offs. Shorter windows react quickly and produce many more extreme readings, most of which lead nowhere, while longer windows are steadier and slower. Any setting that looks perfect on a past chart was chosen with hindsight, which is a well-documented way to fool yourself. Pick a setting for a stated reason and keep it consistent.
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