What the RSI Measures
The Relative Strength Index, developed by J. Welles Wilder in 1978, quantifies momentum by comparing the magnitude of recent gains to recent losses over a set period — typically 14 candles. The result is plotted on a scale from 0 to 100. It answers a specific question: is price moving up or down with conviction, and is that move getting stretched?
A high RSI means recent candles have been dominated by gains — buyers are in control, but the move may be overextended. A low RSI means losses dominate — sellers are in control, potentially to an extreme. The indicator doesn't predict reversals; it describes the current state of momentum, which a trader then interprets in context.
Reading Overbought and Oversold
The classic interpretation uses two thresholds:
- Above 70 — overbought. Price has risen quickly; the move may be due for a pause or pullback. This is not an automatic sell signal.
- Below 30 — oversold. Price has fallen sharply; a bounce may be near. This is not an automatic buy signal.
- Around 50 — neutral. Momentum is balanced; often where RSI sits during consolidation.
The critical caveat that catches beginners: in a strong trend, RSI can remain overbought or oversold for extended periods. During a powerful bull run, RSI can sit above 70 for weeks while price keeps climbing. Selling simply because RSI hit 70 in a strong uptrend is a classic way to exit a winning position far too early. Overbought doesn't mean "reverse now" — it means "momentum is strong," which in a trend is bullish, not bearish.
RSI Divergence — The Real Edge
The most reliable RSI signal isn't the absolute level — it's divergence, where price and RSI disagree:
- Bearish divergence: price makes a higher high, but RSI makes a lower high. Momentum is weakening even as price rises — a warning the uptrend may be running out of fuel.
- Bullish divergence: price makes a lower low, but RSI makes a higher low. Selling pressure is fading even as price drops — a hint the downtrend may be exhausting.
Divergence works because RSI reflects the force behind a move. When price pushes to a new extreme but momentum doesn't confirm, it signals that fewer participants are driving the move — a structural weakness that often precedes a turn. Divergence is most powerful at significant levels and on higher timeframes, where it carries more weight.
Limitations You Must Respect
RSI is a useful tool, but treating it as a standalone signal is a fast way to lose money:
- It lags. Like all indicators built on past prices, RSI confirms momentum rather than predicting it.
- It fails in strong trends. Overbought/oversold readings are most useful in ranging markets and least useful in powerful trends.
- It produces false divergences. Not every divergence leads to a reversal; in strong trends, divergence can persist while price continues.
- It needs confirmation. RSI should support a thesis built on price structure, not replace it. Combine it with support/resistance, trend, and volume.
