What Is RSI — Reading Momentum Without the Jargon
A 0-to-100 scale that tells you whether buyers or sellers have been in control, and whether that control is fading.
The Definition
RSI stands for Relative Strength Index. It was developed by J. Welles Wilder in 1978 and remains one of the most widely used momentum oscillators in trading.
RSI compares the magnitude of recent gains to recent losses over a set period (typically 14 bars) and expresses the result as a number between 0 and 100.
- Above 70: Generally considered overbought. Buyers have been dominant, and the move may be stretched.
- Below 30: Generally considered oversold. Sellers have been dominant, and the move may be exhausted.
- Around 50: Neutral. Neither side has clear control.
The word “relative” is important. RSI measures strength relative to the instrument’s own recent history, not compared to another asset.
Why It Matters
RSI gives you a quick read on the health of a move. Price alone does not tell you everything. A stock can make new highs while the energy behind the move is already fading. RSI catches that.
- Momentum confirmation: If price is rising and RSI is rising with it, the move has energy behind it. The trend is healthy.
- Exhaustion warning: If price is making new highs but RSI is making lower highs, that divergence warns the move is running out of fuel. Smart money often uses these signals to reduce exposure.
- Mean reversion setups: In ranging markets, RSI readings near 30 and 70 can identify attractive entry points for counter-trend trades.
How Traders Use It
- Divergence detection: This is the highest-value use of RSI. When price and RSI disagree, something is shifting beneath the surface. Bearish divergence (price higher, RSI lower) warns of potential reversal. Bullish divergence (price lower, RSI higher) suggests selling pressure is fading.
- Trend filtering: In strong uptrends, RSI tends to oscillate between 40 and 80. In strong downtrends, it tends to stay between 20 and 60. Watching which range RSI operates in tells you the trend character.
- Multi-timeframe analysis: Daily RSI might be overbought while weekly RSI is still mid-range. This context matters. An overbought daily reading within a healthy weekly trend is less concerning than one appearing after a weekly RSI divergence.
A Real-World Example
NAS100 has rallied from 18,500 to 19,800 over two weeks. Daily RSI hit 78 during the first push higher. Now price is at 19,800 (a new high) but RSI reads 71, lower than its earlier peak.
This is classic bearish divergence. Price is higher but momentum is weaker. The rally is losing its buyers. This does not mean NAS100 will crash tomorrow, but it means the risk/reward of new long positions is deteriorating. Experienced traders either tighten stops or wait for a pullback rather than chasing the move.
Common Mistakes
- Selling just because RSI is above 70: In strong trends, RSI can stay overbought for weeks. Selling every time RSI crosses 70 in a bull market is a recipe for missing the best moves. Overbought is not the same as “sell now”.
- Using RSI in isolation: RSI is a supporting indicator, not a standalone signal. It gains its power when combined with price structure, volume, and broader market context.
- Applying it mechanically across all conditions: RSI works differently in trending markets versus ranging markets. In a range, 70 and 30 are reliable boundaries. In a trend, those levels are just noise.