The Relative Strength Index, or RSI, is one of the most popular momentum indicators in trading. Developed by J. Welles Wilder in 1978, it measures the speed and magnitude of recent price changes to evaluate whether an asset is overbought or oversold. Nearly every charting platform includes it by default.
Concept: RSI Calculation Simplified
RSI compares the average size of up moves to the average size of down moves over a set period, typically 14 days. The result is a number between 0 and 100. If all 14 periods closed higher, RSI would be near 100. If all closed lower, it would be near 0.
In plain English: RSI tells you how strong recent buying or selling pressure has been. A high RSI means buyers have been dominant. A low RSI means sellers have been dominant. It does not tell you where price will go — it tells you the current state of momentum.
Overbought and Oversold Levels
Traditionally, an RSI above 70 is considered overbought and an RSI below 30 is considered oversold. When RSI reaches these extremes, the current move may be overextended and due for a pullback or reversal.
However, this is where most beginners go wrong. Overbought does not mean "sell immediately" and oversold does not mean "buy immediately." In a strong uptrend, RSI can stay above 70 for weeks while price continues to rise. In a strong downtrend, it can stay below 30 while price keeps falling.
A better approach is to use overbought readings in downtrends (looking for short entries) and oversold readings in uptrends (looking for long entries). This way you are using RSI to time entries in the direction of the bigger trend, not fighting the trend.
RSI Divergence
Divergence is arguably the most powerful RSI signal. It occurs when price and RSI move in opposite directions. Bullish divergence happens when price makes a lower low but RSI makes a higher low — momentum is improving even though price is still falling. This often precedes a reversal upward.
Bearish divergence is the opposite: price makes a higher high but RSI makes a lower high. The upward momentum is fading even though price is still rising. This warns that the trend may be about to reverse or at least enter a significant pullback.
Example: Bullish Divergence in Action
Tesla shares fall from $250 to $200, with RSI dropping to 28. Price bounces, then falls again to $195 — a new low. But this time, RSI only drops to 35 — a higher low. This is bullish divergence. The selling pressure that drove price to $200 was stronger than the selling pressure at $195, even though the price is lower.
A trader spots this divergence and waits for confirmation — a bullish candlestick pattern at $195. When a hammer forms, they enter long with a stop below $190 and a target at $220. The divergence gave the early warning that momentum was shifting.
Combining RSI with Trend Analysis
RSI is most effective when used as a secondary tool alongside trend analysis. First, determine the trend using price structure or a moving average. Then use RSI to time your entries within that trend.
In an uptrend, watch for RSI to pull back to the 40-50 zone (not necessarily all the way to 30) and then turn back up. This often coincides with a price pullback to a support level. In a downtrend, look for RSI to rally to the 50-60 zone and then turn back down. These mid-range signals are often more reliable than the extreme overbought and oversold levels.
Warning: RSI Alone Is Not a Strategy
Blindly buying when RSI hits 30 and selling when it hits 70 will lose you money over time. RSI is a supplementary tool — it adds context to what you are already seeing on the chart. Always combine RSI readings with price action, key levels, and the overall trend direction. No single indicator is a complete trading system.