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Relative Strength Index (RSI)
RSI is a momentum indicator that measures the speed and magnitude of recent price movements to identify overbought or oversold conditions.
7 min read
RSI is a momentum indicator that measures the speed and magnitude of recent price movements to identify overbought or oversold conditions.
The Relative Strength Index (RSI) is one of the most enduring and versatile momentum oscillators in technical analysis. Developed by J. Welles Wilder Jr. and introduced in his 1978 book New Concepts in Technical Trading Systems, the RSI has stood the test of time because it elegantly solves a core problem: measuring the speed and change of price movements on a standardised 0–100 scale.
Today, it's used by day traders, swing traders, and long-term investors alike to identify overbought/oversold conditions, hidden and regular divergences, and potential trend reversals.
Why "Relative Strength"? The name doesn't refer to comparative strength versus another asset (that's a different concept). Instead, RSI compares the magnitude of a security's recent gains to its recent losses – its own internal strength.
The classic RSI calculation uses a 14-period lookback (usually days, but can be any timeframe). The formula is:
RSI = 100 - (100 / (1 + RS))
Where:
The first calculation uses a simple average of the first 14 periods; then Wilder's smoothing method (a form of exponential moving average) is applied.
Why 14 periods? Wilder chose 14 days after observing that 14‑day cycles appeared frequently in various markets. Many traders still use 14 as the default, but shorter periods (7–9) increase sensitivity, while longer periods (21–25) smooth out more noise.
Assume a 5‑period RSI for illustration (the same logic applies to 14 periods):
| Day | Price | Change | Gain | Loss |
|---|---|---|---|---|
| 1 | 100 | – | – | – |
| 2 | 102 | +2 | 2 | 0 |
| 3 | 101 | -1 | 0 | 1 |
| 4 | 104 | +3 | 3 | 0 |
| 5 | 103 | -1 | 0 | 1 |
Total Gain = 2 + 0 + 3 + 0 = 5 → Average Gain = 5/4 = 1.25
Total Loss = 0 + 1 + 0 + 1 = 2 → Average Loss = 2/4 = 0.5
RS = 1.25 / 0.50 = 2.5
RSI = 100 – (100 / (1 + 2.5)) = 100 – (100 / 3.5) ≈ 100 – 28.57 = 71.43 → Overbought zone.
Wilder defined thresholds:
However, these levels are not absolute. In strongly trending markets, RSI can stay above 70 for long periods (bull trend) or below 30 for extended times (bear trend). Some traders adjust the thresholds to 80/20 or 85/15 depending on volatility.
Common mistake: Treating RSI > 70 as an automatic sell signal. In a powerful uptrend, RSI may reach 80 and keep climbing while price continues higher. Always consider the broader context (trend, support/resistance, volume).
Beyond simple overbought/oversold, the RSI reveals deeper insights when combined with price action.
A divergence occurs when price makes a new high/low but RSI fails to confirm – suggesting weakening momentum and a potential reversal.
Bearish Divergence
Price makes a higher high, but RSI makes a lower high → weakness beneath the surface.
Bullish Divergence
Price makes a lower low, but RSI makes a higher low → hidden buying pressure.
Divergences are most reliable when they occur near overbought/oversold levels (RSI > 70 or < 30) and align with other signals (e.g., support/resistance, candlestick patterns).
A failure swing is a small reversal within the RSI itself:
Bullish Failure Swing
RSI falls below 30 (oversold), bounces above 30, pulls back but holds above 30, then breaks its prior high. → Buy signal.
Bearish Failure Swing
RSI rises above 70, dips below 70, rallies but fails to break 70, then turns down. → Sell signal.
Some traders use the 50 level as a directional filter:
Consider a tech stock in a parabolic run. RSI hits 85, but price keeps rising. Selling at 70 would have missed huge gains. In such cases, use trailing stops and wait for a bearish divergence or a break below a key moving average.
During a healthy uptrend, price retraces but RSI forms a higher low (bullish hidden divergence). This signals that the pullback is losing steam and the uptrend will likely resume – a perfect entry point.
Pro tip: Combine RSI divergences with volume and support/resistance. A bullish divergence at a major demand zone is far stronger than one in the middle of nowhere.
| Period | RSI Sensitivity | Best used for |
|---|---|---|
| 7–9 | Very high, many signals | Scalping, day trading |
| 14 | Standard | Swing trading, general analysis |
| 21–25 | Smoother, fewer whipsaws | Position trading, weekly charts |
Shorter periods generate more extreme RSI values (often >80 or <20) but also more false signals. Smoother RSIs are slower to trigger but can be more reliable for major trend turns.
Golden rule: Never trade based solely on RSI. Use it as a confirmation tool alongside trend analysis, key support/resistance, and at least one other indicator (e.g., MACD, moving averages).
The Relative Strength Index is not a magic bullet, but when used correctly, it provides a disciplined framework to quantify momentum and spot early warning signs of reversals.
Key takeaways:
J. Welles Wilder once wrote: "The RSI is not designed to pick tops and bottoms. It is designed to help the trader determine when a trend is potentially exhausted."
Happy trading, and remember: the trend is your friend, but the RSI can tell you when to tighten your seatbelt.