RSI is a momentum oscillator that measures the speed and magnitude of recent price changes. It helps identify overbought and oversold conditions.
Formula
RSI = 100 - (100 ÷ (1 + RS)), where RS = Average Gain ÷ Average Loss over 14 periods
Example
If average gain over 14 days is 2% and average loss is 1%, RS = 2. RSI = 100 - (100 ÷ 3) = 66.7.
How to Interpret It
RSI above 70 = overbought (may be due for a pullback). RSI below 30 = oversold (may be due for a bounce). RSI divergences from price are powerful signals.
Real-World Example: RSI Divergence
In late December 2022, the S&P 500 made a new low near 3,800, but its 14-day RSI printed a reading of ~28 — higher than the October low's RSI of ~22. This bullish divergence correctly signaled the start of a major rally that pushed the index above 4,700 by mid-2024.
Conversely, Nvidia in March 2024 saw RSI hit 88 (extreme overbought) while price kept rising. The stock didn't immediately crash — it consolidated for 6 weeks before resuming its uptrend. This shows why RSI above 70 doesn't mean "sell immediately" in strong trends.
RSI Thresholds and What They Mean
RSI Range
Signal
Typical Action
0–30
Oversold
Watch for bounce; don't auto-buy
30–50
Weak / Bearish
Trend still down, wait for 50 cross
50–70
Bullish
Healthy uptrend
70–100
Overbought
Strong trend; divergence = danger
Common Mistakes
❌ Selling just because RSI > 70. In strong trends, RSI can stay above 70 for weeks or months. During Nvidia's 2023–2024 rally, RSI was overbought for months while the stock tripled.
❌ Using default 14-period on all timeframes. Day traders often use 9-period RSI for faster signals. Swing traders may prefer 21-period. Match the setting to your strategy.
❌ Ignoring the 50-line. RSI crossing above 50 confirms a trend shift upward; crossing below confirms weakness. Many traders focus only on 30/70 and miss this valuable signal.
❌ Using RSI alone. RSI works best combined with support/resistance levels, moving averages, or MACD. No single indicator should drive a trade decision.
💡 Pro Tip: The strongest RSI signals come from divergences. A bullish divergence (price makes a lower low but RSI makes a higher low) has a roughly 70% success rate for calling tradable bottoms. Bearish divergences (price higher high, RSI lower high) are equally powerful at calling tops.
Yes, in strong trends RSI can stay above 70 for weeks. This is why RSI works best in ranging markets and should be combined with other indicators. During the 2020 bull market, many tech stocks had RSI above 70 for months while prices kept climbing.
What time period should I use for RSI?
The standard is 14 periods (days for daily charts). Short-term traders use 9-period RSI for faster signals. Long-term investors may prefer 21 or 25 periods for smoother readings. Shorter periods generate more signals but also more false signals.
What's RSI divergence?
When price makes a new high but RSI makes a lower high, that's bearish divergence — momentum is weakening. Bullish divergence is the opposite: price makes a new low but RSI makes a higher low. Divergence is one of the most powerful RSI signals and often precedes trend reversals.
To understand how the Relative Strength Index functions, imagine a hypothetical technology stock named TechX. Initially, TechX starts trading at $100 with an RSI of 45, indicating the market is neutral and the price movement is proportional to previous fluctuations. Over the next two weeks, the stock price surges to $120, reflecting strong investor buying pressure. During this rapid ascent, the RSI climbs steadily. When the price reaches $120, the RSI hits 82. In standard trading terms, an RSI above 70 is considered "overbought," suggesting the stock might be overvalued or due for a correction. The investor recognizes this signal, perhaps choosing to take profits on their long position before the price potentially drops. A few days later, the price stabilizes at $118, and the RSI falls to 68, signaling that the stock is no longer in overbought territory, which could invite new buyers back into the market.
One of the most frequent errors investors make is relying on RSI signals without considering the overall market trend. RSI is a momentum indicator, meaning it works best in strong trends rather than choppy or sideways markets. In a sideways market, the index can bounce between 30 and 70 frequently, generating false buy and sell signals that can lead to significant losses if acted upon blindly. Another common pitfall is treating RSI as a direct predictor of price direction rather than a measure of speed and intensity. Many traders expect the RSI to reverse direction exactly when it crosses the 30 or 70 threshold, but price can continue to trend in the same direction for a long time even after the indicator becomes overbought or oversold. Additionally, failing to adjust the time period settings can be detrimental; using a standard 14-day setting during a high-volatility period might produce too much noise, while a shorter period during a slow market might generate too many whipsaws, making it difficult to distinguish genuine signals from random fluctuations.
The Relative Strength Index differs significantly from technical indicators like Moving Averages, which focus on the direction of price rather than its momentum. Moving Averages are lagging indicators that smooth out price data to identify the prevailing trend over a specific period, whereas RSI measures the speed and change of price movements to indicate potential overbought or oversold conditions within the current trend. Another related metric is the Moving Average Convergence Divergence or MACD, which uses two exponential moving averages to show the relationship between the two. While MACD focuses on the crossover of moving averages to signal a change in momentum, RSI is unique because it is calculated using the magnitude of recent price changes. Furthermore, Bollinger Bands utilize a Moving Average combined with standard deviation to create a dynamic range around the price, whereas RSI is plotted on a separate scale entirely. Investors often use RSI in conjunction with these other tools to filter out false signals; for example, a trader might only take a trade suggested by the RSI if it aligns with an upward-trending Moving Average, ensuring they are trading with the trend rather than against it.