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Relative Strength Index: Overbought and Oversold
RSI is a momentum indicator that tells you how stretched a stock's recent gains are relative to its recent losses. It runs on a 0 to 100 scale and is one of the most widely used tools in technical analysis.
Key Takeaways
- The relative strength index compares the average size of up moves to down moves over 14 periods, producing a 0–100 reading.
- Strong uptrends can keep RSI above 70 for weeks; selling every overbought reading fights a trend that is still intact.
- RSI works best combined with a trend filter, above the 200-day SMA, focus on oversold dips to buy, not overbought peaks to short.
- Bearish divergence, price makes a new high while RSI makes a lower high, is an early warning of fading momentum.
Key Takeaways
- The relative strength index compares the average size of up moves to down moves over 14 periods, producing a 0–100 reading.
- Strong uptrends can keep RSI above 70 for weeks; selling every overbought reading fights a trend that is still intact.
- RSI works best combined with a trend filter, above the 200-day SMA, focus on oversold dips to buy, not overbought peaks to short.
- Bearish divergence, price makes a new high while RSI makes a lower high, is an early warning of fading momentum.
What It Is
The Relative Strength Index was introduced by J. Welles Wilder Jr. in his 1978 book New Concepts in Technical Trading Systems. It compares the average size of up moves to the average size of down moves over a lookback window, usually 14 periods. The result is plotted as a single line oscillating between 0 and 100.
Two zones get the most attention. Readings above 70 are traditionally called overbought. Readings below 30 are called oversold. Those labels describe what price has done recently, not what it is about to do.
The Intuition
Price alone does not tell you whether a rally is running out of steam. A stock that climbs 2 percent on a quiet day might be doing so calmly, or it might be the tenth straight day of gains with nothing on the other side of the ledger. RSI answers the second question. When every recent period is an up period, RSI pushes toward 100. When every recent period is a down period, RSI pushes toward 0. Most of the time it sits somewhere in the middle.
The indicator compresses that ratio of recent gains to recent losses into one number so you do not have to stare at individual candles to judge whether momentum is balanced or lopsided.
How It Works
The formula has two steps. First, compute the relative strength (RS), which is the average gain divided by the average loss over the lookback window. Then plug RS into the RSI transform:
RSI = 100 - (100 / (1 + RS))
Where:
RS = average gain over N periods / average loss over N periods
Wilder's original method uses a smoothed average, not a simple mean. After the first 14 periods, each new average gain is calculated as:
new avg gain = ((previous avg gain * 13) + current gain) / 14
The average loss uses the same rule. This smoothing makes the indicator less jittery. Most charting platforms default to a 14-period RSI, but you can shorten it for more sensitivity or lengthen it for slower signals.
Worked Example
Suppose a stock has the following closing prices over 5 days: 100, 102, 101, 104, 103. The daily changes are +2, -1, +3, -1.
Total gains: 2 + 3 = 5 Total losses: 1 + 1 = 2
Simple average gain: 5 / 4 = 1.25 Simple average loss: 2 / 4 = 0.50
RS: 1.25 / 0.50 = 2.5 RSI: 100 - (100 / (1 + 2.5)) = 100 - 28.57 = 71.4
A reading of 71.4 is just above the overbought threshold. In a real 14-period setup, you would repeat this across a longer window using Wilder's smoothed average. The logic is identical.
Common Mistakes
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Treating overbought as a sell signal. An RSI above 70 does not mean "sell now." Strong uptrends can keep RSI above 70 for weeks. In a genuine bull run, buying the dips when RSI cools to 50 often works better than shorting the highs.
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Treating oversold as a buy signal. The mirror of the first mistake. Stocks in serious trouble can ride RSI below 30 all the way down. Oversold is a description of the past, not a promise about the future.
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Using RSI alone without trend context. RSI is most useful when combined with a trend filter. Many practitioners use a 200-day moving average to decide which direction to take RSI signals. Above the 200-day, they look for oversold dips to buy. Below it, they look for overbought rallies to avoid.
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Ignoring divergence. When price makes a new high but RSI makes a lower high, momentum is fading even though price is still climbing. This bearish divergence is a warning, not a guarantee, but skipping it is a common rookie error.
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Using the wrong period for the asset. The 14-period default is a convention, not a law. Short-horizon traders sometimes use 7 or 9 for more sensitivity. Longer-term investors use 21 or higher to filter noise. Copying the default without thinking about your timeframe is a frequent source of false signals.
Frequently Asked Questions
Q: What is the relative strength index in simple terms? RSI is a number from 0 to 100 that shows whether recent gains or losses have been dominant. A reading above 70 means recent gains have been unusually large; below 30 means recent losses have been unusually large.
Q: How does RSI affect investment decisions? Many investors use RSI as a momentum confirmation tool. An RSI rising from 40 toward 60 in an uptrend confirms buying pressure is healthy. A declining RSI while price holds near a high warns that the rally is losing its underlying strength.
Q: What is a real-world example of an RSI signal? During the S&P 500's 2022 bear market, RSI repeatedly reached the 35–40 range and bounced, providing potential oversold buy signals for shorter-term traders. But each bounce failed to push RSI back above 60, confirming the downtrend was still in control.
Q: How can investors use RSI practically? Use the 200-day moving average to filter the signal direction: when price is above the 200-day, only take the oversold RSI dips to buy. When below it, only consider the overbought peaks as potential short or reduce signals. This cuts most false positives.
Q: How is RSI different from the stochastic oscillator? RSI measures the ratio of average gains to average losses over a fixed window. The stochastic oscillator measures where the current close sits inside the period's high-low range. RSI is smoother and less prone to false extremes; the stochastic is faster and cycles more aggressively.
Sources
- Investopedia. "Relative Strength Index (RSI) Indicator Explained With Formula." https://www.investopedia.com/terms/r/rsi.asp
- StockCharts School. "Relative Strength Index (RSI)." https://school.stockcharts.com/doku.php?id=technical_indicators:relative_strength_index_rsi
- Wilder, J.W. (1978). New Concepts in Technical Trading Systems. Trend Research. https://archive.org/details/newconceptsintec00wild
- Fidelity Learning Center. "Relative Strength Index (RSI)." https://www.fidelity.com/learning-center/trading-investing/technical-analysis/technical-indicator-guide/rsi
Disclaimer
This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.
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