i Short answer
RSI (Relative Strength Index) is a momentum oscillator measuring the speed and magnitude of recent price changes on a 0-100 scale, by comparing the average size of recent up-moves to the average size of recent down-moves over a set lookback period, typically 14.
Readings above 70 are conventionally considered overbought, and below 30 oversold, though these are conventions rather than guarantees, RSI can remain in either zone for extended periods during a genuinely strong trend. Many traders also watch for divergence between RSI and price as a more reliable momentum signal than the simple threshold alone.
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RSI: The Core Mechanics
Developed by J. Welles Wilder Jr. and introduced in his 1978 book "New Concepts in Technical Trading Systems."
1. What RSI actually measures
The Relative Strength Index measures the speed and magnitude of recent price changes, expressed as a single number between 0 and 100. It's calculated by comparing the average size of recent up-moves to the average size of recent down-moves over a set lookback period (14 periods by default), producing a reading that reflects how strong or weak recent momentum has been, not the price level itself.
As a momentum oscillator, RSI is designed to complement, not replace, price and support/resistance analysis, it tells you something about the underlying strength of a recent move that price alone doesn't directly show.
2. Overbought and oversold, what these thresholds mean
By long-standing convention, an RSI reading above 70 is considered overbought, suggesting price has risen quickly enough recently that a pause or pullback is statistically more likely than a continuation at the same pace. A reading below 30 is considered oversold, suggesting the equivalent situation on the downside.
| RSI Reading | Interpretation |
|---|---|
| Above 70 | Overbought |
| 30-70 | Neutral range |
| Below 30 | Oversold |
These specific thresholds (70 and 30) are conventions widely used across trading platforms and education, not fixed rules embedded in the indicator's mathematics, some traders adjust these levels (to 80/20, for example) depending on the instrument and timeframe they're analysing.
3. Choosing an RSI period
14 periods is the original default proposed by J. Welles Wilder Jr., who created the indicator, and remains the most widely used setting across trading platforms today. Shorter periods, like 9, make RSI more sensitive and responsive to recent price action, generating more frequent signals but also more noise from insignificant short-term fluctuations. Backtesting different periods against your specific instrument and timeframe is the most reliable way to find a setting that suits your approach.
Longer periods, like 21 or 25, smooth the reading out considerably, producing fewer, generally more significant signals but with correspondingly more lag before the indicator reflects a genuine shift in momentum. Which period suits your approach depends on your specific trading timeframe and how much noise versus responsiveness you're willing to trade off.
4. Why RSI can stay overbought during a strong trend
This is one of the most common, costly misunderstandings among newer traders using RSI. During a genuinely strong, sustained trend, RSI can remain in overbought or oversold territory for an extended period without price actually reversing, since strong trends by their very nature involve sustained directional momentum that keeps pushing the reading into these zones repeatedly.
Treating an overbought reading as an automatic sell signal during a strong, established uptrend is a well-documented, common mistake, one that can mean exiting or shorting a position that continues running considerably further in the trend's direction. RSI generally works better as one input combined with broader trend context, rather than a standalone, automatic trigger applied blindly regardless of the prevailing trend.
5. RSI divergence: a more reliable signal
Divergence occurs when price makes a new high (or low) that RSI does NOT confirm with its own corresponding new high (or low), a signal that the underlying momentum behind the move is weakening even as price itself continues moving in the same direction on the surface.
Many experienced traders consider divergence a genuinely more reliable signal than the simple overbought/oversold threshold crossing alone, since it reflects an actual, measurable change in underlying momentum rather than just an arbitrary numerical level being crossed. Bearish divergence (price higher highs, RSI lower highs) can precede a reversal in an uptrend, bullish divergence (price lower lows, RSI higher lows) the equivalent in a downtrend.
6. RSI versus other momentum indicators
RSI and MACD (Moving Average Convergence Divergence) both measure momentum, but calculate it through entirely different methods, RSI is bounded between 0-100 and focuses specifically on the ratio of average up-moves to average down-moves, while MACD tracks the relationship between two exponential moving averages and isn't bounded to any fixed numerical range.
Many traders use both together rather than relying on either in isolation, since they can occasionally give meaningfully different readings on the exact same price action, providing a useful cross-check that helps filter out signals that only one indicator, but not the other, is flagging. Neither indicator is inherently superior, they simply capture momentum through different mathematical lenses.
Key Takeaways
- RSI (Relative Strength Index) is a momentum oscillator measuring the speed and magnitude of recent price changes on a 0-100 scale, comparing average up-moves to average down-moves.
- Readings above 70 are conventionally considered overbought, and below 30 oversold, though these are conventions, not guarantees, price can remain overbought or oversold during strong trends.
- 14 periods is the original, most widely used default lookback, shorter periods increase sensitivity and noise, longer periods smooth the reading with more lag.
- RSI can remain in overbought or oversold territory throughout a genuinely strong, sustained trend, treating it as an automatic reversal signal in these conditions is a common mistake.
- RSI divergence, where price makes a new high or low that RSI doesn't confirm, is considered by many traders a more reliable signal than simple overbought/oversold thresholds alone.
- RSI and MACD both measure momentum differently and are often used together as a cross-check, rather than relying on either indicator in isolation.
Frequently asked follow-up questions
What does RSI actually measure?
RSI (Relative Strength Index) measures the speed and magnitude of recent price changes on a scale of 0 to 100, comparing the average size of recent up-moves to the average size of recent down-moves over a set lookback period. It's a momentum oscillator, meaning it's designed to show how strong or weak recent price momentum has been, not the price level itself.
What do overbought and oversold actually mean on the RSI scale?
By convention, an RSI reading above 70 is considered overbought, suggesting price has risen quickly enough that a pause or pullback is more likely, while a reading below 30 is considered oversold, suggesting a similarly sharp decline that could see a bounce. These thresholds are conventions, not guarantees, price can remain overbought or oversold for extended periods during a strong trend.
What RSI period should I use?
14 periods is the original, most widely used default, proposed by Welles Wilder who created the indicator. Shorter periods (like 9) make RSI more sensitive and responsive to recent price action, generating more signals but also more noise, while longer periods (like 21 or 25) smooth the reading out, producing fewer, more significant signals with more lag.
Why does RSI sometimes stay overbought or oversold during a strong trend?
This is one of the most common misunderstandings about RSI, during a genuinely strong, sustained trend, RSI can remain in overbought or oversold territory for an extended period without price reversing, since strong trends by definition involve sustained directional momentum. Treating an overbought reading as an automatic sell signal during a strong uptrend is a common, costly mistake, RSI works better as one input among several rather than a standalone trigger.
What is RSI divergence, and why do traders watch for it?
Divergence occurs when price makes a new high (or low) that RSI does NOT confirm with its own new high (or low), suggesting the momentum behind the move is weakening even as price itself continues in the same direction. Many traders consider divergence a more reliable signal than the simple overbought/oversold threshold alone, since it reflects an actual change in underlying momentum rather than just an arbitrary level being crossed.
How does RSI compare to other momentum indicators like MACD?
RSI and MACD (Moving Average Convergence Divergence) both measure momentum but calculate it differently, RSI is bounded between 0-100 and focuses specifically on the ratio of up-moves to down-moves, while MACD tracks the relationship between two moving averages and isn't bounded to a fixed range. Many traders use both together, since they can occasionally give different readings on the same price action, providing a useful cross-check rather than relying on either indicator in isolation.
📚 Sources & further reading
This article draws on established technical analysis education resources. Practice applying RSI on a demo account before using it with real capital.
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