What is the RSI?
The Relative Strength Index (RSI), developed by J. Welles Wilder in 1978, is a momentum oscillator that measures the speed and magnitude of recent price changes to evaluate whether a stock is overbought or oversold.
RSI always outputs a number between 0 and 100. The traditional interpretation:
- RSI above 70: Overbought — price may have risen too far, too fast.
- RSI below 30: Oversold — price may have fallen too far, too fast.
- RSI 40–60: Neutral zone.
How RSI is calculated
RSI uses the ratio of average gains to average losses over a lookback period (default: 14 days):
RS = Average Gain over N periods ÷ Average Loss over N periods
RSI = 100 − (100 ÷ (1 + RS))If a stock rose on 10 of the last 14 days for an average gain of $1.50, and fell on 4 days for an average loss of $0.50, RS = 3.0 and RSI = 75.
Overbought doesn't mean "sell"
The biggest RSI mistake beginners make: treating RSI > 70 as an automatic sell signal. It's not. In a strong uptrend, RSI can stay above 70 for weeks or months while the stock doubles. Nvidia in 2023. Tesla in 2020. Amazon in any good bull year.
What RSI above 70 tells you: the recent price gains have been strong and the market is in a buying frenzy. It increases the probability of a near-term pullback — but only in the context of the broader trend.
Use RSI as a warning light, not a trigger switch.
RSI divergence
One of the most reliable RSI signals is divergence — when price and RSI move in opposite directions:
Bearish divergence: Stock makes a new 52-week high but RSI makes a lower high than it did at the previous peak. The momentum behind the move is weakening — a potential reversal warning.
Bullish divergence: Stock makes a new low but RSI makes a higher low. Selling momentum is fading — the downtrend may be losing steam.
Divergence is not a guarantee of reversal, but it's one of the more meaningful RSI signals available to traders.
Choosing the right RSI timeframe
The default 14-period RSI works well for most investors, but different timeframes change the sensitivity:
- RSI(9): Faster, more sensitive — generates more signals but also more false positives. Used by short-term traders.
- RSI(14): The standard. Balanced between responsiveness and noise.
- RSI(21–25): Slower, fewer signals. Better for longer-term trend confirmation.
The timeframe of the chart also matters. RSI on a weekly chart tells you about multi-month momentum. RSI on a 5-minute chart tells you about the last hour of trading. Same formula, very different signals.
RSI and trend context
RSI is most useful when combined with trend direction:
- In an uptrend: RSI tends to oscillate between 40 and 80. A dip to 40–45 is often a buy opportunity, not a sell signal.
- In a downtrend: RSI tends to oscillate between 20 and 60. A bounce to 55–60 may be a short opportunity, not a breakout.
This is why looking at RSI in isolation is a mistake. RSI of 65 in an uptrend is very different from RSI of 65 in a downtrend.
Combining RSI with other indicators
RSI works best as a confirming signal rather than a standalone trigger:
- RSI + Moving Averages: Look for oversold RSI (<35) when price is still above the 200-day MA. The long-term trend is intact — the dip may be a buying opportunity.
- RSI + MACD: RSI below 30 + MACD histogram turning from negative to positive = stronger reversal signal.
- RSI + Volume: A bounce from oversold territory on rising volume carries more conviction than a bounce on declining volume.
- RSI + Support/Resistance: RSI oversold near a known price support level is a high-probability entry setup.
RSI limitations to keep in mind
RSI is a lagging indicator — it reacts to price, it doesn't predict it. Key limitations:
- False signals in trending markets: In strong trends, RSI can stay overbought/oversold for extended periods without reversing.
- Doesn't account for fundamentals: A stock with RSI at 25 may be oversold for good reason (earnings collapse, fraud, sector rotation). RSI sees price, not value.
- Works better on some assets than others: RSI is more reliable on liquid, high-volume stocks and indices. It can be misleading on thinly traded small caps.
Quick RSI reference
- RSI 70+: Overbought zone — watch for potential pullback
- RSI 50–70: Bullish momentum
- RSI 30–50: Bearish momentum
- RSI below 30: Oversold zone — watch for potential bounce
- Divergence: RSI and price moving in opposite directions — early reversal warning
RSI is one of the most widely used technical indicators for a reason: it's simple, visual, and often right. The key is using it in context — with trend direction, support/resistance levels, and at least one confirming signal — rather than as a standalone buy or sell trigger.