RSI Explained: How the Relative Strength Index Works
RSI is a 0–100 momentum oscillator built from the size of recent gains versus recent losses, used to flag when a move may be overextended.
- RSI compares the average size of up-candles to the average size of down-candles over a lookback period, then converts that ratio to a 0–100 scale.
- Above 70 is read as overbought and below 30 as oversold — but in a strong trend, RSI can stay pinned at those extremes for a long stretch without reversing.
- Divergence, where price and RSI disagree about direction, is often a more useful signal than the raw overbought/oversold reading.
- The default period is 14. Shorter periods (like 9) react faster and give more, noisier signals; longer periods (like 21–25) are smoother and slower.
RSI, the Relative Strength Index, answers a specific question: over the last N candles, how much of the price movement has been up versus down, and how lopsided is that split? It says nothing about where price will go next — it describes how strongly it has already been moving in one direction.
That distinction matters more than it sounds. RSI is a momentum indicator, not a prediction engine, and most of the confusion beginners run into with it comes from treating a description of recent momentum as if it were a forecast.
The RSI Formula
RSI is built in two steps. First calculate the relative strength (RS): the average size of up-moves divided by the average size of down-moves over the lookback period (14 candles by default). Then convert RS to the 0–100 scale:
RS = Average Gain ÷ Average Loss (over 14 periods)
RSI = 100 − [100 ÷ (1 + RS)]
Say over the last 14 candles the average up-move was 1.2% and the average down-move was 0.8%. RS = 1.2 ÷ 0.8 = 1.5. RSI = 100 − [100 ÷ (1 + 1.5)] = 100 − 40 = 60. An RSI of 60 means gains have moderately outpaced losses recently — firmly bullish momentum, but nowhere near the 70 line usually called overbought.
Overbought and Oversold — and Why They Lie to You in a Trend
The standard reading treats RSI above 70 as overbought and below 30 as oversold, implying momentum has stretched further than usual and may be due to cool off. That’s a reasonable starting point in a sideways, range-bound market.
It breaks down in a strong trend. During a genuine, sustained rally, RSI can sit above 70 for weeks, because gains keep outpacing losses by a wide margin the entire time — that’s not a malfunction, it’s exactly what strong, one-sided momentum looks like. Shorting the first RSI reading above 70 in a strong uptrend is one of the most common ways new traders fight a trend and lose.
RSI Divergence
Divergence is when price and RSI tell different stories. If price makes a new low but RSI makes a higher low than its previous one, momentum is quietly strengthening even while price still looks weak — that’s bullish divergence. The mirror case, price making a new high while RSI makes a lower high, is bearish divergence: price is still pushing up, but the strength behind that push is fading.
Bullish divergence: price makes a lower low, RSI makes a higher low
Choosing an RSI Length
14 periods is the default nearly every platform ships with, and it remains the most widely watched setting for exactly that reason — enough of the market is looking at the same number that it tends to matter. A shorter length like 9 reacts faster and flags more overbought/oversold conditions, useful for active day trading but noisier in choppy conditions. A longer length like 21 or 25 smooths the line out further, better suited to swing or position trading where you care less about every short-term wiggle.
RSI vs. Stochastic RSI
Stochastic RSI applies the stochastic oscillator’s formula on top of RSI itself rather than on raw price, which makes it react even faster and swing between its extremes more often. It’s more sensitive, and correspondingly noisier — useful for traders who specifically want an earlier, more aggressive signal, at the cost of more false ones. (See the quick-reference glossary for the full breakdown of the stochastic oscillator.)
Key takeaways
- RSI describes recent momentum, it does not predict future price — treat "overbought" as "strong," not as "about to fall."
- The 70/30 levels work best in range-bound markets and are far less reliable during a strong, sustained trend.
- Divergence between price and RSI is usually more informative than the raw number crossing 70 or 30.
- Match the RSI period to your timeframe: shorter for active trading, longer for smoother, less noisy swing signals.
Common mistakes
- Shorting purely because RSI touched 70, with no regard for the broader trend direction.
- Ignoring that an oversold reading in a strong downtrend can persist for a long time before any bounce.
- Reading RSI in isolation with no volume, trend, or support/resistance context to confirm it.
Frequently asked questions
What is a good RSI level to buy?
There is no single number that works universally. Many traders treat RSI dropping below 30 and then curling back above it as a signal worth investigating, but it should be combined with trend and support context, not used as a standalone buy trigger.
Can RSI stay overbought for weeks?
Yes, and it regularly does during strong bull trends in both stocks and crypto. A persistently high RSI reflects genuinely strong, one-sided momentum, not a guaranteed reversal.
Is RSI at 50 bullish or bearish?
Neither on its own — 50 is the midpoint, meaning gains and losses have been roughly balanced. Some traders use the 50 line as a simple trend filter: RSI consistently above 50 supports a bullish bias, consistently below 50 supports a bearish one.
What is the best RSI setting for crypto?
14 remains the standard starting point on crypto just as it is on stocks, since it’s what most of the market is already watching. Because crypto is more volatile, some day traders shorten it to 9 or 10 for faster signals, accepting more noise in exchange for quicker reactions.
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Disclaimer: this article is educational content, not financial advice. Markets involve risk, and past behavior does not guarantee future results.
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