What Is RSI? The Relative Strength Index Explained

The short answer

RSI (Relative Strength Index) is a momentum oscillator from 0 to 100 that measures how fast and how far price has moved recently. Above 70 is called overbought, below 30 oversold — but those labels are traps in a strong trend. Its best signal is divergence.

RSI rides between 0 and 100; the 70 and 30 lines are the classic zones.

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What RSI measures

RSI compares the size of recent up-moves to recent down-moves and squashes it into a 0–100 reading. High RSI = up-momentum has dominated; low RSI = down-momentum has. It's a speedometer for momentum, not a price target.

Overbought / oversold (and the trap)

The textbook says sell above 70, buy below 30. The reality: in a strong trend, RSI can sit overbought (or oversold) for a very long time while price keeps running. Blindly fading 70/30 in a trend is a classic way to get steamrolled. They're context, not signals.

Divergence — the real signal

The most useful RSI read is divergence: price makes a higher high but RSI makes a lower high (bearish — momentum fading), or price makes a lower low but RSI makes a higher low (bullish). It hints the trend is running out of fuel — best used with structure, not alone.

Hamster's note: I shorted "overbought" RSI in a bull run more times than I'll admit. RSI at 80 in a rip isn't a sell sign — it's the trend flexing. I save RSI for divergences now and sleep better.

Quick check — RSI has been above 70 for days while price keeps climbing. Sell?

Not on that alone. In a strong uptrend RSI can stay overbought for a long time — fading it just because it's above 70 is a common way to lose. Treat it as context; wait for divergence or a structure break for an actual signal.

Key takeaways

RSI = a 0–100 momentum oscillator.
70/30 are overbought/oversold zones — but they persist in strong trends.
Divergence (price vs RSI disagreeing) is its most useful signal.

Glossary: RSI MACD Market structure The Method

Hamster keeps it real: 'Overbought' is not a sell signal — RSI can pin at 80 for weeks in a rip. Indicators describe the past; fading them because a number looks extreme is a classic way to get steamrolled.

FAQ

What is RSI in trading?

RSI (Relative Strength Index) is a momentum oscillator that ranges from 0 to 100 and measures the speed and size of recent price changes. Readings above 70 are called overbought and below 30 oversold, but its most reliable use is spotting divergence between price and momentum.

What do overbought and oversold mean?

Overbought (RSI above 70) means up-momentum has been strong; oversold (below 30) means down-momentum has been strong. They are not automatic sell/buy signals — in a strong trend RSI can remain overbought or oversold for a long time while price keeps moving.

Why does RSI fail in strong trends?

RSI measures momentum, and in a powerful trend momentum stays extreme, so RSI can stay above 70 (or below 30) for a long stretch while price continues. Fading those levels in a trend is a common mistake; they work better as context than as standalone signals.

What is RSI divergence?

RSI divergence is when price and RSI disagree: price makes a higher high while RSI makes a lower high (bearish), or price makes a lower low while RSI makes a higher low (bullish). It suggests momentum is fading and a reversal may be near — best confirmed with market structure.