The Relative Strength Index (RSI) is one of the most widely used momentum indicators in technical analysis. It compresses recent price action into a single number between 0 and 100, helping traders see when a stock has stretched too far, too fast, and when a pause or reversal becomes more likely.

What it is

The Relative Strength Index is a bounded momentum oscillator developed by J. Welles Wilder in the late 1970s and published in his book "New Concepts in Technical Trading Systems". It measures the speed and magnitude of recent price changes and translates them into a single value that can never go below 0 or above 100. That bounded scale is what makes RSI so useful: instead of looking at raw price, you are looking at how forceful the recent buying or selling has been relative to itself.

Because RSI is normalised, it lets you compare momentum across very different instruments - a high-priced index, a small-cap stock, an ETF - on the same scale. A reading of 75 on Apple means roughly the same kind of overextension as a reading of 75 on a thinly traded micro-cap, even though their price charts look nothing alike. That cross-asset comparability is one of the reasons RSI has stayed in the standard toolkit for over forty years.

How it works

Without diving into the formula, the intuition is simple. Over a recent window of bars, RSI looks at every up bar and every down bar. It then asks: how big were the average gains compared to the average losses? When average gains dominate, RSI rises toward 100. When average losses dominate, it falls toward 0. When the two are roughly equal, RSI sits near 50, and momentum is neutral.

The smoothing inside RSI means it does not whip back and forth on every tick. A single big bar will move it, but it takes a sustained imbalance to push it into the extremes. This smoothing is also why RSI lags slightly: by the time it is screaming "overbought", the rally has already happened. RSI is best understood as a confirmation tool, not a crystal ball. The standard lookback is 14 periods, but shorter settings react faster and whipsaw more, while longer settings are calmer and slower.

How to read it

The classic reading levels are 70 and 30. Above 70, the asset is considered overbought - the recent rally has been so one-sided that a pause or pullback becomes more probable. Below 30, the asset is considered oversold - the recent decline has been so sharp that a bounce becomes more probable. These levels are starting points, not absolute thresholds; in strong trends, RSI can stay above 70 or below 30 for long stretches without reversing.

More experienced traders watch for two refinements. The first is divergence: price makes a new high but RSI makes a lower high (bearish divergence), or price makes a new low but RSI makes a higher low (bullish divergence). Divergence often precedes meaningful reversals. The second is the midline cross. A move from below 50 to above 50 is a sign that momentum has flipped from net-negative to net-positive, and many trend-followers use that as an early confirmation.

A simple working checklist:

1. Mark the 30, 50 and 70 levels on the daily chart of a symbol you actually follow.
2. Note whether price is trending or ranging - overbought/oversold reads behave very differently in each regime.
3. Look for divergence between swing highs/lows in price and the matching swings in RSI.
4. Use the 50 midline cross as a momentum-direction filter rather than a standalone trade trigger.

Strengths & limits

RSI is fast to compute, easy to interpret, and works on every timeframe from one-minute charts to weekly charts. It pairs well with trend tools because it tells you something different from a moving average: a moving average tells you direction, RSI tells you intensity. Used together, they help you separate "the trend is up and momentum is healthy" from "the trend is up but momentum is fading".

The biggest limitation is that RSI is a momentum indicator, not a price indicator. In a strong, persistent trend, RSI will print overbought or oversold for many bars in a row, and stubborn contrarians who keep shorting an overbought market will get steamrolled. RSI also struggles in genuinely sideways markets, where it can oscillate between 40 and 60 for days without producing any actionable signal. This is why no serious trader uses RSI in isolation - it needs context from price structure, volume, and the broader regime.

Key takeaway: RSI is a normalised 0-100 gauge of momentum intensity; treat 70/30 as zones to investigate, watch for divergence and the 50 cross, and never trade it without trend context.