RSI

Introduction

Moving averages tell you which way a market is leaning. They say nothing about how forcefully it is moving — and force matters. A market can be rising gently or surging; falling calmly or collapsing. The Relative Strength Index, almost always shortened to RSI, is the most popular tool for measuring that force. It is a momentum oscillator: it turns the speed and size of recent price changes into a single number between 0 and 100.

RSI is genuinely useful and almost universally misunderstood. The phrase "overbought" gets read as "sell" and "oversold" as "buy," and in trending markets that reading is actively harmful. So while this article explains what RSI measures and how it is read, it spends just as much effort on what its readings do not mean.

What RSI Measures

The RSI compares the size of recent gains to the size of recent losses over a lookback window (standardly 14 periods) and expresses the result on a 0–100 scale.

The intuition behind the formula is simple. Over the last 14 periods, add up the average gain on up-periods and the average loss on down-periods. Their ratio is the "relative strength," and RSI maps it onto 0–100:

You do not need to compute RSI by hand — every charting tool does it — but the formula tells you what it is: a normalised measure of how one-sided recent movement has been. A high RSI means recent periods have been dominated by gains; a low RSI, by losses; 50 is the balance point.

Reading The Scale

Three reference levels structure the reading:

• Above 70 — "overbought": recent up-momentum has been unusually strong.
• Below 30 — "oversold": recent down-momentum has been unusually strong.
• The 50 centreline: above it, gains have outweighed losses (bullish momentum bias); below it, the reverse. Some analysts watch the 50 line more than the extremes.

The words "overbought" and "oversold" are unfortunate, because they sound like verdicts on value — they are not. They are purely statements about the strength of recent movement.

The Single Biggest RSI Mistake

Read this section twice. The most common and most costly RSI error is treating overbought as "sell" and oversold as "buy."

Here is why it fails. A strong trend, by definition, keeps producing strong momentum — so in a powerful uptrend, RSI can stay above 70 for weeks while price climbs relentlessly. Selling at the first touch of 70 means fighting the trend again and again. The same is true in reverse: in a sharp downtrend, RSI can stay pinned below 30 while price keeps falling.

Overbought means strong, not finished. That is the whole lesson.

Divergence: When Momentum And Price Disagree

RSI's more sophisticated use is divergence — when the direction of momentum and the direction of price part ways. Because RSI measures force, it can weaken even as price grinds to a new extreme, hinting that the move is running on fumes.

• Bearish divergence: price makes a higher high, but RSI makes a lower high. Price is still rising, but with less force behind each push.
• Bullish divergence: price makes a lower low, but RSI makes a higher low. Price is still falling, but selling momentum is fading.

Divergence is the same momentum-vs-price idea the Pattern Lab detection engine looks for. It is a useful warning that force is fading — but it is not a clock. A market can diverge for a long time, and "hidden" divergences can even signal continuation. Like everything in RSI, it is context, not a command.

A Worked Example

A share rallies hard; RSI pushes to 78 and stays above 70 for two weeks as price keeps climbing. A reader who learned "overbought = sell" would have fought the move repeatedly. Then, on a final push to a new price high, RSI prints 72 — below its earlier 78 (a lower high against a higher price high). That is bearish divergence: price is still making ground, but with visibly less momentum.

An honest description: "a strong uptrend showing its first sign of fading momentum — worth watching for a change in structure to confirm it." What it is not: a signal that price will now fall. Momentum fading is a yellow flag, not a turn; the turn, if it comes, would show up as the kind of lower-high/lower-low structure covered in Reversals. RSI hints; structure confirms.

The Honest Limits

• Overbought is not "sell"; oversold is not "buy." They describe momentum strength. In trends, RSI can stay at an extreme far longer than feels reasonable.
• RSI lags and can mislead in ranges and trends alike. It is a derived, smoothed measure — useful context, never a standalone trigger.
• Divergence is a warning, not a timer. Fading momentum can persist for a long time before — if ever — price responds.

RSI measures the force of a single market's recent moves on one bounded scale. The next indicator, MACD, comes at momentum from a different angle — built from the moving averages you already know — and adds the dimension of momentum turning.