RSI Explained: How the Relative Strength Index Works

The Relative Strength Index (RSI) is a momentum oscillator developed by J. Welles Wilder and introduced in his 1978 book New Concepts in Technical Trading Systems. It measures the speed and size of recent price changes and expresses the result as a single number between 0 and 100. Despite the name, it has nothing to do with comparing one stock's strength to another's — it measures a stock's momentum relative to its own recent price history.

How RSI Is Calculated

Over a lookback period (14 days is the standard default, and what this tool uses), RSI separates each day's price change into a gain or a loss, then smooths those gains and losses separately using Wilder's smoothing method — an exponential moving average with a specific weighting (equivalent to an EMA with alpha = 1/period). The smoothed average gain and average loss are combined into a Relative Strength (RS) ratio:

RS = Average Gain / Average Loss

which is then converted into the 0–100 RSI scale:

RSI = 100 − (100 / (1 + RS))

When average gains dominate average losses, RS is large and RSI pushes toward 100. When losses dominate, RSI pushes toward 0. A reading of exactly 50 means gains and losses over the period have been roughly balanced.

Reading the Levels

RSI is traditionally described using two thresholds:

• Above 70is often called "overbought" — momentum has been strongly positive.
• Below 30is often called "oversold" — momentum has been strongly negative.

The common beginner mistake is treating these as automatic reversal signals — "RSI hit 70, so it must fall now." In a genuinely strong trend, RSI can sit above 70 for weeks while price keeps climbing; in a strong downtrend it can sit below 30 for just as long. Overbought does not mean "about to reverse" — it means "currently strong."

RSI as a Trend-Strength Gauge, Not Just a Reversal Signal

A more useful way to read RSI, especially for trend-following strategies, is as a strength gauge across a middle range. A stock in a healthy uptrend will often see its RSI hold above 40–50 even during pullbacks, only dipping toward 30 in a genuine trend reversal. That difference — a shallow RSI dip versus a deep one — is a reasonable way to distinguish "a normal pause within an uptrend" from "the uptrend is over."

This is also why checking RSI on more than one timeframe matters. A stock can show a weak daily RSI reading during a pullback while its weekly and monthly RSI remain firmly strong — meaning the bigger picture is intact and the daily dip may just be noise. See Multi-Timeframe RSI Analysis for how this scanner uses exactly that combination.

RSI Pullback Entries

One common systematic use of RSI is to look for a shallow pullback — RSI easing from a strong reading down toward the 40 area — within a stock that's otherwise trending up on the higher timeframes. The logic: buying strength on a small dip, rather than chasing a stock after a big move or trying to catch a falling knife during a real breakdown. The RSI 5-Star strategy on this site is a fully mechanical version of that idea.

Limitations

• RSI is a lagging-ish momentum measure — it's calculated from past price changes, so it confirms what has already happened rather than predicting what happens next.
• Like any single indicator, it produces false signals in choppy, directionless markets.
• The standard 14-period setting is a convention, not a law — shorter periods react faster and produce more (and noisier) signals; longer periods are smoother but slower.

This is educational content, not investment advice — see the full disclaimer. To see RSI-based filters applied to real markets, browse today's RSI 5-Star scan results.