Relative Strength Index (RSI)

RSI compares the average gain on up days to the average loss on down days over a specified lookback period (typically 14 days for daily charts). The result is expressed on a 0-100 scale. A reading of 70 means up-day average gains have been approximately 2.3 times larger than down-day average losses over the period -- the stock has been trending up strongly. A reading of 30 means the reverse -- down-days have dominated meaningfully. RSI does not measure price levels or valuations; it measures the internal momentum of recent price action, specifically how one-sided the gains or losses have been.

The 14-period default is not sacred. Shorter periods (7-9 days) produce a more volatile RSI that triggers overbought/oversold readings more frequently -- useful for shorter-term timing. Longer periods (21-25 days) produce a smoother RSI that triggers readings less frequently but with higher conviction when it does reach extremes. Different timeframes also matter: RSI on weekly charts is a very different tool from RSI on daily charts. A stock that is overbought (RSI above 70) on a daily chart but deeply oversold (RSI below 30) on a weekly chart may be experiencing a short-term countertrend bounce within a longer downtrend -- a context that daily-only analysis would miss.

The textbook RSI signal -- buy at 30, sell at 70 -- is the least reliable RSI application. In strongly trending markets, RSI can stay above 70 for weeks or months as momentum stocks grind higher, generating continuous sell signals in a rising market. RSI above 70 in a strong bull market is better interpreted as confirmation of momentum than as a sell trigger. The same principle applies in reverse: RSI below 30 in a bear market can stay oversold as the stock continues to fall, generating false buy signals at successively lower prices.

Bullish and bearish divergence are the most reliable and most underused RSI signals. Bullish divergence occurs when price makes a lower low but RSI makes a higher low -- indicating that selling pressure is weakening even as price is declining, often a precursor to a trend reversal. Bearish divergence occurs when price makes a higher high but RSI makes a lower high -- buying pressure is weakening even as price is climbing. These divergence patterns, particularly when they occur after extended trends and at technically significant price levels, have genuine predictive content that the simple overbought/oversold framework lacks.

RSI as a trend confirmation tool is another high-value application. In a healthy uptrend, RSI consistently reaches 60-70 on rallies and holds above 40-50 on pullbacks. When RSI begins failing to reach 70 on rallies while still touching 30 on pullbacks, it is signaling deteriorating internal momentum even if price is still making new highs. This 'failure swing' pattern -- where RSI's range contracts asymmetrically -- often precedes trend breaks by weeks, giving attentive traders earlier warning than price action alone provides.

RSI levels that indicate extreme conditions vary meaningfully by sector. High-beta technology stocks with high earnings volatility will regularly reach RSI 80+ during momentum phases without representing meaningful overbought conditions. Low-beta utility stocks with stable, predictable cash flows rarely sustain RSI above 70 before some reversion occurs. Applying universal thresholds across all sectors produces systematically biased signals. The calibration should match the typical volatility and momentum characteristics of each sector.

The most powerful RSI applications combine multiple timeframes. A stock oversold on the daily chart (RSI below 30) that is also oversold on the weekly chart (RSI below 35) in a longer-term uptrend, at a key technical support level, with positive fundamental tailwinds is an example where multiple confirming signals converge. Single-signal RSI trades have much lower reliability than multi-timeframe, multi-confirmation setups. Using RSI in isolation is like using one word of a sentence to interpret its meaning.