RSI Indicator
How the Relative Strength Index measures momentum and identifies overbought/oversold conditions.
Written by the SmartRates Academy Team · Reviewed by M. Reyes, Financial Systems Architect & Data Analyst
🎯 Key Takeaways
- RSI is a momentum indicator that oscillates between 0 and 100, based on the size of recent gains versus losses
- Readings above 70 are commonly described as 'overbought,' and below 30 as 'oversold' — though these are conventions, not fixed rules
- RSI can remain in 'overbought' or 'oversold' territory for extended periods during strong trends
- Divergence between RSI and price is another way traders sometimes use the indicator
What RSI measures
The Relative Strength Index (RSI) is a momentum indicator that compares the magnitude of recent price gains to recent price losses over a specified period (commonly 14 periods), producing a value between 0 and 100. It's designed to measure the speed and magnitude of price movements, helping to gauge whether a security might be 'overbought' or 'oversold' relative to its recent behavior.
The calculation involves averaging the size of up-moves and down-moves over the lookback period and converting that relationship into the 0–100 scale — the exact formula is more involved, but the underlying concept is comparing recent buying momentum to recent selling momentum.
Overbought and oversold conventions
An RSI reading above 70 is commonly described as 'overbought' — suggesting that recent price gains have been unusually strong relative to recent losses, which some traders interpret as a sign that a pullback or reversal could be more likely. An RSI reading below 30 is commonly described as 'oversold' — the opposite situation.
These thresholds (70/30) are conventions, not rules derived from any guarantee — some traders use different thresholds (like 80/20) depending on the asset or market conditions, and 'overbought' or 'oversold' readings don't mean a reversal is imminent or certain.
Example: Interpreting an RSI reading
A stock's 14-period RSI rises to 78 during a sustained rally.
Some traders would describe this as 'overbought' and might watch for signs of slowing momentum or a potential pullback.
However, in a strong uptrend, RSI can remain above 70 for an extended period while the price continues rising — treating 'overbought' as an automatic sell signal has historically led to exiting strong trends too early in many cases.
RSI during strong trends
During a strong, sustained uptrend, RSI can stay in 'overbought' territory (above 70) for a long time as the price continues climbing — and during a strong downtrend, RSI can stay 'oversold' (below 30) for a long time as the price continues falling. This is one of the most common pitfalls for traders new to RSI: treating an overbought/oversold reading as an automatic reversal signal, when it may simply reflect the strength of an ongoing trend.
Divergence
Divergence occurs when the direction of RSI and the direction of price disagree. For example, 'bearish divergence' is sometimes identified when price makes a new high but RSI makes a lower high than its previous peak — interpreted by some traders as a sign that the momentum behind the price move is weakening, even though price itself is still rising. 'Bullish divergence' is the mirror-image situation during a downtrend.
Like other technical signals, divergence is an observed pattern that doesn't always lead to the anticipated outcome — prices can continue in their original direction despite divergence for extended periods.
Using RSI alongside other tools
RSI is often used in combination with trend analysis (e.g., moving averages) and support/resistance levels, rather than in isolation — for example, an oversold RSI reading occurring near a known support level might be considered more significant by some traders than an oversold reading occurring in the middle of an established downtrend with no nearby support.
Example: RSI during a sharp decline
A stock drops 15% over five trading days on no specific news, and its 14-day RSI falls to 22 — well below the traditional 'oversold' threshold of 30. Some traders might interpret this as a sign the decline has become stretched and a bounce could be near.
But RSI can stay below 30 for extended periods during a strong downtrend — 'oversold' doesn't mean 'must go up immediately.' A trader relying solely on RSI during the 2008 or 2020 market declines, for example, would have seen the indicator flash 'oversold' multiple times before the eventual bottom, underscoring why RSI is usually paired with trend context rather than used as a standalone timing tool.
Frequently Asked Questions
What time period is RSI usually calculated over?+
14 periods (e.g., 14 days on a daily chart) is the most commonly referenced default, though some traders use shorter or longer periods depending on their time frame and how responsive they want the indicator to be.
Does an RSI of 50 mean anything?+
A reading near 50 is sometimes interpreted as 'neutral' — neither strongly bullish nor bearish momentum — and some traders watch whether RSI is generally trending above or below 50 as a rough gauge of overall momentum direction.
Can RSI be used on any time frame?+
Yes — RSI can be applied to charts of any time frame (minutes, hours, days, weeks), though the overbought/oversold conventions and typical behavior can vary somewhat depending on the time frame and the specific asset.
⚠️ Mistakes to avoid
✕ Selling automatically when RSI hits 70.
→ Strong trends keep RSI high for a long time. It's not a hard signal.
✕ Using RSI alone.
→ Combine with trend and other tools. Momentum is one input.
✕ Ignoring divergence.
→ RSI diverging from price can hint at weakening momentum — a useful extra read.
✍️ Your turn
Read RSI in context
Examine RSI during both a trend and a range.
- Add RSI to a chart.
- Find a strong trend and note how long RSI stayed extreme.
- Find a divergence between RSI and price.
Next recommended lesson
MACD Indicator →
Stock Analysis Academy