CluenexWhat is RSI?
The Relative Strength Index (RSI) is a momentum oscillator, meaning a line that moves up and down within a fixed range. It measures how strong recent price gains have been compared with recent losses, on a scale of 0 to 100.
J. Welles Wilder introduced it in his 1978 book New Concepts in Technical Trading Systems. Most charting platforms plot it in a panel under the price chart.
RSI measures momentum, the speed of price change, not value. A stock can have a high RSI and still be cheap, or a low RSI and still be expensive.
How is RSI calculated?
RSI = 100 minus [100 divided by (1 + RS)], where RS is the average gain divided by the average loss over the lookback period. The default period is 14 bars (14 days on a daily chart).
Here is a simple worked example with made-up numbers. Over 14 days, the average up-day gain is 1.0% and the average down-day loss is 0.5% (each averaged across all 14 days).
- RS = 1.0 / 0.5 = 2.0
- RSI = 100 - [100 / (1 + 2.0)] = 100 - 33.3 = 66.7
If gains and losses were equal, RS would be 1 and RSI would be 50. If there were no losses at all, RSI would be 100. If there were no gains, it would be 0.
Wilder's original method smooths the averages after the first 14 periods, so each new day updates the previous average rather than recalculating from scratch. Most charting software does this automatically, which is why values can differ slightly between platforms.
What does an RSI of 70 or 30 mean?
An RSI above 70 is conventionally read as overbought and below 30 as oversold. It means price has risen or fallen quickly relative to its recent history, not that a reversal is due.
| RSI reading | Common label | What it actually says |
|---|---|---|
| Above 70 | Overbought | Gains have been much larger than losses lately |
| 50 | Neutral | Gains and losses roughly balanced |
| Below 30 | Oversold | Losses have been much larger than gains lately |
The 70 and 30 lines are conventions from Wilder, not laws of nature. Some traders use 80 and 20 for volatile assets or 60 and 40 to describe trend strength instead.
Does a high RSI mean a stock will fall?
No. A strong uptrend can hold an RSI above 70 for weeks, and a steady downtrend can hold it below 30 just as long. A high reading says momentum is strong, which is often what a healthy uptrend looks like.
This is why "overbought, so sell" and "oversold, so buy" are weak rules by themselves. The label describes what has already happened to price. It does not say what happens next.
A more careful use is to treat extreme readings as a prompt to look closer. Ask whether the move was driven by news, whether volume supports it, and whether the business changed.
What is RSI divergence?
Divergence is when price and RSI disagree. In bearish divergence, price makes a new high but RSI makes a lower high. In bullish divergence, price makes a new low but RSI makes a higher low.
The idea is that the push in the new direction is losing force. For example, a stock rises from $100 to $110 with RSI peaking at 78, then rises to $112 with RSI peaking at only 69. Price went higher but momentum behind it was weaker.
Divergence is a hint of fading momentum, not a timing tool. Prices can keep trending for a long time while RSI diverges, and many divergences resolve with no reversal.
Which RSI period should you use?
The default of 14 is the most common. Shorter periods such as 7 react faster and swing more, so they cross 70 and 30 more often. Longer periods such as 21 or 28 are smoother and cross the extremes less often.
| Period | Behavior | Trade-off |
|---|---|---|
| 7 | Fast, jumpy | More signals, more false alarms |
| 14 | Standard | Balanced, widely used |
| 21 to 28 | Smooth, slow | Fewer signals, later |
No period is proven best. Because RSI is a mathematical transform of past prices, tuning the period to fit past charts is easy and rarely holds up going forward. Pick one and keep it consistent so readings are comparable over time.
Where does RSI fail?
RSI fails in strong trends, around news events and on thinly traded stocks. It only uses past closing prices, so it cannot see earnings, lawsuits, debt or anything else about the business.
- Trending markets. Readings pinned above 70 or below 30 give repeated early warnings that turn out wrong.
- Gaps and news. A single large earnings move can push RSI to an extreme for reasons RSI cannot explain.
- Illiquid stocks. Few trades means choppy prices and noisy readings.
- Cherry-picked charts. Looking back, it is easy to find the divergence that worked. Charts rarely show the many that did not.
- Different settings. Platforms differ in smoothing and period, so the same stock can show different numbers.
Separately, no widely quoted win rate for RSI signals should be trusted unless it comes from a test that includes costs, many stocks and long time periods, and covers periods the rule was not tuned on. Published claims of a fixed success rate are usually not like that.
How does RSI fit with other indicators?
RSI measures one thing, momentum, so it works best as a check beside other information rather than a trigger. It overlaps with other momentum tools. For example, see our guide on MACD and RSI together and the overview of moving averages.
Useful questions to pair with it:
- What is the trend on a longer chart?
- Is volume rising or falling with the move? See volume analysis.
- Is the price near an area where it has turned before? See support and resistance.
- What do the company's earnings, debt and valuation say?
Cluenex charts include common technical indicators such as momentum oscillators, shown as context. Cluenex treats chart patterns as context, not as buy or sell signals, because its own tests found no reliable directional edge in them.
What this means for you
If you hold stocks in a brokerage account and have a regular job, RSI is best used as a way to notice what has already happened, not as a trigger for trades.
- Use it to spot stretched moves. A reading above 70 or below 30 is a reason to check the news and fundamentals, not a reason to act.
- Keep one setting. Pick 14 periods on a daily chart and stay with it so readings are comparable.
- Check the bigger trend first. Extremes mean less when a stock is trending hard.
- Decide your plan before the move. Know your time horizon and how much you can lose before looking at an indicator.
- Be skeptical of tidy examples and claimed success rates. Ask whether the test included costs, many stocks and long periods.
- Remember that an indicator cannot see the business. Pair chart context with what the company earns, owes and is worth.
For long-term investors who buy and hold diversified funds, RSI is optional. Frequent trading on short-term signals adds costs and taxes that can outweigh any small edge.