The Relative Strength Index, usually called RSI, is a momentum indicator. It compares the strength of recent upward price changes with the strength of recent downward price changes. Traders use it to study momentum, spot unusually strong moves and identify possible disagreement between price and momentum.
RSI moves between 0 and 100. It does not predict where price must go next. A high reading can remain high during a strong advance, while a low reading can persist during a strong decline. RSI is therefore better treated as evidence about market conditions than as an automatic entry signal.
How RSI is calculated
The common setting uses 14 periods. A period may represent minutes, hours, days or another chart interval. The indicator first separates positive price changes from negative price changes. It then calculates an average gain and an average loss across the selected lookback.
The calculation has two main steps:
- Relative strength equals average gain divided by average loss.
- RSI equals 100 minus 100 divided by 1 plus relative strength.
Most charting software uses a smoothing method associated with the indicator's original formula. After the first calculation, each new value includes part of the previous average. This reduces abrupt jumps, although RSI can still move quickly when price changes sharply.
Worked hypothetical example
Imagine a simplified calculation in which the average gain is 10 points and the average loss is 5 points. Losses are entered as positive quantities for this calculation. Relative strength is 10 divided by 5, which equals 2.
RSI is then 100 minus 100 divided by 1 plus 2. This becomes 100 minus 33.33, giving an RSI of approximately 66.67. The reading shows that gains have been stronger than losses across the sample. It does not mean price has a 66.67 percent chance of rising.
If the average loss were zero, the formula would produce an RSI of 100. If the average gain were zero, RSI would be 0. These extremes reflect one-sided movement within the calculation window, not certainty about the next price change.
Overbought and oversold readings
Traders often label RSI readings above 70 as overbought and readings below 30 as oversold. These labels describe unusually strong recent momentum. They do not prove that an asset is objectively expensive or cheap.
In a range, RSI may move above 70 near the upper part of the range and below 30 near the lower part. Under those conditions, a return through the threshold can support a reversal idea, especially when price also rejects a clear zone.
In a trend, the interpretation changes. Strong upward movement can keep RSI above 70 while price continues rising. Strong downward movement can hold it below 30 while price continues falling. Entering against the trend solely because a threshold was crossed can expose a trader to repeated losses.
Some traders adjust thresholds after testing the instrument and timeframe. For example, they may examine 80 and 20 to reduce signal frequency. Any adjustment should be defined before testing, applied consistently and evaluated across different market conditions.
Understanding RSI divergence
Divergence appears when price and RSI move in different directions. It suggests that momentum is not confirming the latest price swing, but it does not identify an exact turning point.
- Bullish divergence: price forms a lower swing low while RSI forms a higher swing low. Selling momentum may be weakening.
- Bearish divergence: price forms a higher swing high while RSI forms a lower swing high. Buying momentum may be weakening.
The swing points should be clear and comparable. Connecting a major price low to a minor RSI dip can create a misleading signal. Both points should also come from the same timeframe and similar positions in the market structure.
Divergence may continue for several swings before price reverses. It can also fail completely. Traders often wait for separate confirmation, such as a break of a swing point, a rejection pattern or a change in trend structure. Waiting reduces early entries, but it may also produce a less favorable entry price.
Limits and common mistakes
RSI is based entirely on past price changes. It cannot account for future order flow, sudden information or changes in liquidity. Its reading also depends on the chosen timeframe and lookback. A short lookback reacts faster but creates more noise. A long lookback is smoother but responds later.
Common mistakes include:
- Selling immediately above 70 or buying immediately below 30.
- Treating overbought as expensive and oversold as cheap.
- Ignoring whether price is trending or ranging.
- Forcing divergence between unrelated swing points.
- Using several momentum indicators that provide nearly identical information.
- Changing settings after every losing trade.
- Testing signals without spreads, commissions, slippage or realistic execution.
- Entering without a defined exit or position size.
No RSI setting removes trading risk. Even a historically useful signal can fail several times in sequence. Risk per trade should be small enough that one loss, or a cluster of losses, does not seriously damage the account.
A practical RSI checklist
- Identify whether the market is trending, ranging or unclear.
- Keep the RSI lookback and thresholds fixed during a test.
- Mark clear price swings before looking for divergence.
- Require a separate entry condition, such as a structure break.
- Define the invalidation point, stop placement and position size before entry.
- Record the setup, timeframe, RSI reading and result in a journal.
- Review a meaningful sample of trades before changing the rules.
Use RSI as one part of a written process. If the price context, confirmation or risk limit is missing, skip the trade regardless of the indicator reading.
Educational content only, not investment advice. Leveraged trading can lose more than you expect.