The Relative Strength Index, or RSI, is one of the most widely used momentum indicators. It condenses recent price gains and losses into a single number between 0 and 100, and traders use it to judge whether a move is strong, weakening or stretched.
This article explains how the RSI indicator is calculated, works through an example, explains what overbought and oversold readings actually mean, covers divergences, and sets out the limits that cause many RSI-based trades to fail.
How RSI is calculated
RSI was developed by J. Welles Wilder. The standard setting uses 14 periods, which can be days, hours or minutes depending on the chart.
Step 1. For each period, record the gain if the close was higher than the previous close, or the loss if it was lower.
Step 2. Calculate the average gain and average loss over the last 14 periods. After the first calculation, Wilder's method smooths these averages so that each new period carries a fraction of the weight.
Step 3. Relative strength (RS) is the average gain divided by the average loss.
Step 4. RSI = 100 − 100 ÷ (1 + RS).
Worked example. Suppose over 14 days the average daily gain is 1.2 and the average daily loss is 0.8.
RS = 1.2 ÷ 0.8 = 1.5
RSI = 100 − 100 ÷ (1 + 1.5) = 100 − 40 = 60
If gains and losses were equal, RS would be 1 and RSI 50. If there were only gains, RSI would approach 100; only losses, it would approach 0.
Overbought and oversold: what the levels mean
The conventional thresholds are 70 and 30. Readings above 70 are called overbought; below 30, oversold. Some traders use 80 and 20 for stronger signals.
The labels are misleading. "Overbought" does not mean a price must fall; it means recent gains have heavily outweighed recent losses. In a strong uptrend, RSI can stay above 70 for weeks while the price keeps climbing. Selling simply because RSI crossed 70 in such a market means repeatedly stepping in front of a rising trend.
Similarly, in a sharp decline, RSI can stay below 30 while the price keeps falling. "Oversold" stocks can become much more oversold.
A more useful interpretation: RSI measures momentum. High readings show strong buying pressure; low readings show strong selling pressure. Whether that pressure continues or exhausts is a separate question that RSI alone does not answer.
Using RSI with the trend
Many experienced traders use RSI in the direction of the prevailing trend rather than against it.
In an uptrend, RSI often oscillates between roughly 40 and 80. Pullbacks that bring RSI down towards 40 to 50, followed by a turn back up, can mark points where the trend resumes.
In a downtrend, RSI often oscillates between roughly 20 and 60. Rallies that lift RSI towards 50 to 60, followed by a turn down, can mark points where the decline resumes.
The 50 line is sometimes used as a simple momentum filter: above 50, gains have outweighed losses over the period; below 50, the reverse.
Defining the trend first, with a longer moving average or price structure, and then using RSI for timing within it, avoids the most common mistake of fighting a strong move.
Divergences
A bearish divergence occurs when price makes a higher high but RSI makes a lower high. Momentum is weakening even as price rises.
A bullish divergence occurs when price makes a lower low but RSI makes a higher low. Selling momentum is fading even as price falls.
Divergences can warn that a move is losing strength, but they are early and unreliable as timing signals. A trend can show several divergences in a row and keep going. Many traders treat a divergence as a reason to tighten stops or reduce size, not as a reason to take an opposite position on its own.
Divergences are also easy to see in hindsight and easy to miss or imagine in real time, because they depend on which highs and lows you choose to compare.
RSI with options and futures
On the underlying, not the option. RSI is normally applied to the price of the stock, index or future. Applying it to an option's premium mixes in time decay and volatility changes, which distort the reading.
Choosing structures. A trader expecting a pullback in a stretched market might prefer a defined-risk structure, such as a debit spread, rather than selling naked options against a strong trend.
Timeframes. A 14-period RSI on a five-minute chart and on a daily chart can say opposite things. Match the timeframe to how long you intend to hold the position.
Position sizing first. No indicator reading justifies a position too large to survive being wrong.
Limits and good habits
It lags. RSI is built from past prices and confirms what has already happened.
Settings change results. Shorter periods make RSI more sensitive and noisier; longer periods smoother and slower. Tuning settings to past data until they look perfect is a classic way to fool yourself.
It ignores news and fundamentals. A result or policy announcement can overwhelm any momentum reading.
Test before trusting. Write precise rules, apply them to past data with realistic costs, then paper trade. Keep a journal of how RSI-based decisions actually performed.
Combine thoughtfully. RSI and other momentum indicators often say the same thing in different ways. Combining RSI with price levels, trend and volatility information adds more than stacking several momentum indicators.
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Frequently Asked Questions
What is the RSI indicator?
The Relative Strength Index, a momentum indicator ranging from 0 to 100 that compares the size of recent gains with recent losses, usually over 14 periods.
How is RSI calculated?
Divide the average gain by the average loss over the period to get RS, then RSI = 100 minus 100 divided by (1 + RS).
Does RSI above 70 mean a stock will fall?
No. It means recent gains have outweighed losses strongly. In strong uptrends RSI can stay above 70 for long periods while prices keep rising.
What is RSI divergence?
When price and RSI move in different directions, such as a higher price high with a lower RSI high, suggesting momentum is weakening. It is an early, unreliable timing signal.
What is the best RSI setting?
The standard is 14 periods. Shorter settings are more sensitive and noisier; longer ones smoother and slower. Test any setting before relying on it.
Related Reading
- Volatility bands around the average
- The volume-weighted average price
- Defining the trend with an average
- Momentum within swing setups
- Testing indicator rules properly
- Defined-risk directional spreads
Disclaimer: TheFinBaba provides educational content only - this is not investment advice. Trading involves risk of loss.
