 is, how to calculate it, read 70/30 levels, spot divergence and avoid the common mistakes. Guide by Zumedha){kind=link}
Relative Strength Index (RSI): Formula, Levels and How to Use It
A plain-English guide to the most widely used momentum indicator: how it is calculated, what the 70 and 30 levels really mean, and where traders and investors go wrong.
The Relative Strength Index (RSI) is a momentum oscillator that scores how strongly a stock, index or fund has been rising or falling on a scale of 0 to 100. Traders read it to spot overbought and oversold conditions, and investors use it to avoid buying into a stretched price. This guide explains the RSI formula, how to read it on NSE and BSE stocks, and why it works best as a timing aid rather than a verdict.
01What is the Relative Strength Index (RSI)?
RSI was created by J. Welles Wilder Jr. and introduced in his 1978 book New Concepts in Technical Trading Systems. It compares the size of a security’s recent gains with the size of its recent losses and converts the result into a single number between 0 and 100.
The name causes confusion. Despite the word “relative”, RSI does not compare one stock against another or against the Nifty 50. That is “relative performance”. RSI measures a security’s strength against its own recent history.
02RSI formula and step-by-step calculation
The standard RSI uses 14 periods (14 days on a daily chart). It takes two steps: first compute RS, the ratio of average gain to average loss, then convert RS to a 0 to 100 scale.
THE RSI FORMULA
RS = Average Gain / Average Loss
RSI = 100 − [ 100 / (1 + RS) ]
Average Gain and Average Loss are taken over the last 14 periods. Losses are entered as positive numbers.
Worked example with 14 daily closes
Suppose a stock closes at ₹100 and then moves as shown below over the next 14 sessions. Each day’s change is split into a gain (if up) or a loss (if down).
| Day | Close (₹) | Change | Gain | Loss |
|---|---|---|---|---|
| 0 | 100 | – | – | – |
| 1 | 102 | +2 | 2 | 0 |
| 2 | 101 | -1 | 0 | 1 |
| 3 | 104 | +3 | 3 | 0 |
| 4 | 102 | -2 | 0 | 2 |
| 5 | 103 | +1 | 1 | 0 |
| 6 | 107 | +4 | 4 | 0 |
| 7 | 106 | -1 | 0 | 1 |
| 8 | 104 | -2 | 0 | 2 |
| 9 | 106 | +2 | 2 | 0 |
| 10 | 109 | +3 | 3 | 0 |
| 11 | 108 | -1 | 0 | 1 |
| 12 | 109 | +1 | 1 | 0 |
| 13 | 106 | -3 | 0 | 3 |
| 14 | 108 | +2 | 2 | 0 |
| Total | 18 | 10 |
- Average gain = 18 ÷ 14 = 1.286 and average loss = 10 ÷ 14 = 0.714.
- RS = 1.286 ÷ 0.714 = 1.80.
- RSI = 100 − [100 ÷ (1 + 1.80)] = 100 − 35.71 = 64.3.
How later values are smoothed
From the 15th session onward, Wilder’s method smooths the averages instead of recalculating from scratch:
WILDER SMOOTHING
Avg Gain = (Previous Avg Gain × 13 + Current Gain) / 14
Avg Loss = (Previous Avg Loss × 13 + Current Loss) / 14
If the stock rises ₹2 on day 15, the average gain becomes (1.286 × 13 + 2) ÷ 14 = 1.337 and the average loss becomes (0.714 × 13 + 0) ÷ 14 = 0.663. RS is then 2.02 and RSI rises to about 66.8. You will never need to do this by hand, because every charting platform computes it, but knowing the mechanics explains why RSI reacts the way it does.
03How to read RSI: the 70, 50 and 30 levels
Above 70: overbought
Buying has been strong and the price has risen quickly. It signals stretched momentum, not an automatic reversal.
Below 30: oversold
Selling has been heavy and the price has fallen quickly. It signals stretched weakness, not an automatic bottom.
| RSI range | Momentum reading | What it usually suggests |
|---|---|---|
| 70 to 100 | Overbought | Strong uptrend or a stretched rally; watch for divergence |
| 50 to 70 | Bullish momentum | Buyers in control; healthy in uptrends |
| 30 to 50 | Bearish momentum | Sellers in control; common in downtrends |
| 0 to 30 | Oversold | Stretched decline; watch for stabilisation |
Key point. “Overbought” does not mean “expensive” and “oversold” does not mean “cheap”. RSI knows only the price path of the last few weeks. It knows nothing about earnings, balance sheets or valuation.
04Common ways traders use RSI
1. Overbought and oversold reversals
The simplest approach treats a move back below 70 as a warning that upside momentum is fading, and a move back above 30 as a sign that selling pressure is easing. Acting on the cross back through the level, rather than the first touch, avoids selling a stock simply because it is strong.
2. The 50 line as a trend filter
RSI holding above 50 on pullbacks suggests an uptrend is intact, while RSI failing at 50 on bounces suggests a downtrend. Many traders use this as a simple filter to trade only in the direction of momentum.
3. Range shifts in trends
Technical analysts such as Andrew Cardwell and Constance Brown observed that RSI behaves differently by trend. In strong uptrends it tends to range roughly between 40 and 80, and in strong downtrends roughly between 20 and 60. So a reading of 75 in a bull trend is normal, and a dip to 40 can mark a pullback rather than a collapse.
4. Divergence
Divergence appears when price and RSI disagree, and it is one of the most respected RSI signals.
- Bearish divergence: price makes a higher high but RSI makes a lower high, hinting that the rally is losing strength.
- Bullish divergence: price makes a lower low but RSI makes a higher low, hinting that selling is losing strength.
5. Failure swings
Wilder’s own pattern. In a bullish failure swing, RSI drops below 30, bounces, pulls back without falling below 30 again, and then breaks above the earlier bounce high. The bearish version mirrors this above 70. It is a signal drawn from RSI alone, independent of the price chart.
05Best RSI settings and timeframes
| Setting | Behaviour | Typical use |
|---|---|---|
| RSI (9) | Fast, many signals, more noise | Short-term trading |
| RSI (14) | Wilder’s default, balanced | Most daily and weekly charts |
| RSI (21 to 25) | Smooth, fewer signals | Position trading, trend confirmation |
Timeframe matters as much as period. A daily RSI reflects weeks of price action, a weekly RSI reflects months, and a monthly RSI is most relevant to long-term investors. Signals that agree across daily and weekly charts carry more weight than a signal on one chart alone. There is no universally “best” setting: choose one, keep it consistent, and judge it over many occasions rather than a few.
06Limitations and common RSI mistakes
Where RSI helps
- Quick read on momentum strength
- Highlights stretched moves
- Divergence can flag fading trends
- Works on any liquid security and timeframe
Where RSI misleads
- Can stay overbought or oversold for long periods
- Ignores fundamentals and valuation
- Generates false signals in sideways markets
- Less reliable in thinly traded stocks
Mistakes to avoid
- Selling every reading above 70. Strong stocks often stay above 70 while the trend continues.
- Buying every reading below 30. A weak stock can keep sliding while RSI hugs the low end, which is how traders “catch falling knives”.
- Ignoring the trend. The same RSI value means different things in an uptrend and a downtrend.
- Using RSI alone. Pair it with trend, support and resistance, volume and, for investors, valuation.
- Over-tuning the settings. Tweaking the period until it fits past data usually fails on new data.
07RSI and fundamental analysis: how investors can use it
At Zumedha, valuation answers what a business is worth and RSI helps with when the market’s mood is stretched. They are different questions, and mixing them up is a frequent source of error.
- A fundamentally strong company with a monthly RSI deep in overbought territory may deserve patience on fresh purchases, because momentum can mean-revert even when the business is sound.
- A company with weak fundamentals and a low RSI is not cheap by default. Falling prices can reflect real deterioration.
- For staggered investing, a weekly RSI cooling toward the 40 to 50 zone in an uptrend is sometimes used to space out purchases rather than invest all at once.
These are ways to think about timing, not recommendations on any security. The ownership decision should still rest on business quality, balance-sheet strength and valuation.
08RSI frequently asked questions
What is a good RSI value?
There is no single good value. Above 70 is conventionally overbought and below 30 oversold, while 40 to 60 usually indicates balanced momentum. In strong uptrends RSI often ranges between 40 and 80, so context matters more than any one number.
Is RSI above 70 a sell signal?
Not by itself. It shows strong recent buying and a quick price rise. In a strong trend RSI can remain above 70 for weeks, so traders usually wait for confirmation such as a bearish divergence or a break in price structure.
What is the best RSI setting?
The default of 14 periods, proposed by Wilder, suits most charts. Shorter settings like 9 react faster but give more false signals, while 21 or 25 are smoother and signal less often.
Can RSI be used for long-term investing?
RSI is a momentum and timing tool, not a valuation tool. Long-term investors can use weekly or monthly RSI to avoid chasing stretched prices, but the decision to own a business should rest on fundamentals and valuation.
What is the difference between RSI and moving averages?
A moving average smooths price to show trend direction, while RSI measures the speed and size of recent price changes on a 0 to 100 scale to show momentum. They answer different questions and are often used together.
09Key takeaways
IN SHORT
- RSI = 100 − [100 / (1 + RS)], where RS is average gain divided by average loss over 14 periods.
- Above 70 means stretched upward momentum and below 30 means stretched downward momentum. Neither is a standalone buy or sell signal.
- Trend context, divergence and the 50 line make RSI far more useful than the 70/30 levels alone.
- RSI measures price momentum, not value. Pair it with fundamentals and valuation for investing decisions.
Disclaimer: This article is for educational purposes only and does not constitute investment advice, a recommendation or an offer to buy or sell any security. Technical indicators have limitations and past patterns do not guarantee future results. Zumedha Equity Research is an independent research platform and is not a SEBI-registered investment adviser. Please consult a qualified adviser and do your own research before investing.