What Is RSI in Stocks? How the RSI Indicator Works

RSI scores recent gains against recent losses on a 0 to 100 scale.
Above 70 is called overbought, below 30 oversold, with 14 periods the default.
Relative strength here means a stock against itself, not against an index.
A stock can stay overbought for weeks while its uptrend continues.
RSI describes past price moves; it explains nothing about the business.

If you have ever opened a stock chart and noticed a wavy line under the price, there is a good chance it was the Relative Strength Index (RSI). Traders have used it since 1978, mainly because it is so easy to read. Easy to read is not the same as reliable though. RSI tells you how price has moved recently, not where it is going next.

In this article you will learn what RSI measures, which is narrower than most people assume and what the evidence says about trading on it and the best way to interpret and use it.

What Is RSI in Stocks?

The Relative Strength Index is a momentum oscillator. It compares the size of a stock's recent gains with the size of its recent losses. The result is expressed as a single number between 0 and 100. J. Welles Wilder Jr. introduced it in his 1978 book, New Concepts in Technical Trading Systems, alongside several other indicators still in use today.

A high reading means recent moves have been dominated by gains. A low reading means losses have dominated. That is the whole of what the number reports.

What RSI stands for, and what it is not

RSI stands for Relative Strength Index. The "relative strength" refers to a stock's gains relative to its own losses, not to any other stock or index.

This is a common mix-up. Comparing one stock's performance against the S&P 500 is relative strength analysis, which is a different technique with a similar name.

What RSI can and cannot tell you

RSI describes the recent behaviour of the price. It knows nothing about the company, its earnings or its valuation.

A stock can show an RSI of 25 because the business is deteriorating, or because the market overreacted to an ordinary news item. The indicator reads the same in both cases.

How Is RSI Calculated?

The formula compares average gains with average losses over a chosen number of periods, usually 14 days.

The formula

RS = average gain over 14 periods / average loss over 14 periods

RSI = 100 - [100 / (1 + RS)]

A worked example makes it concrete. Suppose that over 14 days a stock's average up-day move was 1.0% and its average down-day move was 0.5%. RS is then 2.0, and RSI works out at 100 minus 33.3, which is 66.7. Gains have been twice the size of losses, and the reading sits below the traditional overbought line. Two properties follow from the maths. The output can never leave the 0 to 100 range, and it responds to the size of moves rather than their direction alone.

After the reading

The first RSI value uses a simple average of the first 14 periods. After that, Wilder's method smooths the averages instead of recalculating them from scratch.

Average gain = (previous average gain × 13 + current gain) / 14
Average loss = (previous average loss × 13 + current loss) / 14

On a down day, the current gain is zero. On an up day, the current loss is zero.

Take the earlier example, with an average gain of 1.0% and an average loss of 0.5%. If the next day is a 2.0% gain, the new average gain is (1.0 × 13 + 2.0) / 14 = 1.07%. The new average loss is (0.5 × 13 + 0) / 14 = 0.46%. RS becomes about 2.31, and RSI rises from 66.7 to about 69.8. One large up day moves the reading close to 70.

Why the period setting matters

A shorter setting reacts faster and produces more extreme readings. A longer setting smooths the line and produces fewer signals.

Setting	Behaviour	Trade-off
7 periods	Very reactive, hits 70 and 30 often	More signals, more false ones
14 periods	Wilder's default, moderate sensitivity	The convention most charts show
21 periods	Smoother, extremes are rare	Fewer signals, later ones

Wilder's 14 is a convention, not a discovery. Changing it changes every signal the indicator generates, which is worth remembering when comparing two charts.

How Do You Read an RSI Chart?

RSI is plotted in a separate panel below the price, with horizontal lines usually drawn at 30, 50 and 70.

Reading	Conventional label	What it describes
Above 70	Overbought	Recent gains have strongly outweighed losses
50 to 70	Bullish territory	Gains have modestly outweighed losses
30 to 50	Bearish territory	Losses have modestly outweighed gains
Below 30	Oversold	Recent losses have strongly outweighed gains

Some traders shift the lines to 80 and 20 for volatile stocks, so extremes are not flagged constantly. The levels are conventions that can be adjusted, not fixed thresholds. The words carry more weight than they deserve. "Overbought" does not mean expensive, and "oversold" does not mean cheap.

RSI behaves differently by trend

The 30 and 70 lines are not equally useful in every market. In a strong uptrend, RSI tends to stay in a higher band. In a strong downtrend, it tends to stay in a lower one.

Trend	Typical RSI range	What it means for the 30/70 lines
Uptrend	Roughly 40 to 80	Pullbacks often stop near 40 to 50. Readings above 70 are common and not a reversal signal on their own.
Downtrend	Roughly 20 to 60	Rallies often stall near 50 to 60. Readings below 30 are common and not a bottom signal on their own.

Technical analysts such as Constance Brown and Andrew Cardwell described these range shifts. The practical point is that an "overbought" reading in an uptrend may only confirm the trend.

Reading RSI on a real stock: Tesla (TSLA)

As of October 5, 2026, according to Financhill, Tesla (TSLA) closed at $378.73 with an RSI(14) reading of 58.8. That puts the reading in the 50 to 70 band. It is neither overbought nor oversold. Over the last 14 sessions, gains have modestly outweighed losses. The wider picture looks different. The price still sat below its 200-day simple moving average of $392.63.

A mid-range RSI did not show that longer-term weakness. The same 58.8 could appear early in a recovery or in a brief bounce within a decline, and the number cannot tell you which.

What Signals Do Traders Look For?

Four readings of the indicator come up most often. Each answers a different question about momentum.

1. Overbought and oversold crossings

The simplest use treats a move above 70 as a sign that buying pressure may be stretched, and a move below 30 as the reverse. Most traders wait for the line to cross back out of the zone rather than acting on entry into it. Entering a zone says momentum is strong, not that it is about to stop.

2. The 50 line

The centre line separates a period where gains dominate from one where losses do. Readings that hold above 50 through pullbacks are read as a sign of a healthy uptrend.

3. Divergence

Divergence occurs when price and RSI disagree. A stock making a lower low while RSI makes a higher low is described as bullish divergence. It is treated as a hint that the move is losing force. Divergence can also persist for a long time without resolving, which is why it is a caution flag rather than a trigger.

Hidden divergence is the reverse. Price makes a higher low while RSI makes a lower low, which some traders read as a sign an uptrend may be continuing. Like regular divergence, it is a caution flag, not a trigger.

4. Failure swings

Wilder considered these the strongest RSI signals. A bullish failure swing happens when RSI drops below 30, recovers above it, pulls back without returning below 30, then breaks its prior peak. The pattern uses only the indicator, ignoring price action entirely.

What Are the Limitations of RSI?

Three limitations matter more than any individual signal.

First, RSI can stay overbought or oversold for a long time. In a strong trend, a reading above 70 often means the trend is working. Selling into it repeatedly means selling early through an entire advance.
Second, it is backward-looking. Every input is a past price change, so the indicator describes what has already happened rather than forecasting what comes next.
Third, the settings are adjustable, which invites fitting. Any period and threshold combination can be tuned until it looks good on a chart you have already seen.

The CFA Institute makes a broader point in its refresher reading on technical analysis. Charts and indicators require human interpretation, and technical traders carry the same biases as everyone else.

Does RSI Actually Work?

The honest answer is that the evidence is mixed and weaker than the marketing around indicators suggests.

The most cited survey of the field is Park and Irwin's review in the Journal of Economic Surveys. Among 95 modern studies of technical trading, 56 reported positive results, 20 reported negative results and 19 were mixed. That sounds encouraging until you read the authors' caveat. They note that most of those studies suffer from testing problems, including data snooping.

Data snooping is the practice of trying many rules and reporting the ones that worked. Their survey also found that technical strategies looked profitable in stock markets mainly in earlier decades, with results weakening later.

None of this says RSI is useless. It says a single indicator is not an edge by itself, and any published win rate deserves scepticism about how it was produced.

How Should Long-Term Investors Use RSI?

As context, not as a decision rule. RSI can tell you a stock has fallen hard and fast, which is worth knowing before you act. What it cannot tell you is why. That is a job for the company's revenue and earnings, its balance sheet and its industry position.

Two practical uses fit a long horizon. RSI can flag that a planned purchase is happening into a sharp run-up, which may argue for spreading the buy over time. It can also show that a holding you were about to sell in frustration has simply moved with the market.
Two uses do not fit. Buying purely because a reading is below 30 ignores the reason for the fall, and selling purely because it is above 70 cuts winners short.

If short-term signals are tempting you into frequent trading, the costs deserve a look first.

Gotrade offers fractional shares from $1, which makes spreading a purchase across several dates practical rather than theoretical. That is a more reliable answer to an extreme reading than trying to time the turn.

Conclusion

Next time you see an RSI reading quoted, ask two questions before acting on it. What period is it using, and what is happening in the business behind the chart? An indicator describes the past behavior of a price, and that is all it claims to do, whether it sits at 25 because a company is genuinely deteriorating or because the market simply overreacted to routine news.

Used as context rather than a decision rule, RSI can flag a run-up worth spreading a purchase across, or a holding that only moved with the market rather than for any reason of its own.

This content is for educational purposes only and is not investment advice or a recommendation to use any indicator or strategy. Investing carries risk, including the possible loss of principal. Technical indicators describe past price behaviour and do not predict future results.