What it means
The mechanics are more approachable than the name suggests. RSI takes the average size of the price rises and the average size of the price falls over the chosen window, compares the two, and converts the ratio into a score out of 100.
A share that has risen on almost every day of the window scores near 100, one that has fallen on almost every day scores near 0, and a share moving sideways sits around 50. The 14 day setting is the convention, though shorter windows such as 7 days make the indicator twitchier and longer ones such as 21 days smooth it out.
Whatever the setting, RSI measures the character of recent price movement, not the value of the underlying business. The overbought and oversold labels cause more confusion than anything else in technical analysis.
An RSI of 80 does not mean a share is expensive or due to fall; it means buyers have dominated recently, and in a strong upward trend a share can hold a high reading for weeks. Experienced users treat extreme readings as a prompt to look more closely rather than as a signal to trade.
The pattern practitioners watch most is divergence, where the price makes a new high but the RSI does not, suggesting the buying pressure behind the move is fading. The same logic applies in reverse at market lows, where a new price low on a rising RSI hints that selling is running out of steam.
For anyone in a corporate finance or investor relations role, RSI is worth understanding as a description of trading behaviour rather than as an investment tool. When a share moves sharply on no news, part of the explanation often lies in traders and algorithms responding to indicators like this one.
In practice
Real-world examples.
Example
A portfolio manager holding a technology share sees its RSI climb to 82 after a strong earnings report. Rather than selling, she notes the reading, checks that the fundamental case is unchanged, and simply tightens her stop loss to protect the gain.
Example
A commodities trader watches an energy share fall for three weeks until its RSI drops to 24. He combines that with news that the sell-off was driven by a one-off tax charge and takes a small position, using the indicator as one input among several.
Example
A treasury team monitoring its own company's shares before a bond issue notices RSI has fallen below 30 on heavy volume with no announcement. The signal prompts a call with the corporate broker, who confirms a large index fund has been rebalancing rather than any investor losing confidence.
Think of it
“RSI measures momentum strength-how strong the recent price moves are.
Formula
Calculation
RS = average gain over the period / average loss over the period
RSI = 100 - (100 / (1 + RS))
Over the last 14 trading days, a share rose on 9 days for a combined gain of $8.40 and fell on 5 days for a combined loss of $2.80. Both totals are divided by the full 14 day window, not by the number of up or down days.
Average gain = $8.40 / 14 = $0.60. Average loss = $2.80 / 14 = $0.20. RS = $0.60 / $0.20 = 3.0.
RSI = 100 - (100 / (1 + 3.0)) = 100 - (100 / 4) = 100 - 25 = 75. A reading of 75 sits above the conventional overbought threshold of 70, telling us that gains have outweighed losses three to one over the fortnight, not that the share is now overvalued.Case study
Seen in the real world.
This is an illustrative and entirely fictional example. Larkspur Capital, an invented boutique investment firm, built a simple rule for one of its funds: buy any holding whose RSI fell below 30 and sell any whose RSI rose above 70.
In the fictional back test the rule looked excellent, because it had been fitted to a period of choppy, range-bound markets where prices repeatedly reverted to the middle. Run live through a strong 18 month bull market, it sold the fund's best performers early and again and again bought into shares that kept on falling.
Larkspur's illustrative conclusion was that RSI describes momentum but says nothing about whether a trend will continue. The firm rewrote the rule so that an extreme reading only triggered a review by an analyst, who then had to find a fundamental reason before any trade was placed.
Watch out
Common mistakes.
- Reading an RSI above 70 as an instruction to sell, when strongly trending shares routinely stay overbought for weeks at a time.
- Dividing total gains by the number of up days instead of by the full period length, which inflates the average gain and distorts the result.
- Using RSI alone as a trading system rather than alongside price trend, volume and the underlying business case.
Questions
People also ask.
What does an RSI of exactly 50 mean?
Average gains and average losses over the period have been equal, so momentum is neutral in both directions.
Does RSI work on anything other than shares?
Yes, it is applied to currencies, commodities, indices and bonds, and to any time frame from one minute bars to monthly data.
Is RSI useful for a long term investor?
Only marginally, since it captures short term price behaviour and says nothing about earnings, cash flow or the value of the business.
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