RSI explained, without the maths
5 min read
RSI — the Relative Strength Index — is the most quoted and most misused indicator in trading. It is worth understanding properly, because most of what people say about it is wrong.
What it measures
RSI compares the size of a stock's recent gains to the size of its recent losses and turns that into a number between 0 and 100. High means recent moves have been mostly up; low means mostly down.
That is all it is. It measures the speed and one-sidedness of a move — not value, not direction, and definitely not whether a stock is cheap.
Why 'oversold' is not a buy signal
The usual rule of thumb is that under 30 is oversold and over 70 is overbought. The problem is that both readings mean the same thing: the move has been strong. In a real downtrend, RSI can sit under 30 for weeks while the stock keeps falling. In a strong uptrend it can sit over 70 for months while the stock keeps climbing.
Buying purely because RSI is low is a good way to buy something that is going down for a reason.
The two uses that hold up
Divergence. Price makes a new low, RSI makes a higher low. The second decline had less force. That is a genuine early warning that momentum is shifting — and it is the RSI read we lean on for bottom setups.
Context inside a trend. In an established uptrend, an RSI dip toward 40 often marks a pullback rather than a breakdown. Same reading in a downtrend means nothing of the sort. The trend tells you how to read the number.
Practical rules
- Never take an RSI reading as a trade on its own. Pair it with price structure.
- Divergence is a heads-up to start watching, not an entry.
- Extreme readings during news events are noise — the maths just reflects one violent day.
- The default 14-period setting is fine. Tuning it mostly manufactures signals that fit the past.
Common questions
What is a good RSI level to buy at?
There is not one. A low RSI plus a low that holds plus a close above the last swing high is a setup; a low RSI on its own is just a fast decline.
Does RSI work on every timeframe?
It calculates the same way on any timeframe, but shorter timeframes produce far more false readings. Daily is the usual middle ground for swing trading.
More guides
A short squeeze in plain English: why a stock can rip higher when the people betting against it are forced to buy back.
What the 0–100 squeeze score measures, what the breakout, give-up and stretch levels mean, and when a setup is actually actionable.
The signs that selling is exhausted, the difference between a base and a falling knife, and how to wait for confirmation instead of guessing.
Note: educational content only, not investment advice. Only invest money you can afford to lose.