RSI explained: what overbought and oversold really mean
RSI is a speedometer for recent price moves, not a measure of value. Learn what it shows, what it cannot tell you, and how to test a rule built on it.
Key takeaways
- RSI turns the last 14 periods of price moves into a number from 0 to 100.
- Above 70 only means price has risen fast; below 30 only means it has fallen fast.
- An RSI reading is a setup to look at, never a buy or sell signal by itself.
- Test any RSI rule on paper, with charges included, before trusting it.
What RSI measures
The Relative Strength Index (RSI) compares how large the recent up-moves were with how large the recent down-moves were, usually over the last 14 periods. On a daily chart that means the last 14 days. The result is a single number between 0 and 100.
Think of it as a speedometer. A high number says price has been rising quickly. A low number says it has been falling quickly. It says nothing about whether the price is cheap or expensive.
How to read 30 and 70
By convention, a reading above 70 is called overbought and a reading below 30 is called oversold. These are only labels for a fast move. A strong trend can keep RSI above 70 for weeks, and a weak one can keep it below 30 for weeks.
So the useful question is not whether RSI crossed 30, but what else agrees with it: the trend, the risk you are willing to take, and the costs of trading it.
An illustration with Stock A
Imagine Stock A falls hard for a week. Its RSI drops below 30. Some traders treat that as a setup worth watching, because a sharp fall can be followed by a pause. Others read the same fall as a trend breaking down and stay out. Both are betting on different ideas, and neither reading is guaranteed to be right.
How strategies use RSI
A mean-reversion rule uses a low RSI as a possible sign that a fall was overdone, usually together with a longer-term trend check. A momentum rule uses RSI staying high or rising as a sign that a move has strength. SmartPaperDesk includes strategies of both kinds, such as RSI Trend Momentum and RSI(2) Mean Reversion. Open a strategy card to read its exact rule.
Common mistakes
Treating 30 as a buy signal and 70 as a sell signal on its own. Ignoring the trend. Forgetting trading costs, which matter more for rules that trade often. Judging a rule on one good example instead of testing it across many trades and periods.
Test an RSI rule on paper
The safest way to learn is to write the rule down, then test it on historical daily data with estimated charges and stops, and look at the number of trades and the worst dip, not only the final number. SmartPaperDesk lets you do this on the free plan with no real money.
Questions people ask
Is RSI below 30 a buy signal?
No. It only says price has fallen quickly. Whether a rule based on it works depends on the trend, the exit and the costs, and must be tested.
What RSI period is standard?
14 periods is the common default, which is 14 days on a daily chart. Shorter periods react faster and give more signals; longer ones are smoother.
Test an RSI rule on the free plan
Keep reading
SmartPaperDesk is an educational paper-trading tool. Examples use a made-up Stock A.
For educational purposes only. This is not investment advice. Past performance does not guarantee future results.