RSI (Relative Strength Index): calculation and reading

RSI compares the strength of up moves and down moves over a period, on a scale of 0 to 100.

Updated：2026-11-08

How it is calculated

For an N-day RSI (this site shows 6-day and 12-day by default):

• Average gain: the average of the gains on up days in the last N days.
• Average loss: the average of the losses on down days in the last N days (as a positive number).
• RS = average gain ÷ average loss.
• RSI = 100 − 100 ÷ (1 + RS).

The original author, Wilder, smooths day by day: new average = (previous average × (N − 1) + today’s value) ÷ N. This site uses that method by default, and you can switch to a simple average.

How to read it

• The value always stays between 0 and 100: higher means recent up moves were relatively stronger; lower means the opposite.
• Common reference lines are 30 and 70: below 30 is called the low zone and above 70 the high zone. This only describes position; in a sustained trend RSI can stay outside the zone for a long time.
• A shorter period (for example 6 days) is more sensitive and swings more; a longer period is smoother.

Limits

RSI uses only the change of closing prices, not volume, and it does not predict. Use adjusted prices.

Origin

RSI was introduced by J. Welles Wilder Jr. in his 1978 book “New Concepts in Technical Trading Systems”. This article explains the calculation in our own words.

This article only explains the calculation and how to read a chart. It is not investment advice.