The Absolute Breadth Index is a market momentum indicator developed by Norman G. Fosback that displays the activity, volatility, and change taking place on the New York Stock Exchange. It calculates the difference between the advancing issues and declining issues and, by dropping the sign (taking the absolute value), discards the actual direction prices are headed.

By discarding market direction, the ABI functions as an "activity index." High values indicate market activity and change, while low readings indicate a lack of change. Fosback has noted historically high values typically lead to higher prices three to twelve months later. One highly reliable variation of the Absolute Breadth Index is to divide the weekly ABI by the total issues traded. If after a ten-week moving average is calculated and readings are above 40%, they are said to be very bullish. Readings below 15% are bearish.

The Absolute Breadth Index is calculated by subtracting the number of declining issues from the number of advancing issues and taking the absolute value of the difference:

ABI = |Advancing Issues - Declining issues|

recalling that |-100| = 100 = |+100|