What it is
The Average Directional Index, or ADX, measures how strong a trend is, whichever way it runs. J. Welles Wilder introduced it in 1978, alongside RSI. It runs from 0 to 100, and the standard setting looks back 14 sessions.
How it is calculated
Each session, the move up is how far the high rose above the prior high, and the move down is how far the low fell below the prior low. Only the larger of the two counts that day. Smoothed over 14 sessions and divided by the average true range, they become +DI and -DI. ADX is the smoothed average of the gap between +DI and -DI, divided by their sum.
How traders read it
Readings under 20 are generally taken to mean no clear trend, readings over 25 a trend worth the name, and readings over 40 a strong one. ADX says nothing about direction: that comes from whether +DI is above -DI.
Because it is an average of an average, ADX is slow. It can fall while a stock holds steady at a new high, only because the price stopped moving much.
ADX on Breakaway
Every public stock page shows ADX 14 and whether +DI is above -DI. Breakaway's regime label, trending, transition, or choppy, starts from ADX and the moving averages, then corrects for the case ADX reads wrong. ADX itself does not count toward the Quality Score.