The Average Directional Index (ADX) measures trend strength, not direction — a single line from 0 to 100 that rises when a trend is strong and falls when price is ranging. As a filter, it answers one question cleanly: is there a trend worth following, or not?
What sub-20 means
An ADX reading below 20 (some use 25) signals a weak or absent trend — price is ranging, chopping, going nowhere directionally. In that environment, trend-following setups (breakouts, momentum entries) fail repeatedly, because there's no trend to carry them. The market is rotating, not trending, and the ADX is telling you so before you take a losing breakout.
How to use the filter
Treat ADX < 20 as a “switch to range tactics” signal: stop looking for breakouts and trends, and favor reversion plays — fade the edges, buy the dips, target the middle. When ADX rises through 20 and climbs, a trend is establishing — that's your cue to switch to trend-following. It's a fast confirmation of what the regime and VWAP slope are telling you.
ADX below 20: no trend, so don't trend-trade. It's a filter that saves you from taking breakouts on a day that has none.
The honest limits
ADX lags — it confirms a trend after it's begun and confirms chop after it's set in, so it's a context filter, not an entry trigger. It also doesn't give direction (a high ADX can be a strong up- or down-trend). Use it to decide whether to trend-trade, get direction from price and the regime, and time entries at levels. As one filter among several — ADX, VWAP slope, the EMA ribbon, the regime — it earns its place.