ADX & DMI

Introduction

Most indicators tell you which way a market is moving. Few tell you something just as important: how strongly. The Directional Movement Index (DMI) and its companion the Average Directional Index (ADX) — both developed by Welles Wilder, the same trader-engineer behind RSI and ATR — fill that gap. The DMI's two lines show which side is winning, while the ADX line measures how strong the trend is, regardless of direction. That combination makes ADX one of the most useful filters in technical analysis: it tells you whether the market is in a state where trend-following tools will work, or a range where they'll chop you up. This lesson explains the DI lines, the ADX line, the conventional thresholds, and how to use them together.

This builds on the moving-averages, trendlines and ATR lessons — ADX is, under the hood, an ATR-normalised measure of directional movement.

Quick Definition

The DMI plots two lines — +DI (upward directional movement) and -DI (downward) — and whichever is higher shows which side is in control. The ADX is a third line, derived from the two, that measures trend strength on a 0-100 scale regardless of direction. Conventionally, ADX above ~25 signals a trending market and below ~20 a weak or ranging one. Direction comes from the DI lines; strength comes from ADX.

The key mental split: DI = direction, ADX = strength. A market can be strongly trending up (high ADX, +DI on top) or strongly trending down (high ADX, -DI on top) — ADX looks the same in both, because it only cares how strong the move is.

The DI Lines: Which Side Is Winning

The +DI and -DI lines come from comparing how much price moves up versus down over the lookback (typically 14 periods), normalised by the ATR so they're comparable across markets:

• +DI rises when upward movement dominates — higher highs being made with conviction.
• -DI rises when downward movement dominates — lower lows being made with conviction.
• Whichever line is higher shows control: +DI above -DI means buyers are winning; -DI above +DI means sellers are.

The crossings matter: when +DI crosses above -DI, it's a bullish directional signal; when -DI crosses above +DI, it's bearish. These crossovers are the DMI's directional trigger — but, as we'll see, they're far more reliable when ADX confirms the trend has strength.

The ADX Line: How Strong Is the Trend

The ADX is built from the spread between +DI and -DI: the more one dominates the other, the stronger the directional move, and the higher the ADX. Crucially, ADX ignores direction — it rises whether the dominant side is buyers or sellers. The conventional readings:

• ADX below ~20: weak or absent trend — the market is ranging. Trend strategies struggle here.
• ADX above ~25: a trend strong enough to trade with. The higher the ADX, the stronger the trend.
• A rising ADX: the current trend is strengthening, whichever way it points.
• A falling ADX: the trend is losing strength — consolidating or potentially reversing — even if price still drifts the same way.

Putting It Together

The two parts combine into a clean reading:

• Strong uptrend: +DI above -DI and ADX rising above 25. Trend-following longs are favoured.
• Strong downtrend: -DI above +DI and ADX rising above 25. Trend-following shorts are favoured.
• Range / no trend: ADX below 20, DI lines tangled and crossing back and forth. Trend strategies are likely to whipsaw; mean-reversion (fading the edges of the range) suits better.

The most valuable use is as a filter. A breakout or moving-average crossover taken when ADX is high and rising is far more trustworthy than the same signal when ADX is low — in a low-ADX range, breakouts fail and crossovers chop. ADX doesn't tell you what to trade so much as whether the market is in a state where your trend tools will work.

Common Misconceptions

• "A high ADX means go long." No — ADX is directionless. A high ADX can mean a strong downtrend. Direction comes from the DI lines (which is on top), not ADX.
• "ADX predicts reversals." It measures strength, not turning points. A falling ADX warns a trend is weakening, but that's not a precise reversal signal — price can keep drifting.
• "DI crossovers are reliable on their own." They whipsaw badly in low-ADX ranges. They're far stronger when ADX confirms the trend has strength.
• "Above 25 is a hard rule." The 20/25 thresholds are conventions, not laws — they vary by market and timeframe. Use them as guides and adjust to what the instrument actually does.

Real-World Application

A trader has a moving-average crossover system that keeps getting chopped up. They add ADX as a filter. Now they only take crossover signals when ADX is above 25 and rising — confirming a real trend is present — and stand aside when ADX is below 20. On a choppy, range-bound stretch, ADX sits at 15, the DI lines tangle, and the filter keeps them out of a series of false crossovers that would have lost money. Then a genuine move begins: +DI crosses above -DI, ADX turns up and pushes through 25, and the filter green-lights the long. They ride a clean trend that the ADX confirmed had strength. A second trader, taking every crossover without the filter, bled money in the range and only broke even on the trend. ADX didn't generate the trades — it told them which market regime they were in, which was the difference.

Key Takeaways

• The DMI gives direction (+DI vs -DI — whichever is higher is winning); the ADX gives strength (0-100, directionless).
• ADX above ~25 = trending (trade with the trend); below ~20 = weak/ranging (trend tools whipsaw). A rising ADX = strengthening trend.
• A bullish signal is +DI crossing above -DI; bearish is the reverse — stronger when ADX confirms strength.
• ADX is most powerful as a filter: it tells you whether breakouts and crossovers are likely to work (high ADX) or fail (low ADX).
• The thresholds are conventions, and ADX measures strength, not direction or precise reversals — read the DI lines for direction.