ADX Indicator: Measuring Trend Strength

Most indicators tell you which way price is moving. ADX tells you whether it's worth trading at all.

The Average Directional Index (ADX), created by J. Welles Wilder in 1978, measures the strength of a trend — not its direction. That makes it the perfect filter: it tells you when to trade trend-following strategies and when to stand aside.

The components

ADX is part of the Directional Movement System, which has three lines:

1. +DI — positive directional movement (bullish pressure)
2. −DI — negative directional movement (bearish pressure)
3. ADX — the average of the difference between +DI and −DI

The two DI lines show direction; ADX shows strength.

The formula (simplified)

+DM = current high − previous high  (if positive, else 0)
−DM = previous low − current low    (if positive, else 0)

Smooth both over 14 periods, divide by ATR to normalise, then:

DX = 100 × |(+DI − −DI)| / (+DI + −DI)
ADX = 14-period average of DX

ADX is always positive and ranges from 0 to 100.

The strength threshold

ADX	Trend strength	Action
0–20	Weak / none	Trade ranges or stand aside
20–25	Forming	Watch for confirmation
25–50	Strong	Trade trend-following strategies
50–75	Very strong	Trend likely extended
75–100	Extreme	Rare; reversal risk

The 25 line is the classic filter. Below it, trend-following strategies underperform; above it, they shine.

How to read it

1. Direction — if +DI is above −DI, the trend is up; if −DI is above +DI, the trend is down
2. Strength — if ADX is above 25, the trend is strong enough to trade
3. Rising/falling ADX — a rising ADX means strength is increasing; a falling ADX means the trend is fading

Worked example

Reading	Interpretation
ADX = 30, +DI > −DI	Strong uptrend — look for longs
ADX = 30, −DI > +DI	Strong downtrend — look for shorts
ADX = 15, +DI > −DI	Weak uptrend — avoid trend-following
ADX rising from 18 to 28	Trend strengthening — switch on

Why ADX is a filter, not a trigger

ADX doesn't tell you when to enter — it tells you whether the environment is right. Pair it with a trigger indicator:

• Use moving average crossovers as the trigger
• Use MACD or RSI for entries
• Use ADX to confirm the trend is worth trading

Common mistakes

1. Treating ADX as a buy/sell signal — it isn't; it's a strength gauge
2. Ignoring the DI lines — ADX alone gives no direction
3. Trading when ADX < 20 — trend strategies fail in weak-trend markets
4. Chasing ADX > 50 — the trend is often exhausted by then

How to start

1. Add the 14 ADX (with +DI and −DI) to a daily chart
2. Mark the 25 line
3. Only take trend-following trades when ADX > 25
4. Below 25, switch to range strategies or stand aside
5. Always set a volatility-based stop with the stop loss calculator

Summary

ADX is the trade filter most beginners skip — and the one they most need. It tells you whether the market is trending strongly enough to follow, sparing you the whipsaws that destroy trend strategies in flat markets. Pair it with a directional trigger and your hit rate improves immediately.