What is the Average Directional Index (ADX)? Definition, Formula, and Example

What is the Average Directional Index?

The Average Directional Index (ADX) measures how strong a price trend is, irrespective of its direction. Developed by J. Welles Wilder and introduced in his 1978 book *New Concepts in Technical Trading Systems*, ADX reads from 0 to 100: values below 20 indicate a weak or absent trend, 20–25 is a transitional zone, 25–40 is a strong trend, 40–60 is a very strong trend, and readings above 60 signal an extreme trend that frequently precedes exhaustion. ADX does not indicate whether price is moving up or down — that information comes from its two companion lines, the Positive Directional Indicator (+DI) and Negative Directional Indicator (−DI).

How ADX is calculated

The computation runs in five steps, all using Wilder's 14-period smoothing (equivalent to an EMA with α = 1/14):

1. True Range (TR): max(High − Low, |High − Prev Close|, |Low − Prev Close|)

2. Directional Movement:

- +DM = (Current High − Prev High) if positive and greater than (Prev Low − Current Low), else 0

- −DM = (Prev Low − Current Low) if positive and greater than (Current High − Prev High), else 0

3. Smooth over 14 periods: Smoothed TR₁₄, Smoothed +DM₁₄, Smoothed −DM₁₄

4. Directional Indicators:

- +DI14 = 100 × (Smoothed +DM₁₄ / Smoothed TR₁₄)

- −DI14 = 100 × (Smoothed −DM₁₄ / Smoothed TR₁₄)

5. ADX: 14-period Wilder smooth of DX, where DX = 100 × |+DI14 − −DI14| / (+DI14 + −DI14)

The result is a single line that lags price by roughly two periods of its smoothing window. On a 14-period daily chart, ADX is approximately 14 days behind developing trend strength.

Worked example

NVDA entered one of the most sustained trending phases in large-cap equity history from November 2023 through March 2024. On the weekly chart, ADX crossed above 25 in early December 2023 when NVDA was trading near $480, confirming trend strength. By late January 2024 ADX reached 45 as the stock pushed through $600. The +DI line remained consistently above −DI throughout the move. Traders using ADX > 25 as a trend-confirmation filter would have held long exposure through the entire $480–$900 advance rather than exiting on normal pullbacks. When ADX peaked above 50 and began curling lower in March 2024, it signaled trend exhaustion — NVDA subsequently corrected 15% over six weeks.

When traders use ADX

• Trend filter: Take only directional setups (breakouts, moving-average crossovers, momentum signals) when ADX > 25 and skip them when ADX < 20. This prevents trading reversions as if they were trends.
• Strategy switching: Rotate between trend-following systems when ADX is rising and mean-reversion systems when ADX is below 20 and flat.
• Exhaustion alert: ADX above 60–70 combined with divergence (price makes new highs but ADX no longer rises) warns that the trend is losing fuel.
• +DI/−DI crossover signals: When +DI crosses above −DI with ADX rising, it confirms an uptrend entry. When −DI crosses above +DI, it confirms a downtrend.

Limitations and common misconceptions

ADX is a lagging indicator — it confirms trends only after they have established themselves, causing late entries and exits. In fast-moving markets, ADX may not cross 25 until a significant portion of the move is over. The indicator also does not distinguish between a trending-up and trending-down market; ADX rises during strong downtrends just as it does during uptrends. The 14-period default is arbitrary: shorter periods (7–10) produce more sensitive but noisier readings; longer periods (20–30) smooth out noise but lag further. Finally, "ADX below 20 means chop" is a rule of thumb, not a law — some assets trend at sustained low ADX levels, particularly in low-volatility regimes.

Related terms

• What is MACD? — momentum indicator often paired with ADX as a directional filter
• What is RSI? — oscillator used alongside ADX to time entries in trending markets
• What is the Parabolic SAR? — also developed by Wilder; used with ADX to trail stops in strong trends
• What is Average True Range? — shares the True Range input with ADX; measures volatility, not trend strength
• What is Bollinger Bands? — bandwidth indicator used in conjunction with ADX to confirm breakouts