ADX Indicator Explained: How to Measure Trend Strength Before You Trade

Most indicators try to tell you which way a stock is going. The Average Directional Index (ADX) answers a different question: is there a trend worth trading at all? That distinction matters, because a breakout strategy used in a choppy, sideways market tends to get chopped up, and a mean-reversion strategy used against a powerful trend tends to get run over.

This guide explains what ADX measures, how +DI and -DI feed into it, how to read the common thresholds, and how to use it as a filter, then how to turn the idea into alerts so you are not watching charts all day.

What Is the ADX?

ADX was developed by J. Welles Wilder and published in his 1978 book New Concepts in Technical Trading Systems, the same book that introduced RSI and the Parabolic SAR. It is plotted as a single line, usually in a panel below the price chart, and ranges from 0 to 100.

The key point: ADX is non-directional. A reading of 35 means the trend is moderately strong. It does not say whether the stock is rising or falling. A stock in a steady decline and a stock in a steady rally can show the same ADX.

How ADX Is Built: +DI and -DI

ADX comes from two companion lines, usually shown with it:

• +DI (positive directional indicator) measures how much of the recent price movement has been upward
• -DI (negative directional indicator) measures how much has been downward

In simplified terms, the calculation works like this:

1. Compare each period's high and low with the prior period's to find the upward and downward movement
2. Smooth those movements (Wilder's smoothing, 14 periods by default) and divide by the smoothed average true range to get +DI and -DI
3. Measure how far apart +DI and -DI are relative to their sum. This is the DX value
4. Smooth DX over 14 periods to get the ADX

When +DI and -DI are far apart, one side dominates, DX is high and ADX rises. When they are tangled together, there is no dominant direction and ADX stays low. You do not need to compute this by hand; every charting tool does it, but knowing the logic explains why ADX lags: it is a smoothed average of a smoothed value.

Reading the Thresholds

There is no official scale. The bands below are a widely used convention, so treat them as starting points to test.

Two readings of the slope are often more useful than the level:

• Rising ADX means the trend is strengthening, whichever direction it points
• Falling ADX means the trend is losing momentum, even if price is still moving the same way

A very high ADX is not a sell signal. It tells you the move has already been strong, and strong trends can persist. It does warn that a sharp pause or consolidation becomes more likely as the move matures.

The +DI / -DI Crossover

The directional lines add the direction ADX lacks:

• +DI crosses above -DI: upward movement has taken over, a potential bullish signal
• -DI crosses above +DI: downward movement has taken over, a potential bearish signal

On its own, the crossover fires constantly in sideways markets and produces many whipsaws. That is exactly why traders pair it with ADX: a crossover is only taken seriously when ADX is above about 20 to 25 and rising. A bullish crossover with ADX at 14 and flat is mostly noise.

ADX as a Strategy Filter

The most practical use of ADX is choosing which playbook fits current conditions:

This works as a gate, not a signal generator: you still need an entry, a stop and a target. ADX simply keeps you from applying the wrong tool to the wrong market.

Combining ADX With Moving Averages

A simple, testable combination:

1. Direction: price above a rising 50-day moving average (or +DI above -DI)
2. Strength: ADX above 25 and rising
3. Entry: a pullback toward the moving average, or a breakout above recent resistance
4. Exit: a close below the moving average, or ADX turning down while price stalls

Read our moving averages guide and golden cross and death cross guide for the moving-average side. Pairing ADX with RSI or MACD is also common, since each measures something slightly different.

Where ADX Falls Short

• It lags. Because of the heavy smoothing, ADX often confirms a trend after a large part of the move has happened.
• It is blind to direction. You always need +DI/-DI, price or another tool to know which way.
• The thresholds are conventions. A level that works on a slow utility stock may not suit a volatile small cap. Test on your own universe. Our guide on how to backtest a trading strategy covers sample size and overfitting.
• A high ADX can follow a crash. A steep decline produces a high ADX just as a rally does.

Turning ADX Into a Workflow With Alerts

ADX works best as a screening and waiting tool, which is exactly where alerts help:

• Screen first. Use the Stock Alarm Pro screener to build a short list of liquid stocks trading above their 50-day and 200-day moving averages, then check ADX on the chart for each.
• Alert on the setup, not the indicator. Set a price alert at the breakout level or at the moving average you want to buy a pullback to.
• Alert on trend damage. Add an alert below the level that would invalidate the trend, such as your stop or the 50-day average.
• Keep a record. Note why you took each trade in a trading journal so you can see whether ADX actually improved your results.

For intraday ideas, see our day trading alerts setup guide.

Key Takeaways

• ADX measures trend strength, not direction, on a 0 to 100 scale
• It is built from +DI and -DI, smoothed over 14 periods by default
• Common readings: below 20 no trend, 25 to 40 strong, above 40 very strong
• A rising ADX means a strengthening trend; a falling ADX means fading momentum
• Use ADX as a filter to choose between trend-following and range strategies
• ADX lags, so pair it with price levels and test thresholds before relying on them
• Price alerts at breakout, pullback and stop levels let you act without watching every tick

This article is educational and not investment advice. Technical indicators do not predict future prices, and trading carries a risk of loss. Thresholds shown are common conventions, not guarantees.