Indicators

What the ADX Indicator Is and How It Works

Astron Team · 09/24/2026 · 4 min read

Many people look at the ADX expecting it to say whether the price will rise or fall. That's not what it's for. The Average Directional Index (ADX) only measures the strength of a move, leaving direction to two other lines that usually come along on the same panel.

This confusion is common because the indicator ranges from 0 to 100, similar to a regular oscillator. But an ADX at 30 can show up equally during a strong rally or a strong decline. Understanding this difference avoids wrong entries right off the bat.

The system's three lines

The ADX never comes alone. It's part of a set of three lines created by J. Welles Wilder:

• +DI: measures how much the upward move dominates the analyzed period.
• -DI: measures how much the downward move dominates the same period.
• ADX: shows the size of the gap between +DI and -DI, meaning the strength of whichever trend is winning.

When +DI is above -DI, the market has been leaning upward more consistently. When -DI is above, selling pressure has been more persistent. The ADX rises when this gap increases and falls when the two lines come back together.

How the calculation works

The process starts by comparing each candle's high and low with the previous candle, within the so-called true range (which also accounts for gaps). This difference generates the positive and negative directional movement, which after being smoothed (usually over 14 periods) form the +DI and the -DI.

From there, the Directional Index (DX) is calculated:

DX = 100 × |+DI − (-DI)| ÷ (+DI + (-DI))

The ADX is the smoothed average of the DX over time. You don't need to do this math by hand — any charting platform plots the indicator automatically —, but understanding the formula helps explain why the ADX reacts with a lag: it only confirms strength after the move has already started.

How to read the ADX scale

• Below 20: weak direction or sideways market.
• Between 20 and 25: a trend may be forming.
• Between 25 and 40: already established trend.
• Above 40: strong trend; worth checking whether the price isn't overstretched.

These ranges are references, not rigid boundaries. An ADX of 24 and one of 26 don't describe different worlds. What matters is the line's slope and the relationship between +DI and -DI around it.

An example with numbers

Imagine a currency pair where, on a given day, +DI is at 32 and -DI at 12, with the ADX rising from 18 to 27 over two weeks. The reading: the upward move gained strength consistently, and the gap between buyers and sellers is widening — not just an isolated spike.

Now switch the scenario: ADX at 26, but -DI at 34 and +DI at 15. The ADX number is similar to the previous example, except now selling pressure is in charge. This shows why looking only at the ADX value, without the directional lines, is half a reading.

Common mistakes when using the ADX

A falling ADX isn't an automatic reversal signal — it can simply indicate the trend lost pace, but the price still keeps moving in the same direction for longer. Another mistake is trading +DI and -DI crossovers when the ADX is low: under these conditions, the market is usually sideways, and the crossovers tend to repeat without generating a real move, just so-called sideways-range noise.

One way to use the ADX more carefully is treating it as a filter before applying another strategy. If you follow trends with moving averages, for example, you can require an ADX above 25 before considering a signal valid, ruling out entries in markets with no clear direction.

Fitting the ADX into your process

In practice, the ADX works best as a context filter than as an entry trigger. It helps answer a question that comes before deciding to buy or sell: is this market actually trending, or am I trying to fit a trend strategy into a sideways environment? Combined with +DI and -DI for direction, and with a support and resistance analysis for timing, the indicator gains real usefulness. Alone, it only tells half the story — and, as with any trade in the financial market, no combination of indicators eliminates the risk entirely.