The ADX Indicator: What It Is and How to Measure Trend Strength

Plenty of indicators try to predict direction; ADX does something else. It answers one question, whether a strong trend is on the chart right now or price is just drifting in a range, and it never points the way. Its main value is to filter out the flat, where trend strategies do not work, and to stop you mistaking a strong trend for random chop.

Many indicators try to predict direction, while ADX busies itself with something different. I value it precisely for the modesty of its task: it does not guess where price will go, it only tells you whether a trend is worth waiting for at all. Let me work through what ADX is, how it measures the strength of a move, and why on its own it gives no signals to enter.

In this article, we'll cover:

• ADX is an indicator of trend strength that does not show its direction;
• high values speak of a strong trend, low ones of a weak move or a range;
• in my experience, the main use of ADX is to screen out the flat, where trend strategies stall;
• ADX gives no entry signals by itself and works only as a filter alongside other tools.

Start with what the ADX indicator even is.

What the ADX indicator is

ADX (Average Directional Index) is an indicator of directional movement strength that gauges how pronounced the current trend is, but does not point to its side, whether up or down. The name translates as the average directional movement index, and it was developed by Welles Wilder back in 1978.

The main feature of ADX is that it measures exactly the strength, not the direction. Its value swings in a range from zero to one hundred: the higher the number, the stronger and more stable the current move, and it makes no difference whether price is rising or falling. It is generally taken that low values point to a weak market or a range, while high ones point to a pronounced and stable trend, and that completely independently of its side, up or down. So ADX answers not the question of where price is moving but of how strongly it is doing so. This fundamentally sets it apart from indicators that try to guess direction. How indicators are built in principle and why almost all of them lag I cover in the piece on indicators in trading.

In short: ADX is a line that measures only the strength of a move, not its side; the scale runs from zero to one hundred, and it is itself a consequence, not a cause: price is moved by volume and the actions of a big participant, while ADX merely reads the result.

How ADX measures trend strength

ADX works by comparing upward and downward pressure on price. Inside the indicator are two auxiliary directional-movement lines, +DI and -DI: one reflects the strength of buyers, the other the strength of sellers. ADX itself is built on the difference between them and shows how confidently one side outweighs the other. When the edge is clear and steady, ADX rises, signalling a strong trend. When the forces are roughly equal and price goes nowhere, ADX falls, signalling a flat. In practice traders lean on threshold values. A common guide is this: an ADX value above twenty-five usually says a trend is present, while below twenty says it is absent and there is a range. The higher ADX climbs above that threshold, the more confident the move. The two internal directional lines also hint at the side: when the buyers' line is above the sellers' line, the edge is with the rise, and the other way round, but the strength itself is shown precisely by ADX.

In practice this combines neatly with other tools. For example, moving averages show direction well but tell a trend from a range poorly, and here ADX covers their weak spot, confirming that the move is genuinely strong rather than a random swing inside a flat. I cover the moving averages themselves separately. For the same reason ADX is often placed beside trend indicators as a filter that screens out false signals in a flat. What a trend even is and what phases it consists of I cover in the piece on the anatomy of trends.

In short: Above twenty-five is usually a trend, below twenty a range; ADX counts how strongly one side outweighs the other, but this is a recalculation of a move that has already happened, so it lags and suits a filter rather than a leading signal.

The +DI and -DI lines and the 20/25/40 levels: how to read the indicator

To use ADX deliberately, it helps to keep three lines apart. ADX itself is trend strength with no direction. Direction comes from two auxiliary lines: +DI and -DI. The +DI line reflects upward pressure (buyer strength), and -DI downward pressure (seller strength). When +DI is above -DI, the edge is with the rise; when -DI is above, with the fall. The crossover of these lines is often sold as a signal, but I treat it the way I treat all of ADX: a hint about the side, not a command to enter.

ADX itself is read by levels, and the rough scale is this: below 20 there is essentially no trend, just a flat; the 20-25 zone is a move being born, still unsure; above 25 it is treated as a full trend; above 40 the trend is strong, but this is often exactly where it runs out of steam, since such a value shows up in the thick of the move, when it is late to enter. So a high ADX is not an invitation to buy but a mark that the move is already mature. That is why I read the levels like a thermometer of the conditions: they say whether a trend exists and how strong it is, but they decide neither the side nor the entry for me.

In short: +DI and -DI show the side (who is stronger, buyers or sellers), while the ADX levels show strength: below 20 a flat, 20-25 a nascent move, above 25 a trend, above 40 strong and often already overheated; all of it is a filter of conditions, not an entry signal.

ADX as a regime filter: trend-following above 25, range setups below 20

The single most useful thing ADX does is sort the market into two regimes. While ADX holds above 25 the tape is trending, and that is when trend-following methods have a chance: riding a move, trading a breakout, leaning on a moving-average continuation. While ADX sits below 20 the market is ranging, and those same trend methods quietly bleed there, whereas range and mean-reversion approaches fit far better. The whole rule is just that, match the approach to the regime. Bolting a simple "only trade when ADX is above 20" onto a moving-average crossover skips most of the chop that drains an account, which usually helps the result more than any clever signal. For me this is nothing exotic; it puts a number on the read I would make from structure anyway, whether the market is trending or stuck.

The same split sharpens a breakout. A break of a level with ADX rising up through 25 has real conviction behind it, while the same break with ADX stuck under 20 is far more likely a false break that snaps back into the range. And the +DI/-DI crossover, near useless on its own because it flips back and forth in a flat, gets steadier when you only act on it while ADX is above 25 and rising, pairing a hint about the side with a filter on the strength. Even then I keep it in its place: it is a lagging confirmation of what price has already shown, so the level and the volume make the call, and the ADX regime only tells me whether this kind of trade fits the day at all.

In short: Use ADX to pick the regime: above 25 the market trends and trend-following and breakouts have a chance, below 20 it ranges and mean-reversion fits; a breakout with ADX rising through 25 has conviction, and a +DI/-DI crossover is only worth acting on with ADX above 25 and rising.

Why ADX is a filter, not an entry signal

The key thing to grasp about ADX: on its own it does not say when to buy or sell. It only reports that a trend is strong, but prompts neither the side nor the entry point. To enter a trade just because ADX is high is a mistake: a strong trend can be either rising or falling, and a high value often appears already in the thick of the move, when it is late to get in.

So I treat ADX as a filter of the conditions, not as a trading signal. Its job is simple: to confirm that there is a trend on the market at all, so as not to apply trend strategies in a flat, where they bring nothing but losses. Another frequent mistake is to take a falling ADX as a reversal signal. In reality a decline of ADX means only a weakening of the trend's strength, not a change of its side: price may simply move into a flat and get stuck there for a long while. So I do not look in ADX for what is not in it, reversals and entry points. The direction and the entry point themselves I read by levels and volume, not by the line of the indicator. Like any indicator, ADX is a lagging auxiliary tool, not the basis of a decision. How to build indicators into a system without turning them into a grail I cover in detail in the course section on indicators in trading, and how to read the market technically in the section on technical analysis.

In short: Do not enter a trade just because ADX is high: it does not know the side and often lights up already in the thick of the move; keep it as a filter of the flat, and take entry and direction from levels and volume.

How ADX fits into the overall picture of reading a chart and where it genuinely helps, I show in the guide to technical analysis.