Indicator library · Trend
ADX, Average Directional Index
Wilder’s Average Directional Index reports how consistently an instrument has been moving one way, and refuses to say which way. That single omission is what makes it a filter rather than a signal.
The calculation
ADX is the third line of a three-line system, and the two beneath it are where direction lives.
- Directional movement. For each bar, measure how far the high exceeded yesterday’s high and how far the low fell below yesterday’s low. Keep only the larger of the two, and only if it is positive, an inside bar contributes nothing to either side.
- DI+ and DI−. Smooth each directional series and the true range with Wilder’s method, then divide: DI+ = smoothed +DM ÷ smoothed TR × 100, and the same for DI−. Expressed against the range, so the two are comparable on any instrument.
- DX, then ADX. DX = |DI+ − DI−| ÷ (DI+ + DI−) × 100, smoothed again with the same method. The absolute value is where direction is deliberately discarded.
Three smoothings are stacked in that description and it explains most of the indicator’s behaviour: it is late, it is steady, and it stays elevated after the thing it is describing has finished.
Direction lives in the two lines below
The complaint that ADX "does not tell you which way" is true and slightly unfair: the system Wilder published tells you, in DI+ and DI−, and most platforms plot all three. What ADX adds is a single number for the gap between them.
| State | What it describes |
|---|---|
| DI+ above DI−, ADX rising | An advance that is becoming more consistent. The ordinary state of a healthy uptrend. |
| DI− above DI+, ADX rising | A decline becoming more consistent, the identical ADX reading, opposite direction. |
| ADX below 20 | The two lines are close together. No consistent direction, and trend-following rules should be expected to perform badly. |
| ADX high and falling | A trend losing consistency. This happens in consolidations as often as at reversals, so it is not a directional call. |
Using it as a switch
The most defensible use of ADX has nothing to do with entries. Trend-following rules (a moving-average crossover, a breakout) behave completely differently depending on whether the instrument is trending, and their published results are largely a statement about how much of the test period trended. ADX addresses that question directly.
Applied as a switch (treat crossover signals as meaningful while ADX is above the threshold, ignore them below it) the lag that ruins its use as a trigger stops mattering very much. The question being asked is about the regime of the last few weeks, not about today, and a late, steady measure is the right tool for a question about a regime.
What the reading cannot see
Two markets can produce the same ADX for opposite reasons, and the calculation cannot distinguish them. A steady, orderly advance of half a per cent a day for six weeks and a violent one-way collapse both push the line into the forties, because both have DI− or DI+ consistently dominating. The number describes consistency of direction, not the size of the move, the volatility around it or the participation behind it.
That is worth holding onto when ADX is used as the switch it is best at. "This instrument has been trending" is a useful thing to know before applying trend rules; it is not the same as "this instrument has been trending in a way I can trade", which also depends on how far it travels in a session and whether there is enough liquidity to act. Those two questions belong to ATR and to the volume beside the chart.
Where it misleads
| Situation | What goes wrong |
|---|---|
| Read as directional | The absolute value in DX discards direction. A collapse and a rally produce the same reading. |
| Used as an entry trigger | Three stacked smoothings. It confirms a trend well after it starts and stays high after it ends. |
| Falling ADX read as a reversal | A consolidation inside a trend does exactly the same thing to the line. |
| Inherited thresholds | 20 and 25 come from 1970s commodity futures. Recompute from the instrument's own distribution before acting on them. |
| Short history | Wilder smoothing carries its seed forward; early values depend heavily on where the data begins. |
| Unadjusted prices | A split creates enormous directional movement and true range on one bar, distorting all three lines for many bars after. |
What volume adds
ADX is computed from highs, lows and closes, so it can describe a trend as strongly consistent without any knowledge of how many participants produced it. A steady drift upward on the lightest volume of the quarter registers exactly as a steady advance on heavy participation.
The pairing worth having is a consistency measure with a participation measure. A rising ADX alongside expanding volume describes a trend that people are actually trading; a rising ADX on thinning volume describes a market that has simply stopped being contested, which historically has been the less durable of the two.
Frequently asked questions
What does ADX actually measure?
How consistently an instrument has been moving in one direction, on a scale from 0 to 100. It says nothing about which direction, a strong advance and a strong decline both push it up, which is the property that confuses people first and the property that makes it useful as a filter. Direction lives in the two lines it is built from, DI+ and DI−, and those are frequently not plotted at all.
How is it built?
Three stages. First, directional movement: for each bar, take how much the high exceeded yesterday’s high and how much the low fell below yesterday’s low, and keep only the larger of the two if it is positive. Second, smooth those two series and the true range with Wilder’s method and divide to get DI+ and DI−. Third, take the absolute difference between the two DI lines divided by their sum — that is DX — and smooth it. The smoothed DX is ADX.
What does a reading of 25 mean?
It is the conventional line between "trending" and "not trending", and it is a convention rather than a property. Below about 20 the two directional lines are close together and the instrument is going nowhere in particular; above 25 one of them is consistently dominating. The numbers came from Wilder’s work on commodity futures in the 1970s, and an instrument or a timeframe with different behaviour deserves a threshold recomputed from its own history.
Why does ADX rise during a decline?
Because the calculation takes the absolute difference between the two directional lines. A market falling steadily has DI− well above DI+, and the gap between them is what ADX measures. This is not a flaw. It is the entire design. If you want to know which way, read the DI lines or look at the price; ADX answers only "how consistently".
How much does it lag?
Substantially, and by construction. There is a smoothing inside DI+ and DI−, then DX is computed from them, then DX is smoothed again, so the reading is an average of an average. ADX typically confirms a trend well after it has begun and stays elevated for a while after it has ended. Anyone using it to enter is using the wrong tool; as a filter that says "this instrument has been trending, so trend rules are appropriate", the lag matters much less.
Is a falling ADX bearish?
No. A falling ADX means the trend is losing consistency, which happens when a market moves sideways as often as when it reverses. In a long advance the line frequently falls back through a consolidation and rises again when the advance resumes. Reading a falling ADX as a sell signal converts a measure of consistency into a directional call it cannot support.
What period should I use?
Fourteen, unless you have a reason. It is Wilder’s own and it is the platform default everywhere, which is the only real property the number has. A shorter period reacts faster and produces more crossings of whatever threshold you set; a longer one is steadier and later. Because there are three smoothings stacked inside the calculation, changing the period changes the behaviour more than the same change would in a simpler indicator.
How is it best used?
As a switch rather than a signal. Trend-following rules (moving-average crossovers, breakouts) perform very differently depending on whether an instrument is trending, and ADX is one of the few measures that addresses that question directly. Using it to decide which rules apply, rather than when to trade, is the use that survives scrutiny.