Another interesting indicator - ADX - Average Directional Index - Seeing if a Trend actually has strength

 

ADX Explained: How to Measure Whether a Trend Actually Has Strength

If you've spent any time looking at stock charts, you've probably noticed that some trends feel unstoppable — the price just keeps grinding higher or lower, day after day — while other "trends" are really just the price chopping sideways, faking you out in both directions. Telling these two situations apart is one of the most useful skills in chart reading, and it's exactly what the Average Directional Index, or ADX, was built to do.

ADX doesn't tell you whether a stock is going up or down. That surprises a lot of beginners the first time they hear it. Instead, ADX tells you how strong the current trend is, regardless of its direction. This matters because the same strategy that works beautifully in a strong trend — like buying dips — can lose money repeatedly in a weak, sideways market. Knowing which environment you're in first can save you from applying the wrong tool at the wrong time.

In this article, we'll break down what ADX is, how it's built from two lesser-known components, how to read it on a real chart, and where it falls short.

What Is ADX?

ADX stands for Average Directional Index. It was developed by J. Welles Wilder (the same analyst who created the RSI indicator) and is typically plotted as a single line below the price chart, moving on a scale from 0 to 100. The key thing to remember is simple: higher ADX readings mean a stronger trend, and lower ADX readings mean a weaker or non-existent trend. A commonly used rule of thumb is that readings below 20 suggest a weak or absent trend, readings above 25 suggest a trend with real strength, and readings above 40 suggest a very strong trend.

ADX is one part of a three-line family that Wilder designed together. The other two lines are called +DI (the Positive Directional Indicator) and -DI (the Negative Directional Indicator). You'll often see all three plotted in the same lower panel of a chart.

How It's Calculated (In Plain Language)

You don't need to memorize a formula to use ADX well, but understanding the intuition behind it makes the indicator far more useful.

Start with +DI and -DI. On each trading day, the indicator looks at how much of that day's price range was "up" movement versus "down" movement compared to the prior day. +DI tracks the strength of upward movement; -DI tracks the strength of downward movement. When buyers are clearly in control, +DI rises above -DI. When sellers are in control, -DI rises above +DI. A crossover between these two lines is often read as an early signal of a potential change in which side is winning.

ADX itself is then calculated from the difference between +DI and -DI, smoothed out over time (traditionally a 14-day period). Here's the intuitive part: ADX doesn't care which line is on top. It only cares how far apart they are. If +DI and -DI are close together, buyers and sellers are roughly fighting to a draw, and ADX stays low — that's a choppy, directionless market. If one of the lines pulls well ahead of the other, one side has clearly won control, and ADX rises — that's a trending market. So ADX is essentially a measure of the "gap" between buying pressure and selling pressure, averaged over time.

Try it yourself: Backtest a trend-strength rule

Reading about ADX is one thing — seeing how a rule built around it would have actually performed is another. TruCharts' backtesting tool lets you test a rule like "only take trend-following trades when ADX is above 25" against years of historical price data, so you can see for yourself whether filtering for trend strength would have improved results before you ever risk real capital.

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A Worked Example: ABC Corp

Let's walk through a hypothetical example using a fictional stock we'll call ABC Corp.

Suppose ABC Corp spends two months trading in a tight range between $48 and $52, bouncing back and forth with no clear direction. During this stretch, +DI and -DI are tangled close together, crossing each other repeatedly, and ADX drifts down to around 14. That low reading is the indicator confirming what the choppy price action already suggests: there's no real trend here to follow, and a trend-following strategy (like "buy when price breaks above a moving average") would likely generate a string of small, frustrating losses as the price whipsaws back and forth.

Now suppose ABC Corp breaks out above $52 on strong volume and begins a steady climb toward $70 over the following three months. As that move develops, +DI climbs well above -DI, and ADX rises from the high teens, through 25, and eventually up toward 45. That rising ADX line is telling you the trend has real conviction behind it — pullbacks within the uptrend are more likely to be temporary rather than the start of a reversal, which is exactly the kind of environment where trend-following approaches tend to perform best.

An example of ADX in action for stock NBIS: it is indicator at the bottom - it is always best never to use a single indicator to make technical decisions on trends - always use a good combination of indicators to confirm your analysis. The indicators in the chart below are my favorite go to indicators.

Practical Limits and Risks

ADX is a useful tool, but it has real limitations and should never be used in isolation.

First, ADX is a lagging indicator. Because it's calculated from an average of past price movement, it typically confirms a trend only after that trend is already underway — it won't warn you at the exact moment a new trend begins, and it can stay elevated for a while even after a strong trend starts losing steam. Second, ADX says nothing about direction. A rising ADX during a sharp decline looks identical in shape to a rising ADX during a sharp rally — you have to check +DI and -DI (or simply look at the price) to know which way the trend is pointing. Third, like most technical indicators, ADX works better on some instruments and timeframes than others, and whipsaw periods (where the market can't decide on a direction) can produce misleading readings in either direction. No indicator, including ADX, can predict the future or guarantee a profitable trade, and it's generally most useful when combined with other tools — price structure, volume, or other indicators — rather than used as a standalone signal.

Coming Up Next

Next week, we're shifting from indicators that measure momentum and trend strength to one of the most fundamental concepts in all of chart reading: support and resistance — the price levels that charts seem to respect again and again, and why that happens.

This article is for educational purposes only and does not constitute financial or investment advice. ABC Corp is a hypothetical example used for illustration only and does not represent any real company. Past performance of any indicator or strategy does not guarantee future results. Please consult a licensed financial advisor before making investment decisions.