Aroon Indicator: Measuring Time Since Highs and Lows
The Aroon indicator measures how recently an instrument made its highest high and its lowest low within a lookback window. It plots two lines between 0 and 100, Aroon Up and Aroon Down, and a reading near 100 means the extreme happened very recently.
What makes it unusual is that it measures time, not price. Most trend indicators ask how far price moved. Aroon asks how many sessions have passed since the last extreme, a different question and sometimes a more useful one.
The formulas
Both lines use the same structure, with n as the lookback period. Fourteen and twenty-five are the usual settings.
Aroon Up = ((n minus periods since the n-period high) divided by n) x 100
Aroon Down = ((n minus periods since the n-period low) divided by n) x 100
An illustrative case on the Nifty 50 with n equal to 25. Say the highest high of the last 25 sessions came 3 sessions ago and the lowest low came 20 sessions ago.
- Aroon Up = ((25 minus 3) divided by 25) x 100 = 88
- Aroon Down = ((25 minus 20) divided by 25) x 100 = 20
If today’s bar sets a new 25-day high, the count is 0 and Aroon Up reads 100. If no new high has appeared all window, the count is 25 and Aroon Up reads 0. The index level never enters the calculation, so a 3 percent rally and a 30 percent rally that both peaked three sessions ago give the same Aroon Up.
How to read the two lines
The standard readings use the 70 and 30 levels plus crossovers.
| Reading | What it means |
|---|---|
| Aroon Up above 70, Down below 30 | New highs are recent and lows are stale, consistent with an uptrend |
| Aroon Down above 70, Up below 30 | New lows are recent and highs are stale, consistent with a downtrend |
| Both lines below 50 | Neither extreme is recent, usually a range or quiet drift |
| Both above 50 and close together | Both extremes are recent, so the range is expanding |
| Aroon Up crosses above Aroon Down | Highs are now more recent than lows, read as a possible trend change |
Some platforms also plot the Aroon Oscillator, which is Aroon Up minus Aroon Down. It runs from minus 100 to plus 100 and crosses zero where the two lines cross, so it is a single line version of the same information.
How it differs from ADX
Both are trend indicators and they are often confused, but the inputs are unrelated.
| Aspect | Aroon | ADX |
|---|---|---|
| What it measures | Time since the highest high and lowest low | Size of directional movement, smoothed |
| Inputs | Position of extremes in the window | Directional movement and average true range |
| Effect of a large move | None beyond a new extreme | Raises the reading directly |
| Direction | Aroon Up versus Aroon Down | Plus DI versus minus DI, ADX is neutral |
The difference shows up in a slow grind. A stock creeping to marginal new highs every few sessions keeps Aroon Up near 100 while ADX stays low, because the daily moves are small. A two-day spike inside an existing range lifts ADX while Aroon barely changes.
When it works and when it fails
Aroon is best at showing that a trend has begun or ended, and at spotting consolidations. Both lines drifting below 50 is an early, clean description of a market going quiet, which matters if your strategy needs movement.
It fails in three ways. In a tight range, tiny new highs and lows keep flipping the lines, so crossovers arrive constantly and mean nothing. It ignores magnitude, so a new high by one paisa counts the same as a strong breakout. And it ignores volume, so a new high on thin smallcap trading looks identical to one on heavy institutional buying.
The specific risk is whipsaw. Because a crossover can trigger on a one-tick new extreme, a mechanical rule generates many trades in a choppy Bank Nifty session, and STT, exchange charges, stamp duty and GST apply to each one. Most practitioners require a second condition, such as a break of a prior swing level or expanding volume.
Frequently Asked Questions
What lookback period should I use?
Twenty-five is the traditional daily setting and fourteen is the faster alternative. A shorter n reacts sooner and flips more, and n must be shorter than the trend you are following or the lines sit pinned at 100 and 0.
Can both Aroon lines read 100 at the same time?
Yes, if one session sets both the n-period high and low, which happens on very wide bars such as Budget day or a policy surprise. Read that as volatility expansion, not a trend signal.
Is Aroon a leading or lagging indicator?
It is best described as coincident. It records extremes that have already happened, though it registers the loss of a trend earlier than a long moving average.
Does Aroon work on intraday charts?
It works on any timeframe, but whipsaw worsens on short bars where new extremes are set constantly. On 5-minute charts, treat the 70 and 30 zones as context rather than triggers.
Key Takeaways
- Aroon Up equals ((n minus periods since the n-period high) divided by n) times 100, and Aroon Down uses the low the same way.
- Readings above 70 mean the relevant extreme is recent, and below 30 mean it is stale.
- Unlike ADX, Aroon ignores the size of price moves and measures only time since extremes.
- Both lines under 50 is a clear description of a range or a quiet market.
- Crossovers can trigger on a one-tick new extreme, so add a confirming condition before acting.
