What is the Aroon Indicator? Definition, Formula, and Example
The Aroon indicator measures how recently the highest high and lowest low occurred within a lookback window, plotting two lines that oscillate between 0 and 100 to gauge trend strength and direction.
Plain-English Definition
The Aroon indicator, introduced by Tushar Chande in 1995, measures the time elapsed since the highest high and lowest low within a lookback window. It plots two lines — Aroon Up and Aroon Down — that oscillate between 0 and 100. Aroon Up tracks the freshness of new highs; Aroon Down tracks the freshness of new lows. The name comes from the Sanskrit word for "dawn's early light," reflecting Chande's intent to detect new trends early, before momentum oscillators confirm them.
How It Is Calculated
For a lookback period of n bars (default 25):
- Aroon Up = ((n − bars since n-period high) / n) × 100
- Aroon Down = ((n − bars since n-period low) / n) × 100
- Aroon Oscillator = Aroon Up − Aroon Down (range −100 to +100)
If the highest high occurred today, "bars since high" = 0 and Aroon Up = 100. If it occurred 25 bars ago, Aroon Up = 0. Each line is independent — both can read high simultaneously when a stock makes a new high and a new low within the same window, signaling expanded range rather than trend.
Standard signal thresholds:
- Strong uptrend: Aroon Up > 70 and Aroon Down < 30
- Strong downtrend: Aroon Down > 70 and Aroon Up < 30
- Consolidation: both lines below 50
- Trend change: Aroon Up crosses above Aroon Down (bullish) or below (bearish)
Worked Example
Suppose NVDA is in the middle of a rally. Over the past 25 sessions:
- Highest high occurred 3 bars ago → Aroon Up = ((25 − 3) / 25) × 100 = 88
- Lowest low occurred 22 bars ago → Aroon Down = ((25 − 22) / 25) × 100 = 12
- Aroon Oscillator = 88 − 12 = +76
The reading confirms a strong uptrend: new highs are recent, new lows are stale, and the oscillator sits near its upper bound. A swing trader using Aroon as a filter would maintain longs only while the oscillator stays positive and exit on a cross back below zero.
Contrast with a ranging market: if NVDA's highest high was 11 bars ago and lowest low was 9 bars ago, Aroon Up = 56 and Aroon Down = 64 — both lines below 70, oscillator near zero. The chart is choppy and trend-following strategies will whipsaw.
When Traders Use It
The Aroon indicator is used as a regime filter — to determine whether the market is trending or chopping before applying a trend strategy. Pair it with a momentum oscillator like RSI or MACD to avoid taking overbought signals during a strong trend. Quantitative systems use the Aroon Oscillator as a position-sizing input, scaling exposure in proportion to oscillator magnitude.
Limitations and Misconceptions
Aroon is a timing indicator, not a magnitude indicator. Two stocks with identical Aroon readings can have radically different volatility and trend depth — the indicator measures recency of extremes, not size of the move. The 25-period default lags fast reversals; cutting to 14 periods produces more signals but more whipsaws. Aroon also generates false signals in low-volume drift markets where a single tick can create a new "high" inside the window. Use Aroon alongside Average True Range to filter out signals from low-range periods.