What is the High-Low Index? Definition, Formula, and Example
The High-Low Index is a market breadth indicator that measures the percentage of stocks making new 52-week highs versus new 52-week lows, smoothed by a 10-day moving average.
Plain-English Definition
The High-Low Index measures market breadth by comparing the number of stocks hitting fresh 52-week highs against those hitting fresh 52-week lows, expressed as a smoothed percentage. It answers a simple question: is the rally being driven by broad participation across hundreds of names, or by a narrow handful of mega-caps masking weakness underneath? Readings above 70 indicate broad bullish leadership; readings below 30 indicate broad bearish pressure. The indicator is computed separately for the NYSE, Nasdaq, and S&P 500, and it's one of the cleanest tools for detecting hidden divergences between an index price and the health of its underlying constituents.
How It's Calculated
The High-Low Index is built in two steps. First, the Record High Percent is calculated daily:
Record High Percent = New 52-Week Highs / (New 52-Week Highs + New 52-Week Lows) × 100
Then the High-Low Index applies a 10-day simple moving average to smooth out daily noise:
High-Low Index = SMA(10, Record High Percent)
The output oscillates between 0 and 100. Values above 70 mean new highs dominate; below 30 means new lows dominate; near 50 means balance. Some platforms publish the raw daily Record High Percent and a 10-day smoothed version side-by-side. Bull markets generally sustain readings above 50 for extended periods; bear markets stay pinned below 50.
Worked Example
On 2026-04-30, the NYSE recorded 187 new 52-week highs and 24 new 52-week lows. Record High Percent = 187 / (187 + 24) × 100 = 88.6%. Over the prior 10 sessions, the daily readings averaged 81.4 — placing the High-Low Index firmly in bullish-breadth territory and confirming the SPY rally that month. Contrast with 2026-03-14, when the index sat at 28.2 during a broad-market pullback: only 41 NYSE stocks made new highs against 312 new lows that day, signaling that the SPY decline was backed by genuine breadth deterioration, not just mega-cap weakness.
When Traders Use It
Trend-following investors use the High-Low Index to confirm or reject index moves. An SPY new all-time high paired with a High-Low Index above 70 confirms a healthy, broadly participated rally. The same SPY high with the High-Low Index at 45 signals narrow leadership and elevated reversal risk — a classic divergence setup. Active managers use crossings of the 50 line as regime signals: above 50 favors long exposure, below 50 favors defensive positioning. The indicator is also useful for spotting washout bottoms: extreme low readings (below 10) often mark capitulation lows and align with strong forward returns over the following 3-6 months.
Limitations and Misconceptions
The High-Low Index is a lagging confirmation tool, not a leading signal. By the time the indicator turns, the underlying breadth shift has already begun. It's also distorted around year-end as the 52-week window rolls past prior extremes — January readings can spike or collapse based purely on calendar mechanics. The indicator works best on exchange-wide universes (NYSE, Nasdaq composite); applied to narrow indices like the QQQ top-100, it loses statistical power. Finally, persistent extreme readings (above 90 or below 10) are not automatic reversal signals — they often mark the *strongest* portion of trends, not the end.