Mass Index

Volatility

What it measures

The Mass Index, developed by Donald Dorsey, looks for "reversal bulges" — a specific pattern where the trading range (high minus low) widens significantly and then narrows again — without caring about price direction at all. Dorsey's insight was that trend reversals are frequently preceded by a period of range expansion, regardless of whether the reversal turns out to be bullish or bearish.

How readings are interpreted

The Mass Index itself doesn't indicate direction — it only flags that a reversal-associated range expansion pattern has occurred. You need a separate directional tool to determine which way the anticipated reversal is likely to go once the Mass Index bulge signal fires.

Conventional levels

Mass Index rises above 27, then falls back below 26.5 (the classic "reversal bulge"): warns that a trend reversal may be imminent
Mass Index staying below 27: no reversal bulge pattern present, no signal
The specific 27/26.5 threshold pair is the traditional convention, though some practitioners adjust it slightly per security

Where it works, and where it does not

A narrow-purpose tool — it does one specific job (flagging range-expansion-then-contraction) and nothing else. Best used as a supplementary alert layered on top of a primary trend or momentum strategy, not as a standalone system.

Commonly read alongside

ADX/Moving Averages: Once a Mass Index bulge fires, use a directional tool to determine which way the anticipated reversal is likely to break
ATR/NATR: All three relate to trading range, though Mass Index specifically looks for the expand-then-contract pattern rather than raw volatility level
Support/Resistance: A Mass Index bulge signal occurring near a major support or resistance level is more actionable than one in the middle of open space

Known limitations

Gives no directional information whatsoever — it must be paired with something else to be tradeable
The classic bulge signal is a fairly specific pattern that doesn't fire often, limiting signal frequency
Less intuitive and less widely followed than mainstream volatility tools like ATR or Bollinger Band Width

In practice

Treat a Mass Index bulge as an alert to pay closer attention, not a trade signal on its own
Conventionally paired with a directional indicator to establish which way to position once a bulge fires
Because signals are infrequent, this is better suited to a background watchlist alert than an active scanning tool
Most valuable at existing support/resistance levels, where a bulge-flagged reversal has a clear technical level to react against

Plot it yourself. Add this indicator to a chart, change every parameter and watch the line move, then backtest how the rule would have behaved on historical data — free on the S&P 500 ETF, no card.

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This is reference material, not advice. It describes what each indicator measures and how its readings are conventionally interpreted. Nothing here is a recommendation to buy or sell anything, and no indicator predicts future prices. GU Analyser is an analytical tool — no money is ever traded here.