Mass Index

What it is

The Mass Index hunts for trend reversals by watching the high-low range "breathe". When ranges widen sharply and then contract — a pattern Dorsey called the reversal bulge — the prevailing trend, in either direction, has an elevated chance of turning.

How it is calculated

Each bar's high-minus-low range is smoothed with an EMA over emaPeriod (default 9), that EMA is smoothed again with a second EMA of the same length, and the ratio of single-smoothed to double-smoothed EMA is summed over the last period bars (default 25). When ranges expand quickly the ratio exceeds 1 and the sum inflates; as ranges settle, it deflates.

How to read it

The pane shows the mass_index line and a horizontal setup guide at 27. With the default 25-bar sum the line normally hovers in the low-to-mid 20s. The classic sequence is two-step: first the line rises above 27 (the "setup" — ranges have bulged), then it drops back below 26.5 (the "trigger" — the bulge has deflated). Only after the trigger does the indicator suggest the current trend is vulnerable; the direction of the reversal must be read from price or a directional tool, because the Mass Index itself is direction-blind.

What to look for

• Turning patternContext

  The reversal bulge: mass_index rises above 27 and then falls back below 26.5. Dorsey's rule treats this as a warning that the trend in force — up or down — may reverse.

• ConfirmationContext

  Dorsey paired the bulge with a short EMA of price to supply the direction: after a trigger, trade in the direction the EMA then turns.

• SqueezeContext

  A low, flat mass_index means ranges are compressed and stable — no reversal setup is forming, and trend-following signals face fewer volatility ambushes.

When to use it

Designed for daily charts of trending instruments — equities and indices in Dorsey's original work — where range expansions are meaningful events. The 27/26.5 thresholds were calibrated for the default settings; changing periods invalidates them. It is non-directional and unusual in that it deliberately ignores price direction entirely, so it must always be paired with a directional tool. Avoid it on choppy, thin symbols where ranges bulge randomly.

Works well with

Settings explained

emaPeriod

The length of both range EMAs (default 9, Dorsey's original). Changing it alters the ratio's sensitivity and shifts the line away from the calibrated 27/26.5 levels.

period

How many ratio values are summed (default 25). The classic thresholds assume 25; a longer sum raises the line's typical level, a shorter one lowers it.

Limitations and common mistakes

• It never indicates direction — a bulge before a bottom looks identical to one before a top.
• The 27/26.5 thresholds only apply to the default 9/25 settings and to markets with behavior similar to those it was calibrated on.
• Reversal bulges are infrequent; the indicator can stay silent for long stretches.
• Not every bulge resolves in a reversal — trends sometimes survive the volatility episode.

References

• Origin: Donald Dorsey, 1992.
• Mass index — Wikipedia (opens in a new tab)

This page explains how an indicator is calculated and read. It describes past data, not what price will do next, and it is not a recommendation to trade. Read the disclaimers.

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