Volatility & bandsMass Index · MI
A range-expansion measure that flags reversals when the high-low range bulges then contracts.
Works in most conditionsEngine-computed on a fixed sample series
What it is
The Mass Index is a volatility indicator created by Donald Dorsey in the early 1990s to answer a single, narrow question: is the distance between each bar's high and low expanding in a way that historically precedes a trend reversal? It does not measure direction, momentum, or volume — it measures the width of the trading range and how that width is changing over time. The intuition is that many important turns are preceded by a swelling of range, a burst of volatility as the crowd argues over price, followed by a contraction as one side wins. Dorsey packaged that observation into a single line that rises when ranges bulge and falls when they settle back down. Because it ignores whether price is going up or down, the Mass Index is best thought of as an alarm bell that says a reversal is likely soon, leaving the job of picking the direction to another tool.
How it is calculated
The calculation begins with the raw range of each bar, high minus low, which is a pure measure of how far price traveled that period. Dorsey first smooths that range with a 9-period exponential moving average, then smooths that smoothed value again with a second 9-period EMA, producing a double-smoothed range. He then takes the ratio of the single EMA to the double EMA; this ratio grows above one when the recent single-smoothed range is pulling away from its own slower average, which is exactly what happens when volatility is expanding. Finally, the Mass Index sums that ratio over the last 25 periods, so the line accumulates the recent history of range expansion into one number. The single-versus-double smoothing is the clever core: it isolates the acceleration of the range rather than its raw level, which is why a quiet market with a naturally wide range does not falsely trip the indicator.
Reading it, step by step
The Mass Index is read by level, not by crossovers of two lines, and the level that matters is Dorsey's signature reversal bulge. First the index must climb above 27, which signals that ranges have expanded meaningfully relative to their recent norm. Then, and only then, you wait for the index to fall back below 26.5; that drop marks the range beginning to contract again after the bulge. The combination — a push above 27 followed by a slide under 26.5 — is the actual signal that a reversal of the prevailing trend is likely at hand. A reading that pokes above 27 but never falls back has not completed the pattern, and a Mass Index drifting in the low 20s is simply telling you volatility is average and no bulge is forming. Nothing in the line tells you whether the coming reversal is up or down, so you never trade the Mass Index alone.
Best timeframes and settings
Dorsey designed the Mass Index on daily charts of stocks and indices, and that remains its natural home; the 9-period EMAs, the 25-period sum, and the 27 / 26.5 thresholds are his conventions and were tuned to daily data. It can be applied to weekly charts for position trading or to intraday charts for active trading, but the thresholds do not automatically transfer — instruments with different range behavior may routinely sit above or below 27, so you should study the indicator's own history on your chart before trusting the classic levels. Shortening the EMA lengths makes the index more sensitive and quicker to bulge, at the cost of more false alarms; lengthening them smooths the line and delays the signal. The 25-period sum is the memory of the indicator, and shrinking it makes each bulge briefer and sharper. As a rule, keep the defaults on daily equities and only recalibrate deliberately when you move to a very different instrument or timeframe.
When and where to use it
The Mass Index earns its keep in markets that trend and then reverse in identifiable turns, which describes most equities, index futures, and many commodities on the daily timeframe. It is a reversal-anticipation tool, so it is most valuable when you already hold a position or a directional bias and want an early warning that the move may be exhausting. It is far less useful in a persistently choppy, low-conviction market where ranges expand and contract without leading anywhere, because the bulge will fire on noise. Avoid leaning on it in thin or synthetic instruments whose high-low ranges are erratic, and be cautious around scheduled events like earnings, where a one-off range explosion can trip the indicator without implying a genuine trend change. Treat it as a timing filter layered onto a directional framework rather than a signal you act on in isolation.
Strategies that use it
The canonical Dorsey strategy pairs the reversal bulge with a 9-period EMA of price to supply direction: when the bulge completes, look at the EMA — if price is above a falling EMA in a prior downtrend, prepare for a reversal up, and if price is below a rising EMA in a prior uptrend, prepare for a reversal down. A practical entry is to wait for the bulge to complete, then trade in the direction opposite the trend that preceded it once a confirming candle or a break of a short-term swing level appears. A second approach uses the Mass Index purely as an exit filter: if you are riding a trend and a reversal bulge fires, you tighten your trailing stop or take partial profits rather than adding to the position. A third, more conservative method requires the bulge plus a divergence or an overbought or oversold reading from a separate oscillator before committing, which cuts the number of trades but improves their quality.
Combining it with other indicators
Because the Mass Index is direction-blind by design, its most natural partner is a trend tool: a moving average, the ADX, or simple market structure that tells you which way the market has been leaning and therefore which way a reversal would go. Momentum oscillators such as RSI or the Stochastic add confluence by showing whether the market is stretched at the moment the bulge completes, making a turn more probable. Support and resistance or Fibonacci levels give the reversal a logical price location to occur, so a bulge that fires precisely at a major level is far more actionable than one in open space. Volume indicators can corroborate the exhaustion story, since climactic volume often accompanies the range expansion that drives the bulge. The through-line is that the Mass Index answers when, and you always borrow the where and the which-way from another study.
Where it fails
The most common failure is treating the bulge as a signal in itself and trading it without a directional overlay, which turns a reversal-timing tool into a coin flip. The 27 and 26.5 thresholds are conventions fitted to daily equities and can be simply wrong for other instruments, causing the index to either never reach the trigger or to sit above it perpetually. A single volatile session — a gap, a news spike, a limit move — can inflate the range and manufacture a bulge that reflects an event, not a genuine change in trend. The indicator also lags, because it sums 25 periods of smoothed data, so by the time the bulge completes the early part of the reversal may already be underway. Finally, in strong, persistent trends the market can expand its range and keep going, so the anticipated reversal never materializes and the bulge becomes a costly false alarm.
A worked example
Imagine a stock that has fallen steadily for two months, and its daily high-low ranges, which had been narrow near the lows, begin to widen sharply as sellers capitulate and bargain hunters fight back. The single 9-period EMA of the range pulls away from the double-smoothed EMA, the ratio climbs above one on many recent bars, and the 25-period sum lifts the Mass Index from around 24 up through 27. Over the next several sessions the violent swings settle, the ranges contract, the ratio eases back toward one, and the Mass Index slips below 26.5 — the reversal bulge is now complete. You consult a 9-period EMA of price and see that price has just closed back above a flattening EMA after the long decline, pointing to a reversal upward. You enter long on the first higher close, place a stop beneath the recent capitulation low, and target the first significant resistance overhead, having used the Mass Index only to time the turn and the EMA to choose its direction.