How it works
The Mass Index looks for one specific thing: a bulge in the high-low range that is not explained by direction. Its logic starts from an observation about how trends end. A move that is running out of participants often shows a widening range first, as two-sided activity increases and the market stops moving in one direction with quiet bars. The index measures that widening by comparing a smoothed range with a doubly smoothed version of itself, so persistent expansion accumulates and ordinary noise cancels out.
Nothing in the calculation touches direction. Only the distance between the high and the low of each bar is used, so the indicator is genuinely agnostic about whether the market is rising or falling. That is deliberate: the claim is that range expansion precedes a turn in either direction, and the trader supplies the directional read from elsewhere, usually from the prevailing trend that is about to be reversed.
The canonical signal is the reversal bulge. The index rises above 27 and then falls back below 26.5, and that sequence, rather than the level alone, is what the author defined as the setup. The two thresholds mean a reversal is only flagged once expansion has both occurred and started to subside, which avoids acting during the expansion itself. The direction of the expected reversal comes from the trend in place when the bulge formed: a bulge inside an uptrend warns of a decline and vice versa.
It is a niche tool and it deserves to be understood as one. In a portfolio of indicators it occupies a slot nothing else fills, because ATR and BandWidth describe the level of volatility while the Mass Index describes the shape of a change in range relative to its own recent behaviour. But its thresholds were calibrated on daily equity data decades ago, they do not travel well to intraday charts or to markets with very different range dynamics, and the bulge fires often enough in strong trends to require an independent confirmation before anything is done with it.
Calculation
The arithmetic in words, in the order it happens.
For each bar compute the range as the high minus the low. Take a 9-period exponential moving average of that range, then take a 9-period exponential moving average of the result, giving a doubly smoothed range. Divide the single-smoothed value by the double-smoothed value to get a ratio that is above 1 when the range has been expanding and below 1 when it has been contracting. The Mass Index is the sum of the last 25 of those ratios, so it oscillates around 25 and rises as expansion persists. The classic reversal bulge is defined as the index rising above 27 and then falling back below 26.5.
Source
An AlgoBeamScript implementation of the formula above, written by us from the arithmetic so the code and the calculation agree line for line.
Runs unchanged on the platform and in the AlgoBeamTS runtime. The language reference is in the documentation.
Inputs
Defaults are the values most charting packages ship with. They are conventions, not optimal settings — the right length depends on your instrument and your holding period.
| Input | Default | What it changes |
|---|---|---|
| Sum length | 25 | Number of ratios accumulated. This sets the scale of the whole series, so changing it invalidates the 27 and 26.5 thresholds and any rule written against them. |
| EMA length | 9 | Smoothing applied twice to the raw range. Shorter values make the index react to a single wide bar; longer values require the expansion to persist across a swing before it registers. |
| Bulge threshold | 27 | Level the index must exceed for a reversal bulge to be armed. Raising it makes signals rarer and reserves them for pronounced range expansion. |
| Trigger threshold | 26.5 | Level the index must fall back through to complete the bulge. Widening the gap between the two thresholds demands a clearer contraction before the signal fires. |
How to read it
What practitioners take from the plot. Read these as descriptions of market state, not as entry signals.
- Rising above 27
- Ranges are expanding persistently relative to their own recent behaviour. A reversal setup is arming, but nothing has triggered yet.
- Falling back below 26.5 after exceeding 27
- The reversal bulge is complete. The expected direction is opposite to the trend that was in place while the bulge formed.
- Sitting near or below 25
- Range behaviour is stable or contracting. No reversal warning, and often a quiet trending or consolidating regime.
- Repeated bulges without a price turn
- A high-volatility regime in which range expansion is the norm. The indicator is effectively saturated and its signals should be ignored until conditions normalise.
Limitations
Where this indicator misleads. None of these are fixed by a better parameter.
- It is directionless, so the bulge only becomes actionable once a trend read is supplied from elsewhere, and in an ambiguous market that read is exactly what is missing.
- The thresholds are absolute numbers calibrated on daily equity charts of an earlier era. On intraday data and on instruments with different range behaviour they fire either constantly or never.
- Range expansion accompanies plenty of continuations, not just reversals, so the signal has a substantial false positive rate in strong trends.
- Two layers of exponential smoothing plus a 25-bar sum means the index responds slowly; by the time a bulge completes, a good deal of the move it is describing has already happened.
Educational reference. This page explains how an indicator is built and how it is commonly read. It is not investment advice, not a recommendation and not a signal service. No indicator is profitable on its own — each is a way of describing a market, and any rule built on one has to be tested with realistic costs before it is traded.