Mass Index Indicator: Using Range Expansion to Spot Reversal Bulges

Most indicators on your screen are obsessed with where price closes. The mass index indicator doesn’t care about the close at all. It watches only the gap between each day’s high and low, and it waits for that gap to swell and then shrink again. Donald Dorsey, who built it, believed that this swelling and shrinking of the daily range often comes just before a trend turns. He called the pattern a “reversal bulge”.

That makes it an odd tool, and a useful one. It won’t tell you whether to buy or sell. It tells you that a stock’s behaviour is changing, and that the trend you’ve been riding (or fighting) may be running out of road. We’ll go through how it’s calculated, what the famous 27 and 26.5 levels mean, why most traders lower those levels, and a full worked example with a trade plan.

What the mass index indicator actually measures

According to StockCharts’ ChartSchool, the Mass Index uses the high-low range to identify trend reversals based on range expansions, and it is a volatility indicator with no directional bias. Investopedia’s entry on the Mass Index says the same thing in different words: it examines the range between high and low prices over a period, and it suggests a reversal is likely when that range widens beyond a point and then contracts.

Think about what a widening range means on a real chart. A stock that has been drifting down for weeks in tidy ₹8-10 daily ranges suddenly starts printing ₹25 and ₹30 candles. Buyers and sellers are fighting harder. Something has changed, whether it’s results, a block deal, a sector rerating or just exhaustion. When the range then narrows again, the fight has been settled, often in favour of the side that wasn’t winning before. That’s the idea Dorsey was trying to capture.

How the Mass Index is calculated, step by step

ChartSchool breaks the calculation into four parts:

1. Single EMA: a 9-period exponential moving average of the high-low differential.
2. Double EMA: a 9-period EMA of that 9-period EMA.
3. EMA Ratio: the Single EMA divided by the Double EMA.
4. Mass Index: the 25-period sum of the EMA Ratio.

The ratio is the clever part. The Double EMA is a slower, smoother version of the Single EMA. When ranges suddenly expand, the Single EMA jumps first and the Double EMA lags behind, so the ratio rises above 1. When ranges settle, the two catch up with each other and the ratio drifts back towards 1. Summing 25 of those ratios then smooths the line again, which is why ChartSchool describes that final step as acting like a moving average.

Here is one simple consequence to remember. If ranges are steady, each ratio sits close to 1, and 25 ratios of about 1 add up to about 25. That’s why the indicator spends most of its life in the mid-20s, which ChartSchool also points out. A reading of 27 means the recent ratios have, on average, been noticeably above 1. In plain words: ranges have been expanding faster than their own smoothed average.

A note on the EMA weight

An exponential moving average gives more weight to recent data. The multiplier most people use, as Investopedia’s EMA explainer sets out, is 2 ÷ (number of observations + 1). For a 9-period EMA that’s 2 ÷ 10, or 0.2. So each new day’s value gets a 20% weight and the previous EMA keeps the other 80%.

Worked example: computing one day’s reading

Let’s take a made-up mid-cap, Konkan Cables Ltd, and walk through a single day. All figures here are invented for the illustration.

• Today’s high is ₹612 and low is ₹584, so the high-low range is ₹28.
• Yesterday’s Single EMA of the range was ₹18.00.
• Yesterday’s Double EMA was ₹17.50.

Step 1, the new Single EMA: (28 × 0.2) + (18.00 × 0.8) = 5.60 + 14.40 = ₹20.00.

Step 2, the new Double EMA: this smooths the Single EMA, not the raw range. So (20.00 × 0.2) + (17.50 × 0.8) = 4.00 + 14.00 = ₹18.00.

Step 3, the ratio: 20.00 ÷ 18.00 = 1.111.

Step 4, the sum: say the other 24 ratios in the 25-day window add up to 25.42. Today’s Mass Index is 25.42 + 1.111 = 26.53.

Notice what one wide day did. A ₹28 range against a smoothed average of ₹18 pushed today’s ratio to 1.111, and that alone lifted the sum by more than 0.1 over a “neutral” day. String a few of those days together and the line climbs quickly. You’ll never do this by hand in practice, since any charting platform will plot it, but doing it once shows you why the indicator reacts the way it does.

The Mass Index doesn’t predict direction. It tells you the stock has stopped behaving the way it did, and that’s often the first honest clue a trend is tiring.

The reversal bulge: Dorsey’s 27 and 26.5 rule

Dorsey’s signal has two parts, and both have to happen. ChartSchool describes it this way: a bulge occurs when the Mass Index moves above 27, but that alone does not complete the signal. Dorsey waited for the bulge to reverse with a move back below 26.50. Investopedia gives the same two levels.

Here’s how a sequence might look on our made-up stock, again with invented readings:

Day 4 sets up the bulge. Day 8 completes it. Day 7 is the one that catches impatient traders, because the line has clearly turned, yet Dorsey’s rule hasn’t been satisfied.

Which way is the reversal? Look at the trend before the bulge

This is where people trip up. The Mass Index has no idea whether the next move is up or down. ChartSchool’s rule of thumb: a downtrend followed by a reversal bulge suggests a bullish reversal, and an uptrend followed by a reversal bulge suggests a bearish one. The direction comes from the trend that was in place, not from the indicator.

ChartSchool’s sample scans use the 200-day moving average to define that trend. The bullish scan looks for stocks trading below their 200-day moving average, a long-term downtrend, with the Mass Index moving below 26.5. The bearish scan does the reverse, with stocks above their 200-day average. That’s a sensible, simple filter you can copy for Nifty stocks.

Even then, a completed bulge only sets your bias. ChartSchool says that once the reversal bulge is complete, traders should use other techniques to work out the direction of the next move. Its own example pairs the Mass Index with TRIX, which it describes as the one-period rate of change of a triple-smoothed EMA, and suggests TRIX or similar indicators such as MACD or PPO for the directional trigger. If you’re comfortable with momentum tools, our guide to the rate of change indicator covers the building block behind TRIX.

Investopedia makes the same point with a car analogy: the Mass Index is like the speedometer, telling you how fast you’re going, and you still need a compass to know whether you’re heading north or south.

Why 27 is often too strict

Here’s the honest part. ChartSchool says the Mass Index rarely exceeds 27, and it takes exceptional volatility to push it there. Investopedia also notes that an index of 27 represents a rather volatile stock, so some traders set a lower baseline.

ChartSchool’s suggestion is to compare Mass Index levels over time for the stock you’re studying, and treat a move near the high end of its own historical range as the bulge. In its International Paper example, the Mass Index touched 27 once during a very volatile market in August 2011, but 26 looked like the more useful threshold for that stock.

For Indian stocks, we think this matters even more. A heavyweight banking or FMCG stock with deep liquidity may almost never reach 27 outside a market-wide shock, while a small-cap can get there on one lumpy results week. One fixed number can’t serve both. A practical approach:

• Pull up two or three years of the indicator for the stock.
• Note where the bigger peaks topped out. That’s your stock-specific bulge zone.
• Pick a retreat level a little below it to stand in for Dorsey’s 26.5.
• Check how often past bulges near that level came before a genuine change in trend. If the answer is “rarely”, the indicator isn’t a good fit for that stock.

Dorsey’s 27 is a starting point, not a law. The stock’s own history tells you where its bulges really sit.

A trade plan built around a completed bulge

Let’s carry on with Konkan Cables. The stock had been trading below its 200-day moving average for months, so a completed bulge on day 8 gives us a bullish bias. We don’t buy on day 8. We wait for the momentum trigger, say TRIX crossing above its signal line, which happens two sessions later with the stock at ₹598. Again, these are made-up numbers.

• Entry: ₹598, after the TRIX crossover.
• Stop-loss: just below the recent swing low at ₹571. That’s a risk of ₹27 per share.
• Capital: ₹5,00,000, risking 1% per trade, which is ₹5,000.
• Position size: ₹5,000 ÷ ₹27 = 185 shares (rounded down).
• Money deployed: 185 × ₹598 = ₹1,10,630.
• Actual risk if stopped out: 185 × ₹27 = ₹4,995, before brokerage and taxes.

The reason to size this way is simple. Volatility indicators fire around exactly the moments when ranges are wide, and wide ranges mean wide stops. If you size by “a round number of shares” instead of by risk, the Mass Index will quietly push you into your biggest losses.

Where the Mass Index goes wrong

• Event days distort it. A results day, a budget session or a policy surprise can widen ranges for reasons that have nothing to do with trend exhaustion. One big candle can lift the line, and the “bulge” may just be the news being absorbed.
• Strong trends can bulge and keep going. ChartSchool’s own Advanced Auto Parts example notes that one bearish bulge signal led to a trading range rather than a fall. A reversal of trend can mean sideways, not opposite.
• It lags. With a 25-period sum on top of two layers of 9-period smoothing, the signal shows up after the range expansion has already happened.
• Low-liquidity stocks are noisy. Thinly traded counters print erratic highs and lows, which can make the high-low range a poor measure of real participation.

Pairing it with other tools

Because the Mass Index only reads volatility, it works best next to something that reads direction or structure. Volatility bands are a natural partner; our explainer on the Keltner Channel indicator shows another way to judge whether ranges are expanding around a trend. And if you want the broader map of how these tools fit together, start with our technical analysis guide for Indian stocks.

A combination we like for swing trades: the 200-day moving average for trend, the Mass Index for “something is changing”, and a momentum crossover for timing. Each one answers a different question, and none of them is asked to do the others’ job.

The short version

• The Mass Index sums 25 days of a ratio between a 9-period EMA of the high-low range and a 9-period EMA of that EMA.
• With steady ranges it sits around 25; it rises when ranges expand and falls when they narrow.
• Dorsey’s reversal bulge is a move above 27 followed by a drop below 26.5.
• It has no direction. The prior trend sets the bias, and a momentum indicator such as TRIX or MACD gives the trigger.
• 27 is rarely reached, so most chartists set thresholds from the stock’s own history.

Disclaimer: This article is for educational purposes only and is not investment advice. The calculation and signal rules are summarised from StockCharts’ ChartSchool and Investopedia pages linked above, checked on October 21, 2026. Konkan Cables Ltd and every price, range and indicator reading attached to it are made up to show the method. Trading in equities involves risk of loss. Please do your own research and consult a SEBI-registered investment adviser before making any investment decision.