Bollinger Bandwidth: Measuring the Squeeze Setup

Key Takeaways

• Bandwidth equals the distance between the upper and lower bands divided by the middle band.
• A bandwidth at a six-month low is John Bollinger's classical definition of the squeeze setup.
• Falling bandwidth signals contracting volatility; rising bandwidth signals expanding volatility.
• The most common error is reading low bandwidth as a directional signal rather than a setup for the next move.

What It Is

Bollinger bandwidth is a volatility gauge derived directly from Bollinger Bands. The numerator is the absolute price gap between the upper and lower band; the denominator is the middle band, which is just the 20-period simple moving average. The result is a unitless ratio comparable across stocks and timeframes.

The indicator is plotted in a separate pane below price. It does not tell you direction. It tells you whether the bands are contracting, expanding, or holding steady, and by how much relative to the underlying price level.

The Intuition

Volatility clusters. Quiet periods tend to be followed by quiet periods until something breaks, and active periods tend to be followed by more active ones. Bollinger framed this as a cycle: volatility contractions resolve into expansions, and expansions revert to contractions.

The width of Bollinger Bands captures the volatility part of that cycle directly because the bands are two standard deviations wide. When standard deviation falls, the gap between the bands tightens. Dividing by the middle band makes the measure comparable across price levels, since a 1-point band gap means something very different on a $10 stock than on a $1000 stock.

How It Works

The Bollinger bandwidth formula is:

upper_t   = SMA(close, 20)_t + 2 * stdev(close, 20)_t
lower_t   = SMA(close, 20)_t - 2 * stdev(close, 20)_t
middle_t  = SMA(close, 20)_t
BW_t      = (upper_t - lower_t) / middle_t

The 20-period and 2-standard-deviation defaults follow Bollinger's original specification. Bandwidth scales linearly with the standard deviation of returns. A doubling of realized volatility produces a doubling of bandwidth, all else equal.

Bollinger's classical squeeze rule looks at the lowest bandwidth value in the past 125 trading days, roughly six months. When current bandwidth equals that historic low, the asset is said to be in the squeeze. The breakout that follows can be in either direction, so the squeeze itself is a setup, not a trade.

Worked Example

Suppose a stock has a 20-day SMA of 100, an upper band at 104, and a lower band at 96. Bandwidth equals (104 minus 96) divided by 100, which is 0.08 or 8 percent.

Over the next month volatility falls. The SMA drifts to 102, the upper band sits at 103.50, and the lower at 100.50. Bandwidth is now (103.50 minus 100.50) divided by 102, which is 0.0294 or 2.94 percent. The bands have contracted to roughly a third of their prior width. If 2.94 percent is the lowest bandwidth reading of the last six months, the squeeze setup is active.

A week later price breaks above the upper band on heavy volume. Bandwidth jumps from 3 percent to 7 percent as the standard deviation expands. The price direction was not knowable in advance from bandwidth, but the move was foreshadowed by the prior contraction. Bollinger's recommended approach is to wait for a band break to define the breakout direction, not to anticipate it from bandwidth alone.

Common Mistakes

1. Trading bandwidth direction. Bandwidth is omnidirectional. A falling reading does not predict price will fall; it predicts that volatility will fall, then likely expand. Direction comes from price breaking a band.
2. Comparing raw band gap, not bandwidth. A 4-point spread on a $40 stock is very different from a 4-point spread on a $400 stock. Bandwidth normalizes by the middle band for exactly this reason.
3. Using the wrong lookback for the squeeze. Bollinger specified 125 trading days for the six-month squeeze. Some platforms default to 20-day or 50-day lookbacks, which find squeezes that are not historically meaningful.
4. Ignoring the head fake. Bollinger documented a common pattern in which price feints in one direction at the start of a squeeze release and then reverses sharply. Without that awareness, traders get whipsawed on the breakout.
5. Forgetting bandwidth tracks the same data as %B. %B and bandwidth are derived from the same bands and the same SMA. Using both as independent confirmation is double counting.

Frequently Asked Questions

What is the Bollinger bandwidth in simple terms? The Bollinger bandwidth measures how far apart the upper and lower Bollinger Bands are, expressed as a fraction of the middle band. Falling bandwidth means volatility is contracting; rising bandwidth means it is expanding.

How does the Bollinger bandwidth affect investment decisions? Traders use a six-month low in bandwidth to flag potential breakout setups. The signal does not say which direction price will move, only that a meaningful move is statistically more likely after the contraction.

What is a real-world example of Bollinger bandwidth? On a stock that has been ranging for two months, bandwidth often drifts down to a multi-month low just before a sharp directional break. The same pattern shows up on indices like the S&P 500 ahead of major macro events.

How can investors use Bollinger bandwidth effectively? Define a clear historical lookback for the squeeze, wait for price to actually break a band before acting, and prepare for the head fake. Combine with a volume confirmation rule to avoid trading false starts.

How is Bollinger bandwidth different from %B? %B normalizes price within the bands and tells you where price sits. Bandwidth measures the width of the bands themselves and tells you how compressed or expanded volatility is. The two answer different questions.

Sources

1. StockCharts ChartSchool. Bollinger BandWidth. https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-indicators/bollinger-bandwidth
2. StockCharts ChartSchool. Bollinger Bands. https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-overlays/bollinger-bands
3. John Bollinger. Bollinger Bands. https://www.bollingerbands.com/bollinger-bands
4. TradingView Support. Bollinger BandWidth (BBW). https://www.tradingview.com/support/solutions/43000501972-bollinger-bandwidth-bbw/

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.