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Bollinger Bands: Volatility Envelopes Explained
Bollinger Bands wrap a moving average in a pair of volatility-based envelopes. They show at a glance whether a stock is trading at a statistically unusual price relative to its own recent behavior.
Key Takeaways
- Bollinger Bands use a 20-period SMA as the middle line with upper and lower bands set two standard deviations above and below.
- When band width narrows to a multi-month low, a squeeze, it often precedes a sharp directional move, though it gives no direction signal.
- A stock in a strong uptrend can ride the upper band for weeks; treating every upper-band touch as an automatic sell signal repeatedly fights a winning trend.
- The bands adapt to volatility: they widen in volatile periods and contract in quiet ones, making the overbought/oversold definition relative to recent behavior.
Key Takeaways
- Bollinger Bands use a 20-period SMA as the middle line with upper and lower bands set two standard deviations above and below.
- When band width narrows to a multi-month low, a squeeze, it often precedes a sharp directional move, though it gives no direction signal.
- A stock in a strong uptrend can ride the upper band for weeks; treating every upper-band touch as an automatic sell signal repeatedly fights a winning trend.
- The bands adapt to volatility: they widen in volatile periods and contract in quiet ones, making the overbought/oversold definition relative to recent behavior.
What It Is
Bollinger Bands are a volatility indicator developed by John Bollinger in the early 1980s. The indicator plots three lines on a price chart: a middle line (a simple moving average), an upper band set a number of standard deviations above the average, and a lower band set the same number of standard deviations below.
The default parameters are a 20-period simple moving average with bands at plus and minus two standard deviations. Bollinger chose these values because, over a 20-period window, they produced bands that contained most of the price action for the stocks he studied. Most charting platforms ship with this 20 / 2 default.
The Intuition
A moving average tells you where price has been on average. It says nothing about how wildly price has been swinging around that average. Two stocks can share the same 20-day mean yet behave very differently: one drifts in a tight range, the other whips up and down by several percent a day.
Bollinger Bands solve that gap by scaling the envelope to recent volatility. When a stock gets quiet, the bands contract and hug the price. When a stock gets noisy, the bands widen. A move to the upper band is therefore not a fixed dollar distance from the average; it is a statistically scaled distance that adapts as conditions change.
How It Works
The three lines are computed as follows:
Middle Band = SMA(close, 20)
Upper Band = Middle Band + (2 * stdev(close, 20))
Lower Band = Middle Band - (2 * stdev(close, 20))
Where stdev is the population standard deviation of the same 20 closing prices used in the moving average. John Bollinger specifies the simple moving average (not exponential) because the standard deviation calculation itself is built around a simple mean.
Two companion indicators are often shown alongside the bands. %B places price on a 0 to 1 scale relative to the bands:
%B = (close - Lower Band) / (Upper Band - Lower Band)
A %B of 1.0 means price sits exactly on the upper band, 0.0 on the lower band, and 0.5 on the middle line. Readings above 0.80 or below 0.20 flag price near the edges.
BandWidth measures how wide the envelope is relative to the middle:
BandWidth = (Upper Band - Lower Band) / Middle Band
A falling BandWidth reveals a contracting range (a "squeeze") that often precedes a sharp move. A rising BandWidth reflects expanding volatility.
Worked Example
Assume a stock's last 20 closes produce a simple moving average of 100 and a standard deviation of 2.50. With the default 20 / 2 settings, the bands sit at:
- Upper Band: 100 + (2 x 2.50) = 105
- Middle Band: 100
- Lower Band: 100 - (2 x 2.50) = 95
If today's close is 104, %B equals (104 - 95) / (105 - 95) = 0.90. Price is near the upper band but has not broken through. BandWidth equals (105 - 95) / 100 = 0.10, or 10 percent of the mean.
A week later, suppose the stock has gone sideways and the standard deviation has fallen to 0.80. The new bands are 101.6 and 98.4, BandWidth drops to 3.2 percent, and the chart shows a classic squeeze. Traders watching for a volatility expansion would flag this compression as a setup to monitor.
Common Mistakes
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Shorting because price tags the upper band. The single most common error. A stock in a strong uptrend can "ride the band" for weeks, with every dip stopping at the middle line rather than the lower band. Treating an upper band touch as an automatic sell signal leads to repeated losses in trending markets.
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Assuming the lower band is a floor. The mirror mistake. In a real downtrend, price stays pinned to the lower band as it falls. The 2-standard-deviation envelope is not a support or resistance line, it is a statistical description of recent behavior.
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Treating 20 / 2 as sacred. The defaults are a convention chosen by John Bollinger for a particular purpose. Short-term traders often shorten the window; long-term investors lengthen it. The right parameters depend on the timeframe you actually trade.
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Using the bands in a dead market. When volatility collapses, the bands narrow so far that every small tick pushes price outside them. In that regime, band touches are noise, not signal. BandWidth helps you see the squeeze before you act.
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Confusing %B with a momentum oscillator. %B looks like RSI on a 0 to 1 scale, but it is not a momentum indicator. It tells you where price sits inside its own volatility envelope, nothing more. Pair %B with a separate momentum tool rather than treating it as one.
Frequently Asked Questions
Q: What are Bollinger Bands in simple terms? Bollinger Bands are three lines drawn on a price chart: a 20-period moving average in the middle, and two envelopes set two standard deviations above and below it. They show how far price has moved from its recent average relative to how volatile that security has been.
Q: How do Bollinger Bands affect investment decisions? They provide context for whether a price move is unusual or normal. A close above the upper band during a breakout confirms the move is statistically significant. A band squeeze tells a trader to prepare for a large move but wait for direction confirmation before entering.
Q: What is a real-world example of Bollinger Bands in use? After a low-volatility consolidation, Bollinger Band width on a stock contracts to its narrowest point in six months. Three days later the stock breaks out on heavy volume above the upper band. The squeeze, visible days before the move, gave advance warning that a setup was forming.
Q: How can investors use Bollinger Bands practically? Use BandWidth to identify squeezes and %B to track where price sits within the envelope. A simple rule: in trending markets, use the middle band as a dynamic support level to buy pullbacks; in ranging markets, fade moves to the outer bands with tight stops outside them.
Q: How are Bollinger Bands different from ATR-based channels like Keltner Channels? Bollinger Bands scale their width using standard deviation of closing prices, which responds quickly to volatility spikes. Keltner Channels use ATR, which includes the full high-low-close range and reacts more smoothly. When Bollinger Bands contract inside Keltner Channels, it is a refined squeeze signal.
Sources
- John Bollinger. "A Complete Explanation of Bollinger Bands." https://www.bollingerbands.com/bollinger-bands
- StockCharts ChartSchool. "Bollinger Bands." https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-overlays/bollinger-bands
- StockCharts ChartSchool. "%B Indicator." https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-indicators/b-indicator
- StockCharts ChartSchool. "Bollinger BandWidth." https://chartschool.stockcharts.com/table-of-contents/technical-indicators-and-overlays/technical-indicators/bollinger-bandwidth
- Fidelity Learning Center. "Bollinger Bands." https://www.fidelity.com/learning-center/trading-investing/technical-analysis/technical-indicator-guide/bollinger-bands
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.
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