What it means
The middle band is usually a 20-period simple moving average, meaning the average closing price over the last 20 days, weeks or minutes depending on the chart. The upper and lower bands sit two standard deviations either side of that average, so roughly speaking most recent prices will have fallen inside them.
The bands are recalculated every period, which is why they breathe in and out as the market changes. The reason finance people care is that the width of the bands is a direct picture of volatility.
When the bands squeeze together, the asset has been trading in a tight range and something is likely to break that calm; when they flare apart, the market is already moving violently and the risk of a sharp reversal is higher. In practice, a price touching the upper band does not mean sell and a price touching the lower band does not mean buy.
Strong trends ride along one band for weeks, so the bands are usually read alongside other signals such as volume or momentum indicators rather than being used on their own. Two derived measures do most of the analytical work.
Bandwidth expresses the gap between the bands as a percentage of the middle band, giving a comparable volatility number across different assets, and %B tells you where the current price sits within the bands on a scale where 0 is the lower band and 1 is the upper band. Outside pure trading, treasury teams borrow the same maths to monitor commodity or currency exposures.
A procurement manager watching a widening band on a key input price has an early warning that hedging costs are about to rise, which is a business decision rather than a trading one.
In practice
Real-world examples.
Example
An equity analyst at a wealth manager notices that a utility stock's bandwidth has fallen from 14% to 4% over six weeks. She flags the squeeze to the portfolio managers as a sign that the quiet period is unlikely to last, and they reduce position size ahead of the results announcement.
Example
A commodity buyer at a food producer plots Bollinger Bands on the copper price he uses as a proxy for packaging costs. When the price closes above the upper band three days running, he brings forward his quarterly hedge rather than waiting for the usual review date.
Example
A private investor sees a technology share sitting on the lower band and buys, assuming a bounce. The share keeps sliding along that band for another month, teaching him that a band touch describes where the price is, not where it is going next.
Think of it
“Bollinger Bands show volatility range-bands widen and narrow with price swings.
Formula
Calculation
Middle band = simple moving average of the last 20 closing prices
Upper band = middle band + (2 x standard deviation of those 20 closes)
Lower band = middle band - (2 x standard deviation of those 20 closes)
Suppose a listed engineering firm's last 20 closing prices add up to $1,000.00, so the middle band is $1,000.00 / 20 = $50.00. The standard deviation of those same 20 closes works out at $1.50, so the upper band is $50.00 + (2 x $1.50) = $53.00 and the lower band is $50.00 - (2 x $1.50) = $47.00.
Bandwidth is ($53.00 - $47.00) / $50.00 = $6.00 / $50.00 = 12%. If today's close is $52.00, then %B = ($52.00 - $47.00) / ($53.00 - $47.00) = $5.00 / $6.00 = 0.83, meaning the price sits 83% of the way up the channel and is close to, but not through, the upper band.Case study
Seen in the real world.
This is an illustrative and entirely fictional example. Harborline Capital, an invented boutique fund, ran a small systematic strategy that bought any holding whose price closed below the lower Bollinger Band and sold when it returned to the middle band. For two calm years the rule worked well, producing a steady stream of small gains from shares that drifted back to their averages.
The strategy then met a sustained market decline. Because the bands recalculate every day, they simply travelled downwards with the falling prices, and the fictional fund kept buying into a trend rather than catching a reversal. Losses accumulated far faster than the earlier gains.
Harborline's imaginary risk committee added a second condition: no purchase unless bandwidth was below its own 12-month average, filtering out high volatility periods. The strategy traded far less often afterwards, but its worst month improved sharply, which the committee treated as the better outcome.
Watch out
Common mistakes.
- Treating a touch of the upper band as an automatic sell signal, when strong trends routinely hug that band for long stretches.
- Assuming exactly 95% of prices must fall inside the bands, which relies on a normal distribution that real market prices do not follow.
- Leaving the settings at 20 periods and 2 standard deviations for every asset without checking whether they suit that market's rhythm.
Questions
People also ask.
Why do the bands sometimes squeeze so tightly together?
Because recent prices have been unusually similar, which lowers the standard deviation and pulls the outer lines towards the average.
Do Bollinger Bands predict direction?
No, they describe volatility and relative position; direction has to come from trend, momentum or fundamental analysis.
Can a non-trader use them?
Yes, treasury and procurement teams use the same construction to monitor how volatile a key input price or exchange rate has become.
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