Bollinger Bands: What They Measure and Why Touching a Band Decides Nothing

Bollinger Bands are a moving average (usually 20 periods) with two lines set a multiple of the standard deviation away (usually 2). They measure volatility: they widen when price moves and narrow when it's calm.

What it is

John Bollinger designed the bands in the 1980s as a way to judge price in relative terms: is it high or low compared with recent volatility? The author himself stresses that touching a band isn't a signal.

It's often said that about 95% of prices fall within the ±2σ bands. That's only true for a normal distribution, and markets have fat tails: extreme moves happen more often than that assumption implies.

How people use it

selling at the upper band and buying at the lower band in consolidation
the “squeeze” – narrowing bands as a sign of a coming breakout
“riding the band” in a strong trend
the 20/2 settings carried over to every timeframe

What the research and evidence say

The bands are a description of volatility, not a strategy, so it's hard to “test” them as a whole. Mean-reversion strategies based on the bands work in consolidation and lose in trends, and history makes it hard to say in advance which regime is coming. Here's the classic mistake: the bands are judged on examples that look good after the fact.

Weaknesses

in a trend price can “ride” the outer band for a long time
assuming a normal distribution understates the risk of extreme moves
a squeeze tells you something will happen, but not in which direction
calculated from closing prices only, with no volume information

What order flow shows

A band tells you price is far from its average. Order flow shows whether the move has fuel: whether volume and delta are rising in the direction of the move, or absorption and a lack of follow-through appear at the extreme. VWAP deviation bands calculated from the start of the session follow similar statistical logic, but anchored in volume.

Indicators vs order flow

Every indicator and every pattern in this section is built from the same data: a candle's open, high, low, close and sometimes volume. They're derivatives of price, showing the effect rather than the cause. Order flow looks at what price is made of: who was buying and selling aggressively (delta), where limit orders were waiting (the order book) and at which prices the market accepted trade (the volume profile).

On CME futures this data is real and comes from one central market. Spot forex doesn't have it: platforms show tick volume, the number of price changes at a single provider.

To be fair: order flow isn't the Holy Grail. It takes learning, it can be noisy, big players hide some of their orders, and an edge only comes from a process and risk control. But it gives you information that simply isn't on a candlestick chart.

How to use it sensibly

use the bands to judge whether the market is calm or volatile
don't treat a band touch as a signal
on a trend day, don't automatically trade a return to the mean

Glossary: Bollinger Bands
Glossary: Standard deviation
Glossary: VWAP deviation bands
Atlas of biases: Cherry-picking
Atlas of biases: Hindsight bias
Order flow: VWAP and deviation bands: what they tell you and what they don't
Also under the microscope: RSI
Also under the microscope: MACD
Also under the microscope: The stochastic oscillator and Stochastic RSI

Frequently asked questions

What are the Bollinger Band settings?

The standard is 20 periods and 2 standard deviations. The author allows changes, but settings don't change the fact that the bands describe volatility, not direction.

What does a Bollinger Band squeeze mean?

That volatility has fallen. Calm is often followed by movement, but the bands don't tell you which way.

Do Bollinger Bands work in a trend?

In a trend price often moves along the outer band. Trading bounces off it on such a day means trading against the market.

Educational content, not investment advice. No signals, entries, stops or position sizes.