Bollinger Bands Explained: How the Indicator Works

Bollinger Bands are a volatility indicator made of three lines: a middle simple moving average and an upper and lower band placed a set number of standard deviations away. Created by John Bollinger in the 1980s, the bands widen when volatility rises and contract when it falls, framing price within its recent volatility.

What are Bollinger Bands?

Bollinger Bands are a volatility-based overlay developed by John Bollinger in the early 1980s. They sit alongside the other technical indicators traders rely on, but instead of measuring momentum like the Relative Strength Index, they measure how much price is moving around its own average.

The indicator answers a practical question: is price high or low relative to its recent volatility? It draws a moving average through the middle of price action, then places two bands above and below it at a distance set by standard deviation, a statistical measure of dispersion. When price swings get larger, the bands move apart; when price calms down, they close in. Because the band width adapts to the market in real time, Bollinger Bands read the same way across different assets and conditions.

How are Bollinger Bands calculated? (the formula)

Bollinger Bands use three lines built from a simple moving average and standard deviation, with the default settings of a 20-period average and bands two standard deviations away.

Middle Band = 20-period Simple Moving Average (SMA)
Upper Band  = Middle Band + (2 x 20-period standard deviation)
Lower Band  = Middle Band - (2 x 20-period standard deviation)

The middle band is a 20-period SMA of closing prices. The upper and lower bands sit two standard deviations of price above and below that average. Standard deviation rises when price becomes more volatile, which is what makes the bands expand and contract automatically.

A worked illustration: suppose a stock's 20-period SMA is $50 and the standard deviation of its closes over those 20 periods is $2. The upper band sits at 50 + (2 x 2) = $54 and the lower band at 50 - (2 x 2) = $46. If volatility then doubles to a $4 standard deviation, the bands widen to $58 and $42 while the average is unchanged. Because price stays within two standard deviations of its mean most of the time, the bands tend to contain the bulk of price action.

What is a Bollinger Band squeeze?

A Bollinger Band squeeze is when the bands contract to an unusually narrow width, reflecting a stretch of low volatility. It is one of the most-watched Bollinger setups because periods of low volatility often give way to periods of higher volatility.

The logic is that markets cycle between calm and active phases. When the bands pinch tightly together, price is moving in a tight range and volatility is compressed, which historically tends to precede an expansion. The key limitation is direction: a squeeze tells you that a larger move may be coming, but not which way it will break. Acting on a squeeze before price actually moves is essentially guessing on direction, so traders typically wait for a breakout and confirm it against the broader trend and volume rather than anticipating the break.

What does it mean to "walk the band"?

Walking the band describes price hugging the upper or lower band through a strong trend, repeatedly tagging one edge without reversing. It is the single most important reason a band touch is not a reversal signal.

In a strong uptrend, price can ride the upper band for many periods, printing touch after touch while continuing higher. The same happens along the lower band in a downtrend. John Bollinger himself stressed that tags of the bands are not buy or sell signals: a touch of the upper band only means price is high relative to recent volatility, and a touch of the lower band only means it is low. Treating every band tag as a reversal point is how traders get run over in trending markets. The bands describe location relative to volatility, not a turning point.

What are %b and bandwidth?

%b and bandwidth are two companion indicators John Bollinger built to read the bands numerically rather than by eye. They turn the visual relationship between price and the bands into values you can compare and test.

• %b shows where price sits within the bands, equaling 1 at the upper band, 0 at the lower band, and 0.5 at the middle. It is derived from the stochastics formula and is used for pattern recognition and confirmation, for example spotting when a new price high prints with %b lower than the prior high.
• Bandwidth measures how wide the bands are, normalized by the middle band, so it can be compared over time. A low bandwidth reading is the quantified version of a squeeze, and a high reading flags an unusually volatile, stretched market.

Used together, %b locates price inside the envelope while bandwidth tracks how compressed or expanded that envelope has become, which lets traders define squeezes and band positions with numbers instead of judgment.

What are the best Bollinger Band settings?

The standard setting is a 20-period SMA with two standard deviations, written as (20, 2), and it works across daily, hourly, and intraday charts. John Bollinger recommended adjusting only in small steps if the defaults do not fit an asset.

Lengthening the average smooths the middle line and the bands, while shortening it makes them more reactive. Bollinger advised that if you lengthen the period you should widen the standard-deviation multiplier slightly, and if you shorten it you should narrow the multiplier, to keep roughly the same share of price inside the bands. The defaults suit most situations, and any change should be reviewed against the specific asset before you rely on it.

Common Bollinger Band mistakes and limitations

Bollinger Bands are widely used but easy to misread, and most mistakes come from treating a band touch as an instruction. They describe volatility and location, not direction.

1. Reading band tags as reversals. A touch of the upper or lower band is not a sell or buy signal; in trends, price walks the band.
2. Trading the squeeze early. A squeeze flags a likely volatility expansion but not its direction, so anticipating the break is guessing.
3. Ignoring the trend. Mean-reversion plays off the bands fail in strong trends, where price keeps pushing against one edge.
4. Using bands alone. Bollinger himself recommended pairing the bands with a non-correlated indicator, such as a momentum tool like the MACD, rather than relying on them in isolation.
5. Forgetting the bands lag. The middle band is a moving average of past prices, so the whole envelope reacts after a move begins.

Putting Bollinger Bands in context

Think of Bollinger Bands as a volatility frame around price, not a signal generator. They tell you whether price is stretched or calm relative to its recent behavior and when volatility is compressing or expanding, but they cannot tell you on their own which way price will go. The strongest reads come from combining the bands with the broader trend, key price levels, and a non-correlated indicator for confirmation. Used that way, Bollinger Bands become a disciplined way to read volatility rather than a shortcut to direction.