What are Bollinger Bands?

What a touch of the band means

The most repeated rule attached to this indicator says that price touching the upper band is overbought and touching the lower band is oversold. It is wrong often enough to be expensive.

Two standard deviations would hold roughly 95 percent of observations if prices followed a normal distribution. They do not. On real markets with the default settings, closer to 88 percent stays inside, so the band is touched about twice as often as the textbook figure suggests. It is still uncommon, which is the whole point of the line. But unusual is not the same as unsustainable: in a strong trend, price walks along the upper band for candle after candle, and each of those candles is another instance of the rule failing.

The honest reading of a band touch is that price is at the edge of its recent distribution. Whether that means exhaustion or strength is decided by what the market is doing, not by the band. In a range, band touches often mark the turns. In a trend, walking the band is the trend.

The squeeze, and the thing it does not tell you

When volatility falls, the bands narrow. A pronounced narrowing is called a squeeze, and it is the most useful observation the indicator produces, because quiet periods do tend to be followed by active ones. Pronounced compared to what is a question the eye answers badly. The reading that answers it properly is bandwidth: the gap between the bands divided by the middle one, read against its own range over the last hundred and twenty five periods.

Here is the limit, and it is absolute. The squeeze says nothing about direction. It says the market has been quiet and that quiet does not last. A trader who reads a squeeze as a signal to buy has added a direction the indicator never supplied.

The first move out of a squeeze is also frequently a false one, precisely because so many traders are positioned around a tight range with stops just outside it. Where those stops sit is a liquidity question, and it explains the fake break better than the bands do.

The middle band is the one people forget

The centre line is an ordinary twenty period moving average, and it carries the information most traders skip past on the way to the outer bands.

Its slope is the trend of the window being measured. Flat middle band means the outer bands are describing a range, and the touch rules have their best chance of working. Steeply sloped middle band means the market has direction, and the band touches will keep failing as reversal signals.

Read that way, the middle band tells you which rule applies, which is more useful than either rule alone. The guide on moving averages covers what the slope of that line does and does not say.

What the settings actually change

Twenty periods and two deviations is the default and the one almost everyone is looking at. Changing them changes the question:

• A shorter period makes the bands hug price and react faster, and it gives fewer touches, not more: the average sits closer to price, so two deviations contain more of it. Bollinger's own advice is to lower the deviation setting when you shorten the period, and raise it when you lengthen it.
• A wider deviation setting, two and a half or three, makes touches rarer and more meaningful, at the cost of far fewer of them.
• Nothing you do to the settings adds information. The bands are still a description of recent dispersion around a recent average.

The mistakes that cost the most

Fading every band touch. In a trend this is a way of betting against the trend repeatedly.

Reading the squeeze as bullish. A squeeze is directionless. Anything else is imported from somewhere.

Treating the bands as targets. They move with every candle, so a target on the band is a target that runs away from you.

Adding them to an already crowded chart. Bands, an average and a momentum indicator that all read recent price will agree most of the time, and the agreement feels like confirmation when it is repetition.

Questions people ask

What does it mean when price touches the upper Bollinger Band?

That price is at the top edge of its recent distribution. In a range that often marks a turn. In a trend price can walk the band for a long time, and each candle along it is another failure of the reversal reading.

What is a Bollinger Band squeeze?

A period when the bands narrow sharply because volatility has fallen. It suggests an expansion is likely to follow. It gives no indication at all of which way.

What are the best Bollinger Band settings?

20 periods and 2 standard deviations, for the same reason the defaults matter on every indicator: it is what most traders are looking at. Wider deviations give rarer, more meaningful touches; shorter periods give more, noisier ones.

Are Bollinger Bands better than the RSI?

They answer different questions. The bands measure dispersion around an average, the RSI compares recent gains to recent losses. Running both mainly produces two versions of the same observation about recent price.

Reading volatility in context

Trading AI reads a photo or a screenshot of any chart and returns the indicator readings visible in the image, alongside the market structure, the key levels and a full trade plan with its risk. The analysis works from the image, so bands that are not drawn on the chart cannot be read.

The line at the centre of the bands, and what its slope means: What is a moving average?

Why a break out of a squeeze so often runs the wrong way first: What is liquidity in trading?

The bounded momentum reading these bands are most often paired with: What is the RSI?

The same narrowing drawn with two lines instead of arithmetic: What is a triangle pattern?

This page is educational. It is not financial advice, and nothing here is a recommendation to buy or sell anything.