Bollinger Bands Explained

By Laurel Tincher · February 22, 2022 · 7 minute read

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Bollinger Bands Explained

What Are Bollinger Bands?

Bollinger Bands® are popular tools used in technical analysis of securities. They are a set of three bands that measure the relative high or low of a security’s price in relation to previous trades.

These defined trend lines are the simple moving average (SMA) of the price of the security plus plotted lines two standard deviations away from the SMA. The bands are plotted positively and negatively from the SMA, which measures the volatility of a security, and the trader can adjust them based on their particular use case. When the security becomes more volatile the bands widen, when it becomes less volatile they get closer together.

Bollinger Bands were created to help investors understand whether a security is currently oversold or overbought, to help determine whether it is likely to increase or decrease in value over time. When the upper band is close to the SMA, generally traders see this as an overbought security. When the lower band is close to the SMA, the security is considered to be oversold.

The bands and a set of 22 rules about using them for trading were developed in the 1980s by John Bollinger, a well-known technical trader.

How Do Bollinger Bands Work?

Bollinger Bands are plotted using two parameters, period and standard deviation.

Period is found by calculating the simple moving average of the security a trader is interested in. The calculation generally uses a 20-day SMA, an average of a security’s closing prices over a 20 day period — or roughly a month of trading days. The first data point on the graph would be the average of the first 20 days being tracked. The second data point would be the next 20 days, and so on.

That line shows the SMA over time, and the Bollinger Bands are then placed above and below it by calculating the standard deviation of the security’s price along each data point. The standard deviation is a calculation of the average variance of the SMA value, which shows how far apart values are from the SMA.

The standard deviation is calculated by first finding the square root of the variance, which is the average of the squared differences of the mean. After finding the square root of the variance, that number is generally multiplied by two, although the number can be adjusted. The resulting value is then added and subtracted from each SMA data point to form the upper and lower Bollinger Bands.

Key Things to Know About Bollinger Bands

A few key things to understand about Bollinger Bands:

•   When volatility is low, the bands get closer together. This indicates that volatility may increase in the future and therefore there could be a significant price movement up or down.

•   When volatility is high, the bands get farther apart. This indicates that an existing price trend could be coming to a close in the future.

•   Generally the security’s price movements stay within the two bands. And once they touch one band they start moving towards the other band. But the price can also bounce off the band multiple times or it can cross over the band. If the price hits one of the bands and then crosses over the SMA line, that is an indicator that it is heading toward the opposite band’s price level.

When the price crosses to the outside of the bands, this is a strong indicator of a trend in that direction.

Formula for Bollinger Bands

Below is the formula to plot Bollinger Bands:




BOLU=Upper Bollinger Band

BOLD=Lower Bollinger Band

MA=Moving average

TP (typical price)=(High+Low+Close)÷3

n=Number of days in smoothing period (typically 20)

m=Number of standard deviations (typically 2)

σ[TP,n]=Standard Deviation over last n periods of TP

How Do You Read Bollinger Bands?

Bollinger Bands help traders understand whether a security’s price is relatively high or low so that they might make trades based on trends. Bollinger Bands can indicate uptrends and downtrends as well as possible upcoming price reversals.

Trends can last for minutes, hours, days, weeks, months, and even years, so traders should understand how to set up the bands based on the timeline of their trading strategy. Here are some patterns and indicators traders might want to learn.


Traders can use Bollinger Bands to see whether there is a bullish trend in a security’s market price. If the center line hits the upper band multiple times, this indicates an uptrend. If the price hits the upper band, decreases but stays above the center line, then hits the upper band again, that is a strong indicator of an uptrend. If the price then hits the lower band, it may indicate a reversal or a loss of strength in the uptrend.


The lower band can indicate a downtrend or an upcoming reversal towards an uptrend. If the price hits the lower band continuously and stays below the center line, this indicates a downtrend. Traders typically avoid making trades during downtrends, but if there is an indicator of a reversal they might choose to buy.

The Squeeze

When the bands are close together, this is known as a squeeze. The squeeze happens when the security has low volatility, but it indicates that the security will probably have increased volatility in the future. Traders look for high volatility periods to find trading opportunities, so the squeeze indicates that those opportunities may be showing up soon.

Traders typically like to exit trades during periods of lower volatility, so they look for far-apart bands as a clue that volatility may soon decrease. The squeeze is not used as a trading signal and doesn’t show whether a security will increase or decrease in value, but it may help traders figure out the potential timing of upcoming trades.


The SMA line doesn’t always stay between the Bollinger Bands — it can also move above or below the bands. Around 90% of price changes do happen between the bands, so if the price has a breakout above or below the bands it’s a significant event. However, breakouts are not used as trading signals and are not indicators that the security price will move in a particular direction in the future.

Bollinger Band Trading Strategies

Financial analyst Arthur Merrill, who identified a set of 16 trend patterns that have M patterns and W patterns. Here are two key patterns.

M Top

The M top pattern indicates that the security price may decrease to a new low. It forms an M pattern at the upper band, where the price nearly hits or hits the upper band but doesn’t cross over it, then decreases to below the low in the center of the M pattern.

W Bottoms

W patterns can be used to identify W Bottoms, which is when the second low is lower than the first low but neither low goes below the lower band. If the security rises above the high in the center of the W, this is an indicator that the price will likely reach a new high.

Combining Bollinger Bands With Other Indicators

John Bollinger recommended that traders use Bollinger Bands in conjunction with other non-correlated indicators, such as the relative strength indicator (RSI) and the Stochastic Oscillator, in order to gain a comprehensive understanding of the security being assessed. While Bollinger Bands help traders understand price volatility and can show opportunities for upcoming trades, they aren’t strong indicators of potential upcoming price movements.

Drawbacks of Bollinger Bands

There are a number of caveats to consider when it comes to Bollinger Bands. In particular, they are best used with other stock indicators, to form a fuller picture.

•   They show old security price data with equal importance to new data, so data that is outdated may be counted with too much importance.

•   They are more of reactive indicators than predictive indicators, so they show current market conditions and can indicate trends, but are not strong indicators of what will happen to a security’s price in the future.

•   The standard settings of 20-day SMA and 2 standard deviations is an arbitrary measurement that doesn’t convey relevant information for every security and trading situation, so it’s important that traders understand how to adjust the band calculations for their particular situation.

Using Bollinger Bands for Crypto Trading

Bollinger Bands have become a popular tool for crypto traders to track volatility and trends. They can be used for trading crypto in a similar way to stocks, but some traders choose to use a 28 or 30 SMA instead of 20, to better represent a month of trading days, since the crypto markets are open 24/7.

The Takeaway

Bollinger Bands are a useful tool for technical analysis of stocks, which measure the relative high or low of a security’s price in relation to previous trades over typically the past 20 trading days. One of many trend indicators, Bollinger Bands are also sometimes used in crypto trading.

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Photo credit: iStock/blackCAT

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