Watch price trends plus volatility.
Source: StockCharts.com. Annotations by EncyclopĂŠdia Britannica, Inc.
Bollinger Bands are a popular technical analysis tool used by stock market traders to assess price volatility and identify potential buy or sell signals. Developed in the early 1980s by John Bollinger, a renowned trader and Chartered Market Technician (CMT), Bollinger Bands provide insights into market conditions by combining the concepts of a moving average and a volatility measure in one indicator.
According to Bollinger, his namesake indicator is not just for stocks, but can also be used for futures, commodities, and currency trading. In addition, Bollinger Bands are time frame agnostic, meaning they apply to all time period charts.
Key Points
- Bollinger Bands track both market volatility and directional trends.
- The indicator consists of a simple moving average and an upper and lower band representing standard deviations from the moving average.
- Bollinger Bands can be used on any actively traded financial asset and across multiple time frames.
The mechanics of Bollinger Bands
Bollinger Bands combine a simple moving average (SMA) and a measure of price volatility via standard deviations (SD). The calculation of Bollinger Bands involves three main components:
- Simple moving average. The SMA is calculated by summing up a certain number of closing prices and dividing the result by the chosen period. For example, a 20-day SMA adds up the closing prices of the past 20 trading days and divides the total by 20.
- Standard deviation. The standard deviation measures how spread out prices are from the average. It quantifies the volatility of the price series. The standard deviation is usually calculated using the same period as the SMA.
- Upper and lower bands. Bollinger Bands are typically constructed by adding and subtracting a certain number of standard deviations (usually two) from the SMA. This creates an upper band and a lower band that envelop the price series, forming a channel that expands and contracts as volatility increases and decreases.
Hereâs the formula for calculating Bollinger Bands (at two standard deviations) based on a 20-day simple moving average:
- Upper band="20-day" SMA + (20-day SD x 2)
- Middle band="20-day" SMA
- Lower band="20-day" SMA â (20-day SD x 2)
Figure 1: TRADING BAND-AID? Shares of electric vehicle maker Tesla (TSLA) were among the most volatile stocks in the S&P 500 Index (SPX) in 2022 and 2023. Note the wide upper and lower Bollinger Bands (solid purple lines) around the moving average (dotted purple line). For educational purposes only.
Source: StockCharts.com. Annotations by EncyclopĂŠdia Britannica, Inc.
Comparing Bollinger Bands with other indicators
Some technical indicators attempt to measure volatility, like average true range (ATR), standard deviations, or the CBOE Volatility Index (VIX). Others are designed to identify trends, such as moving averages, parabolic SAR, and average directional index (ADX).
Bollinger Bands are unique in that they comprise both a moving average and standard deviations. By combining these two elements, Bollinger Bands capture both the trend (through the moving average) and volatility (through the standard deviations). This makes them particularly useful for identifying periods of high or low volatility and potential price reversals.
Because Bollinger Bands are a particularly visual technical indicator, those dynamic changes in volatility are easy to see on a price chart, especially at the extremes. Some traders incorporate these visual cues into their strategies by analyzing band widthâas well as where price is in relation to the bandsâand then combining this analysis with other indicators.
Using Bollinger Bands in a trading strategy
Bollinger Bands can help traders identify overbought and oversold conditions based on the concept of mean reversion, which suggests that, over time, an asset will tend to converge on its average or mean price.
When price is trading near the upper band, it indicates that a stock is overbought and may be due for a price correction. When the price is near the lower band, it suggests that the stock is oversold and may be ripe for a rebound.
This can provide a strategy for range-bound markets, with the trader going long when price touches the lower band and short when it touches the top band. However, when a stock is moving in a strong trend, these signals may not work as well. Trending prices can continue to âwalkâ along the upper or lower bands for an extended time.
One way traders can adapt a Bollinger Bands strategy in a trending market is by identifying the direction of the trend and then taking only corresponding trades. For example, if the trend is up, a trader would buy when price touches the bottom band, but would not short when price touches the top band. Instead, they might use the next touch of the (rising) lower band to add to their position.
Bollinger Bands can be a powerful technical tool for stock market traders. By combining a moving average with a measure of price volatility, Bollinger Bands offer insights into potential price reversals, overbought or oversold conditions, and market volatility.
Remember that no single indicator can guarantee successful trading outcomes. Traders should always exercise caution and conduct thorough analysis before making investment decisions.
This article is intended for educational purposes only and not as an endorsement of a particular financial strategy. EncyclopĂŠdia Britannica, Inc., does not provide legal, tax, or investment advice.
This article was most recently revised and updated by Brian LundThis article was most recently revised and updated by Karl Montevirgen
