Bollinger Bands

What Bollinger Bands mean

Bollinger Bands are a chart overlay created by the analyst John Bollinger in the 1980s. They consist of three lines drawn on the price chart. The middle line is a moving average. The upper and lower lines sit a fixed number of standard deviations above and below it. Standard deviation is a statistical measure of how widely values are spread around their average, so it works here as a measure of volatility.

The bands adapt to market conditions. When prices swing widely, standard deviation rises and the bands move apart. When the market is quiet, the bands tighten. This is the main difference from fixed envelopes that sit a set percentage above and below an average. Because the bands expand and contract, many traders use them as a visual gauge of how volatile an instrument is relative to its recent behaviour.

Traders apply Bollinger Bands to currency pairs, shares, indices, commodities and crypto charts, on any timeframe. They are a descriptive tool. They do not predict where price will go next, and they are not a trading signal on their own.

How it works

The standard settings are a 20-period simple moving average and 2 standard deviations. The calculation runs in four steps.

1. Compute the 20-period simple moving average of closing prices. This is the middle band.
2. Compute the standard deviation of the same 20 closing prices.
3. Upper band = middle band + 2 × standard deviation.
4. Lower band = middle band − 2 × standard deviation.

With a bell-shaped distribution, about 95 percent of values would fall within two standard deviations of the mean. Prices are not that tidy, so touches of the outer bands are not rare and price can travel along a band for a long time in a strong trend.

Two derived readings are common. Bandwidth is the distance between upper and lower band, often divided by the middle band, and it shows squeezes and expansions. Percent B shows where price sits within the bands, with 0 at the lower band, 1 at the upper band and 0.5 at the middle.

A worked example

These numbers are illustrative. For simplicity, use a 5-period version on a daily share chart with closes of 100, 102, 101, 103 and 104.

• Average = (100 + 102 + 101 + 103 + 104) ÷ 5 = 510 ÷ 5 = 102.
• Deviations from the average are −2, 0, −1, 1 and 2.
• Squared deviations are 4, 0, 1, 1 and 4, which sum to 10.
• Variance = 10 ÷ 5 = 2, and standard deviation = square root of 2 = about 1.41.
• Upper band = 102 + 2 × 1.41 = 104.82. Lower band = 102 − 2.82 = 99.18.
• Bandwidth = (104.82 − 99.18) ÷ 102 = 5.64 ÷ 102 = about 5.5 percent.

If the next close is 106, it sits above the upper band. A trader would note that price is stretched relative to its recent range. That might precede a pause, or it might be the start of a strong trend. The bands alone cannot tell which.

Why it matters

Bollinger Bands give context. They show when volatility is compressed or expanded, and how far price has moved relative to its recent average. Squeezes, where bandwidth reaches a low for the recent period, often precede a larger move, though not its direction. Band walks, where price repeatedly closes near the upper or lower band, can mark persistent trends.

They also help with risk planning. In a wide-band environment, tight stops are likelier to be hit by noise, and spreads may be wider. In a narrow-band environment, breakouts can be sudden. Combined with tools like ATR, RSI or support and resistance, the bands can help structure analysis.

How to use it in practice

• Begin with the 20 and 2 defaults and see how price has behaved around the bands on your instrument.
• Note bandwidth lows as squeezes, and wait for confirmation rather than guessing direction.
• In range-bound markets, traders watch for reversion toward the middle band. In trends, they watch for band walks instead. Identify which regime you are in first.
• Combine with a momentum tool to judge whether a band touch has strength behind it.
• Use a longer lookback or wider multiple on volatile assets to reduce false touches.
• Test settings on historical data, while allowing for spread and slippage.

Common mistakes and misconceptions

• Treating a touch of the upper band as a sell signal and the lower as a buy signal. In strong trends, price can stay at a band for many periods.
• Believing 95 percent of prices must stay inside the bands. Market returns have fat tails.
• Ignoring the regime. A mean-reversion approach fails in a trend, and a breakout approach fails in a range.
• Expecting a squeeze to show direction. It shows low volatility only.
• Using the default on every timeframe without checking it.
• Forgetting the bands are built from past closes and therefore lag.

Risks and limitations

Bollinger Bands are descriptive and not a trading signal. They are based on historical data, so they lag, and sudden news can send price far outside any band. Different settings give different pictures, which allows hindsight bias. False signals are common in sideways markets. Using them on leveraged CFDs can magnify losses, and rules and available indicators differ between platforms.

How Brokeli looks at it

Bollinger Bands are a trader's tool rather than a broker feature, so our interest is in the platform. We check whether charting packages include standard indicators with adjustable parameters, and we compare spreads and commissions that apply when volatility widens, based on published fee schedules and the research snapshot.