Bollinger Bands are a volatility indicator consisting of three lines plotted around price: a middle simple moving average and two outer bands set a fixed number of standard deviations away from it. John Bollinger developed the indicator in the early 1980s, and the default settings — a 20-period simple moving average with bands placed two standard deviations above and below it — remain the standard on nearly every charting platform today. The bands widen when volatility increases and narrow when volatility decreases, which makes Bollinger Bands one of the few indicators that adapts its own shape to changing market conditions rather than staying fixed. Traders use this behavior to spot low-volatility periods that often precede sharp moves, and to gauge whether a current price level is statistically stretched relative to its recent average.

Unlike RSI or MACD, which measure momentum, Bollinger Bands measure dispersion — how far price has moved from its own average. This distinction matters because a price near the upper band is not automatically overbought in the way an RSI reading above 70 is conventionally interpreted; it may simply reflect a strong trend where price consistently trades near the outer band for extended periods. Understanding this difference is the starting point for using Bollinger Bands correctly rather than misreading them as a simple overbought/oversold oscillator.

The indicator uses three formulas applied to a rolling window of closing prices, typically 20 periods.

1. Middle band = 20-period simple moving average (SMA) of the closing price.

2. Upper band = Middle band + (2 × 20-period standard deviation of price).

3. Lower band = Middle band − (2 × 20-period standard deviation of price).

Standard deviation measures how much individual closing prices vary from the 20-period average. A larger standard deviation means prices have been more dispersed recently, which pushes the bands wider apart. A smaller standard deviation means prices have clustered tightly around the average, which pulls the bands closer together. A numerical example clarifies this: if a stock's 20-period SMA sits at $100 and its standard deviation over that window is $2, the upper band sits at $104 and the lower band sits at $96. If volatility later drops and the standard deviation falls to $0.80, the bands tighten to $101.60 and $98.40 around the same $100 average, visually compressing even though the average price has not moved.

Trading platforms recalculate all three lines automatically with every new price bar, so a trader never performs this math manually. Understanding the formula still explains an important property of the indicator: because the bands are built from a statistical measure of dispersion rather than a fixed price distance, they automatically widen during volatile news-driven periods and automatically narrow during quiet, directionless conditions, adapting to the market without requiring a trader to manually adjust the settings.

Two calculated values extend the basic three-line indicator and give traders more precise ways to quantify what the bands are showing.

Bandwidth quantifies the squeeze directly, letting a trader compare current band width to its own historical range rather than relying on a visual read of the chart. A Bandwidth reading at its lowest level in the past six months signals an unusually tight squeeze, often flagged as a setup worth watching for a breakout. The %B metric translates price's position into a single number: %B equals 0 when price touches the lower band, 0.5 when price sits exactly on the middle band, and 1.0 when price touches the upper band, giving a precise numerical alternative to eyeballing where price sits relative to the bands.

This strategy treats a period of contracting bands as the setup and a decisive move outside the bands as the trigger. A squeeze occurs when Bandwidth reaches a multi-month low, indicating that volatility has compressed to an unusual degree and often precedes an expansion. Traders wait for a candle to close above the upper band, accompanied by rising volume, to confirm a bullish breakout, or a close below the lower band with rising volume to confirm a bearish breakout.

Volume confirmation matters because a squeeze can resolve as a false breakout — a brief poke outside the band that quickly reverses back into the range. Requiring volume at or above 150% of the recent average on the breakout candle filters out many of these false signals, since a genuine expansion in volatility is typically accompanied by increased participation from market participants. A stop-loss placed near the opposite band, or just beyond the most recent swing low or high, gives the trade a defined risk level from the outset.

A practical example illustrates the setup: suppose a currency pair trades in a tight $50-pip range for two weeks, with Bandwidth sitting at its lowest level in six months. Price then closes 40 pips above the upper band on a volume spike well above the recent average, confirming a bullish breakout. A trader entering on that close might place a stop-loss near the middle band, roughly 90 pips below entry, targeting a move equal to at least the width of the prior squeeze range as a first profit objective.

In a strong trend, price does not simply touch the outer band once and reverse — it can "walk" along the band, repeatedly touching or slightly exceeding it for many consecutive periods while the trend remains intact. This behavior is precisely why touching the upper band should not be treated as an automatic sell signal: in a strong uptrend, price walking the upper band actually confirms the strength of that trend rather than signaling exhaustion. Traders following this strategy hold a position as long as price continues walking the band and the middle band continues sloping in the trend's direction, exiting only when price closes back inside the bands or the middle band flattens.

This strategy applies specifically to range-bound markets rather than trending ones, treating the outer bands as statistical extremes that price tends to revert from. A trader enters a long position when price touches or closes below the lower band while the middle band is relatively flat, targeting a return toward the middle band or the upper band. A short entry follows the mirror pattern at the upper band. This approach carries higher risk in a trending market, since price walking the band in a strong trend will keep triggering entries against the dominant direction, producing a string of losses until the trend exhausts itself.

These are two-part reversal patterns identified using the bands rather than price alone. A W-bottom forms when price makes a low outside the lower band, bounces, pulls back to a second low that stays inside the band, then breaks above a prior minor high — the second low holding inside the band signals weakening selling pressure compared to the first low. An M-top forms the mirror pattern at the upper band, with a second high staying inside the band signaling weakening buying pressure. Both patterns function as an early warning of a potential reversal, similar in spirit to divergence signals on RSI or MACD, and traders typically wait for the confirming breakout of the interim high or low before entering.

Bollinger Bands measure volatility and relative price position but say nothing directly about momentum or trend direction, which is why combining them with a second tool improves signal quality. Pairing a squeeze breakout with RSI adds a momentum filter: an RSI reading above 55 at the moment of an upside breakout favors taking the long side, while an RSI reading below 45 at a downside breakout favors the short side. This combination reduces the number of breakouts taken in the wrong direction relative to underlying momentum.

Traders applying Bollinger Bands across multiple markets often look for the same squeeze pattern regardless of instrument. Someone monitoring FxPro's forex trading pairs, for example, might scan several currency pairs for the tightest recent Bandwidth reading before focusing attention on the one closest to a genuine breakout, rather than watching every pair with equal attention. The same squeeze logic applies just as well to index markets; a trader tracking FxPro's indices might wait for a squeeze on a major index ahead of a scheduled economic release, since compressed volatility ahead of a known catalyst often resolves into a sharp directional move once the data is published.

The default 20-period, 2 standard deviation setting works well for daily charts and swing trading, but other combinations suit different trading styles and timeframes.

Shortening the period makes the bands more responsive to recent price changes, generating more frequent squeeze and breakout signals but also more false ones during choppy conditions. Widening the standard deviation multiplier makes the outer bands harder to reach, which reduces the number of touches but increases the significance of the touches that do occur. Testing a specific combination of period and standard deviation against historical price data for the instrument in question remains the most reliable way to validate the settings before trading them live.

The right setting also depends on the specific instrument's typical volatility profile rather than the timeframe alone. A commodity or currency pair prone to sharp, sudden price spikes may benefit from a wider standard deviation multiplier, such as 2.5, to avoid triggering breakout signals on every minor volatility spike. A comparatively stable large-cap index, by contrast, may work well with the standard 2.0 multiplier, since its price moves more gradually and genuine breakouts are less frequently masked by noise.

Traders frequently misread Bollinger Bands in ways that lead to avoidable losses.

• Treating every touch of the upper or lower band as an automatic reversal signal, without checking whether the market is trending or range-bound.

• Entering a squeeze breakout without waiting for volume confirmation, resulting in frequent false breakout losses.

• Applying a mean reversion strategy during a strong trend, fighting a market that keeps walking the band in one direction.

• Ignoring the middle band's slope, which provides a simple visual cue for the prevailing trend direction.

• Using Bollinger Bands in isolation without a defined stop-loss or a second confirming indicator.

Bollinger Bands translate price volatility into a visual, adaptive range that widens and narrows with market conditions, giving traders a way to spot compressed volatility before a breakout and to gauge whether a current price move is stretched relative to its recent average. The squeeze breakout, walking the bands, mean reversion, and W-bottom/M-top patterns each interpret the same underlying volatility data from a different angle, and recognizing which market condition — trending or range-bound — applies at a given moment determines which of these approaches fits. Bollinger Bands work best when paired with a momentum indicator like RSI or MACD and a clearly defined risk management plan, rather than as a standalone trigger for every band touch or squeeze.