Bollinger Bands indicator: settings and signals
Bollinger Bands show how far the current price has deviated from its moving average, measured in units of statistical dispersion.
What the indicator looks like on a real chart
Data: EUR/USD, daily bars, 2025-06-16 — 2026-07-20. The indicator is computed over 400 bars; the chart shows the last 180. Quote source: Binance Spot REST API (api.binance.com/api/v3/klines).
| Regime | Score |
|---|---|
| Trending market | 33 |
| Range | 71 |
| High volatility | 44 |
| Low volatility | 52 |
What the indicator actually measures
The construction is simple: the central line is an ordinary SMA, and the bands sit two standard deviations of price away from it over the same period. The key point is that the standard deviation is recalculated on every bar, so the channel width is not a setting but a measurement: it expands on a volatile market and contracts on a calm one on its own.
John Bollinger originally objected to reading the bands as overbought and oversold levels. A band only means that price is in a statistically unusual position relative to its recent average. In a strong trend such a position is the norm, and price can travel along the upper band for a long time, which is clearly visible on the chart.
The most meaningful signal comes not from a band touch but from a squeeze of the channel. Periods of low volatility tend to give way to high volatility, so a narrowed channel often precedes a sharp move — though without indicating the direction.
Formula
Two standard deviations are chosen because for a normal distribution roughly 95% of values fall within that interval. Real prices are not normally distributed and have heavier tails, so breaches of the band happen more often than statistics predict.
Standard settings
| Parameter | Value |
|---|---|
| Moving-average period | 20 |
| σ multiplier | 2.0 |
| Multiplier for ranges | 1.5 |
Period 20 and a multiplier of 2 were proposed by the author himself for daily bars. Shortening the period makes the channel jumpy and price breaches the bands almost constantly; lengthening it smooths the channel and it stops reacting to changes in regime. A multiplier of 2.5 is used on volatile instruments to reduce the number of false breaches.
Implementation code (Python)
def bollinger_bands(close, period=20, mult=2.0):mid = close.rolling(period).mean()std = close.rolling(period).std()return mid + mult * std, mid, mid - mult * std
When the indicator stops working
- In a trending market price can travel along a band without returning to the average — trading the bands against the trend produces a string of losses.
- Channel width lags: the standard deviation reflects volatility that has already happened, not future volatility.
- On instruments with frequent gaps the deviation calculation is distorted by discontinuities that the continuous model does not assume.
Common mistakes in use
- Treating a touch of the upper band as a sell signal. This is the most common and most expensive misconception about Bollinger — the author of the indicator warned against it specifically.
- Ignoring the central line. A return to the SMA is a more meaningful event than a band touch, and it more often serves as the target for a pullback.
- Trading a channel squeeze in a particular direction. A squeeze predicts rising volatility but says nothing about the direction of the coming move.
- Using identical settings for instruments with fundamentally different volatility while expecting identical indicator behavior.
Who needs this and why
A tool for those who assess context rather than look for ready-made entry points: band width answers the question of market regime, and the position of price inside the channel answers how unusual the current move is.
Frequently asked questions
Why does price travel along the upper band for a long time without reversing?
Because the band shows statistical unusualness, not a limit. In a sustained trend every new move up raises both the average and the dispersion, so the band rises together with price. A prolonged move along the band is a sign of trend strength, not of its exhaustion.
What does a strong narrowing of the channel mean?
That volatility has dropped to an unusually low level. Quiet periods statistically give way to active ones, so a narrowing often precedes an impulse. The indicator does not show the direction of that impulse — a separate tool is needed for that.
How do Bollinger Bands differ from Keltner Channels?
In how they measure width. Bollinger uses the standard deviation of closing prices, Keltner uses the average true range. Because of this Bollinger reacts more sharply to spikes in individual bars, while Keltner gives smoother bands. They are often overlaid: Bollinger Bands moving outside the Keltner Channel is considered a sign that a strong move is beginning.