Bollinger Bands are a volatility envelope drawn around a 20-period simple moving average, with an upper and lower band set two standard deviations away. They widen when price swings grow and narrow when the market goes quiet. Traders use them to spot squeezes before breakouts, trend strength during band walks, and stretched moves in ranges.
John Bollinger designed the bands in the 1980s to answer a simple question: is price high or low relative to its own recent behaviour? A fixed percentage envelope can’t do that, because a 2% move is huge for one market and noise for another. Standard deviation adapts automatically. Like every indicator, the bands summarise price — they don’t add new information — but they make volatility visible at a glance.
How Bollinger Bands are calculated
Three lines, all based on closing prices over N periods (default N = 20):
- Middle band = 20-period simple moving average (SMA)
- Upper band = middle band + 2 × σ
- Lower band = middle band − 2 × σ
σ is the standard deviation of the same 20 closes: take each close’s distance from the SMA, square it, average the squares, and take the square root. Bollinger uses the population version (divide by N, not N − 1).
Two derived readings are worth knowing:
- %B = (close − lower band) ÷ (upper band − lower band). 1.0 means price sits on the upper band, 0 on the lower band, 0.5 on the middle.
- Bandwidth = (upper band − lower band) ÷ middle band. This is the number you watch for a squeeze.
Worked example
Suppose the last 20 daily closes of a stock average $100.00 and their standard deviation is $2.50.
- Middle band = $100.00
- Upper band = 100 + 2 × 2.50 = $105.00
- Lower band = 100 − 2 × 2.50 = $95.00
- Bandwidth = (105 − 95) ÷ 100 = 10%
Today the stock closes at $103.00. %B = (103 − 95) ÷ (105 − 95) = 0.80 — in the upper part of the envelope but not stretched.
Now imagine a quiet month where σ falls to $0.80. The bands become $101.60 and $98.40, and bandwidth drops to 3.2%. If that is the lowest bandwidth in six months, you are looking at a squeeze.
Statistically, about 95% of closes would fall inside ±2σ if prices were normally distributed. They aren’t — markets have fat tails — so closes outside the bands are more common than the textbook suggests. Treat the bands as a reference for “unusual”, not as hard limits.
The squeeze: low volatility before expansion
Volatility moves in cycles. Calm periods are followed by violent ones, and vice versa. The squeeze captures the calm phase: bandwidth contracts to a multi-month low while price coils in a tight range, often forming a symmetrical triangle or a bull flag.
What the squeeze tells you:
- A large move is becoming more likely. Bandwidth rarely stays at extremes for long.
- Nothing about direction. The first push out of a squeeze is frequently a fake-out — the “head fake” Bollinger himself warned about.
How to handle direction: wait for a close outside the band, check that volume expands on that candle, and prefer the side that agrees with the higher-timeframe trend. A breakout that immediately closes back inside the bands is a classic false breakout.
Band walks: when touching the band is strength
Beginners often read the upper band as “overbought, sell”. In trends, that is exactly backwards. During a strong advance, price can close on or near the upper band candle after candle — a band walk — while the middle band slopes up underneath.
Signs of a healthy band walk:
- Closes cluster between the middle and upper band (%B mostly above 0.5).
- Pullbacks stop at or near the middle band (the 20 SMA) rather than the lower band.
- Bandwidth is expanding, not shrinking.
Signs it is ending:
- A close below the middle band after a long walk.
- A new price high that prints inside the upper band (%B lower than at the previous high) — a form of divergence.
- Bandwidth starts contracting while price still pushes.
Mean reversion vs breakout: which regime are you in?
The same touch of the lower band can be a buying opportunity or the start of a collapse. The deciding factor is regime.
| Regime | What the bands look like | How touches behave | Typical approach |
|---|---|---|---|
| Range | Flat middle band, steady bandwidth | Outer-band touches revert toward the middle | Fade the extremes, target the middle band |
| Squeeze | Bandwidth at multi-month low | Unreliable until a close outside | Wait for a confirmed close + volume |
| Trend | Sloped middle band, expanding bandwidth | Price walks one band | Trade pullbacks to the middle band |
A quick test: if the middle band is flat and price has crossed it several times in the last 20 candles, you are probably in a range. If it is clearly sloped and price hasn’t crossed it in weeks, you are in a trend, and fading the outer band is fighting the market.
Settings cheat sheet
| Use case | Period | Std dev | Notes |
|---|---|---|---|
| Default (most charts) | 20 | 2.0 | The shared reference most traders see |
| Shorter-term, more responsive | 10 | 1.9 | Bollinger’s suggested pairing for shorter periods |
| Longer-term trend context | 50 | 2.1 | Wider period needs slightly wider bands |
| Squeeze detection | 20 | 2.0 | Watch bandwidth at a 6-month low |
| Mean reversion in ranges | 20 | 2.0–2.5 | Wider bands reduce premature fades |
Period counts are candles, not days, so the same settings work across timeframes. What changes is how much each signal is worth: a squeeze on the daily chart matters more than one on a 5-minute chart.
How to combine Bollinger Bands with other tools
Bollinger Bands measure where price sits relative to its recent volatility. They work best paired with tools that measure something else.
With RSI. The bands say price is stretched; RSI says whether momentum agrees. In a range, a lower-band touch with RSI below 30 that turns up is a stronger mean-reversion setup than the touch alone. In a trend, an upper-band walk with RSI holding 60–80 confirms strength. A new high at the band with a lower RSI high is a warning. The RSI, MACD, ATR and EMA guide covers the momentum side in detail.
With volume. A close outside the bands on two to three times average volume after a squeeze is a far better breakout signal than the same close on light volume. See the volume analysis guide for how to measure that.
With levels. Bands move every candle; support and resistance don’t. Fading the lower band works best when it coincides with a known support level, and your stop belongs beyond that level, not beyond the band.
With ATR for stops. The bands are not a stop-loss tool. Use ATR to add a buffer beyond structure, then size the trade with the position size calculator.
A combined example
A stock has ranged between $48 and $54 for two months. The 20 SMA is flat at $51; the lower band sits at $48.20. Price drops to $48.30, near range support at $48, while RSI prints 28 and then turns up to 34. Volume on the drop is below average — sellers aren’t committing.
Plan: entry $48.60, stop below support at $47.40 (support minus 0.5 × an ATR of $1.20), target the middle band at $51. Risk $1.20, reward $2.40, R:R = 2:1. Invalidation: a daily close below $47.40, or a high-volume close below the lower band, which would suggest the range is breaking rather than holding.
Common mistakes
- Selling every upper-band touch. In trends, the band is where strength lives.
- Assuming a squeeze tells you direction. It only tells you that a move is coming. Wait for the close.
- Ignoring the head fake. The first break out of a squeeze often reverses. Confirmation with volume and a close matters.
- Treating ±2σ as a hard boundary. Fat tails mean price can close outside the bands repeatedly in a strong move.
- Tuning settings to the last move. A 17-period, 1.8σ band that “fit” last month is curve-fitting.
- Using the bands alone. They describe volatility, not direction, momentum or levels.
Read it automatically with SnapPulse
SnapPulse turns a photo or screenshot of a candlestick chart into a structured read in about five seconds: the detected pattern with a directional bias and confidence, key support and resistance levels, RSI, MACD, ATR and EMA explained in plain English, and a plan with entry zone, stop, target and risk/reward. A Steelman card lays out the opposite case. It’s a useful second opinion to set beside your own Bollinger Band read. You can download SnapPulse and try it on charts from TradingView, MT4/MT5 or your broker app.
Educational content — not financial advice.
