Guide · Indicators
What are Bollinger Bands?
The short answer
Bollinger Bands are a volatility envelope: a middle band that is a moving average, usually a 20-period simple moving average, with an upper and lower band placed a set number of standard deviations of price above and below it, by default two. Because standard deviation measures how far recent prices have strayed from their average, the bands widen when volatility rises and contract when it falls. They tell you how stretched or calm price is relative to its own recent behaviour, and, above all, how volatile it is now. They do not tell you which way price goes next, and a touch of a band is not a buy or a sell.
John Bollinger devised the bands in the 1980s, and the single most useful thing to understand about them is what they are made of: standard deviation. That one ingredient is why the bands breathe, why a "squeeze" can precede a big move, and why the reflex to sell the upper band and buy the lower one gets traders run over in a trend. This guide builds the construction exactly, introduces the two companion readings, %B and bandwidth, then spends most of its length on the parts most explanations skip: what the squeeze does and does not tell you, and why price "walks the band" instead of turning at it.
The construction: an average, plus and minus its own volatility
Bollinger Bands are three lines, and each one is defined by a short formula. The middle band is a simple moving average of closing prices over N periods, where N is 20 by default. The upper and lower bands are that same average shifted up and down by K times the standard deviation of price over the identical N periods, where K is 2 by default. Standard deviation is the statistical yardstick of dispersion: when recent closes are scattered far from the average it is large, and when they cluster tightly it is small. That is the whole mechanism. The envelope is not drawn at a fixed rupee distance; it is drawn at a distance proportional to how volatile the instrument currently is, so it inflates and deflates on its own.
| Component | Formula (default N = 20, K = 2) | What it shows |
|---|---|---|
| Middle band | 20-period simple moving average of close | The trend baseline, the centre of the envelope |
| Upper band | Middle band + 2 × standard deviation of price | Two standard deviations above the average |
| Lower band | Middle band − 2 × standard deviation of price | Two standard deviations below the average |
| %B | (Price − lower band) ÷ (upper band − lower band) | Where price sits within the bands: 1 at the top, 0 at the bottom, 0.5 at the middle |
| Bandwidth | (Upper band − lower band) ÷ middle band | The normalised width of the envelope, so volatility as one number |
Why "two standard deviations", and the honest caveat
The choice of two standard deviations is not arbitrary, and it is not a law either. For a variable that follows a normal, bell-shaped distribution, about 95 percent of observations fall within two standard deviations of the mean. Carry that statistic across to price and you get the familiar claim that price spends most of its time inside the bands, so a move beyond them is unusual and worth noticing. That is the intuition the default encodes.
The caveat is that price is not normally distributed. Real returns have fat tails, meaning extreme moves happen far more often than a bell curve predicts, and they cluster: calm begets calm and turbulence begets turbulence. So the clean 95 percent does not hold in practice. On the standard 20 and 2 setting, published descriptions of the indicator note that the bands tend to contain closer to 88 to 89 percent of price action, not 95. That gap is not a flaw to fix; it is the honest character of markets. Read the containment figure as a rule of thumb that makes an out-of-band move significant, never as a probability you can bank on. Anyone quoting a precise hit rate off the bands is over-claiming what a standard-deviation envelope can deliver.
A statistical envelope, not a probability machine. The bands import the language of the normal distribution, standard deviations and 95 percent, onto data that violates its assumptions. That is useful as a relative gauge of how stretched price is, and misleading if taken as a literal forecast of how often price "should" escape. The value is in the relative reading, not in the percentage.
What the bands show: relative price, and above all width
Read correctly, the bands answer two different questions. The first is relative position: price near the upper band is high relative to its recent range, price near the lower band is low relative to that range. This is a statement about where price sits against its own recent history, nothing more. The tidy way to express it is %B, which places price on a 0-to-1 scale between the bands: 1 means price is exactly at the upper band, 0 at the lower, 0.5 at the middle average, and readings can spill above 1 or below 0 when price pushes outside the envelope entirely.
The second question, and the more important one, is how wide the envelope is, because the width is the live measure of volatility. That is captured by bandwidth: the distance between the upper and lower bands, divided by the middle band to normalise it so different instruments and price levels can be compared. When bandwidth is high, the market is volatile and the bands are far apart; when bandwidth collapses to an unusually low reading, volatility has drained out and the bands have pinched together. That low-bandwidth state has a name and a following of its own.
| What you see | What it suggests | The caveat |
|---|---|---|
| Price tags the upper or lower band | Price is high, or low, relative to its recent range (%B near 1 or 0) | Not a signal. In a trend price can keep tagging the same band without turning |
| Bands contract to a narrow neck (a squeeze) | Volatility has fallen; an expansion, a larger move, often follows | Direction is not indicated. The break can go either way |
| Price clings to one band bar after bar (a walk) | A strong, persistent trend in that direction | The opposite of a reversal. Fading it means fighting the trend |
| Bands flare wide, price swinging inside | High volatility; the envelope is stretched | A state, not a forecast; wide bands can precede either continuation or exhaustion |
The squeeze: a coil that promises a move, not a direction
The squeeze is the most celebrated Bollinger idea, and the one most often mistold. It is simply the state where bandwidth falls to a local extreme low: volatility has compressed so far that the two outer bands draw into a tight neck around the average. The logic that makes it interesting is the empirical tendency for volatility to revert and cycle: extended calm is unusual and tends to resolve into a burst of movement. So a pronounced squeeze historically precedes a volatility expansion, a decisive move that throws the bands back apart.
This is the honest boundary of the concept. A squeeze is a statement about timing and likelihood, that a move is more probable than usual, and it is silent on direction. The same tight coil can erupt upward or downward, and there are even false starts where price pokes out of the neck one way before reversing. Using the squeeze well means treating it as a heads-up to pay attention, then turning to price structure for the direction, not asking the bands to supply an answer they do not contain.
The core failure: a band touch is not a buy or a sell
Here is the mistake that defines the difference between using Bollinger Bands and misusing them. The bands are drawn where price is statistically stretched, so it is tempting to read a touch of the upper band as "too high, sell" and a touch of the lower band as "too low, buy". John Bollinger himself rejects this. In his own rules he states that tags of the bands are just tags, not signals: a tag of the upper band is not, in and of itself, a sell signal, and a tag of the lower band is not a buy signal. The bands mark relative position; they do not mark a turn.
The reason the fade fails is walking the band. In a strong trend, price does not oscillate between the bands, it clings to the outer one. In a powerful uptrend price rides the upper band, tagging it again and again for many bars as the whole envelope drifts higher; in a downtrend it walks the lower band down. Every one of those tags looks like an "overbought" sell to someone fading the band, and every one of them is wrong, because the tag is confirming strength, not exhaustion. Selling each upper-band touch in a trend means shorting a market that keeps going up, which is exactly how a mean-reversion reflex gets run over.
Where the reflex burns people. Fading band touches has some logic in a genuinely sideways, range-bound market, where price does swing from band to band. The damage comes from applying that same reflex in a trend, where the market is walking the band and every fade is a fight against the dominant move. The bands cannot tell you which regime you are in; only price structure can. A band touch is relative-volatility context, never a standalone mean-reversion trigger.
Mean reversion versus trend: the same touch, two opposite meanings
The reason Bollinger Bands confuse people is that the identical event, a tag of a band, means opposite things in the two market regimes, and the bands themselves do not label which regime is in force. In a range, the envelope acts like a rubber container and price tends to revert from the edges toward the middle average. In a trend, the envelope is a moving conveyor and price rides its leading edge. Knowing which story you are in is upstream of the indicator, and it is the entire game.
| Behaviour at the band | In a range (mean reversion) | In a trend (band walk) |
|---|---|---|
| Price tags the upper band | Often near the top of the swing; price may revert toward the middle | Confirmation of strength; price frequently tags again and continues up |
| Price tags the lower band | Often near the bottom of the swing; price may revert up | Confirmation of weakness; price frequently tags again and continues down |
| Role of the middle band | A magnet price returns to across the range | Dynamic support in an uptrend, resistance in a downtrend |
| What defeats a naive user | A range that suddenly breaks into a trend | Fading every tag and shorting or buying against the move |
| What actually decides the reading | Price structure and trend context, which sit outside the bands and must be judged first | |
This is why Bollinger Bands are a context tool, not a trigger. They are excellent at one honest job: showing you, at a glance, how volatile an instrument is right now and where price sits inside that volatility. They are silent on the question that actually decides a trade, which regime you are in and which way the next move breaks. That upstream judgement, reading trend and structure before you read any indicator, is exactly what the method we teach is built around. The indicator is the easy part; the context is the skill.
Where Bollinger Bands fit, and where they mislead
A fair summary is that Bollinger Bands do one thing precisely and are routinely asked to do a second thing they cannot. The thing they do is measure volatility and relative price: the width tells you how turbulent the market is, %B tells you where price sits in that turbulence, and the squeeze flags when calm has stretched to an extreme. Used that way, as a volatility lens read alongside trend, they are a genuinely useful part of a chart. Like every indicator built from past prices, they summarise what has happened; they do not forecast what will, and John Bollinger frames them not as a source of buy and sell signals but as a framework for spotting setups where the odds may be more favourable.
The mislead is always the same: treating a band as a line the market must respect. The band touch is not a sell, the lower band is not a buy, the squeeze does not name a direction, and the clean 95 percent is a rule of thumb that real, fat-tailed prices routinely violate. Read plainly, Bollinger Bands hand you a well-calibrated picture of volatility and stretch, and then hand the actual decision straight back to your reading of price. That is not a weakness of the tool; it is an accurate account of what a standard-deviation envelope can and cannot know.
Common Questions
Frequently Asked Questions
What are Bollinger Bands in simple terms?
+Bollinger Bands are a volatility envelope drawn around price. The middle band is a moving average, usually a 20-period simple moving average, and the upper and lower bands sit a fixed number of standard deviations of price above and below it, by default two. Because standard deviation measures how far recent prices have strayed from the average, the bands widen when volatility rises and contract when it falls. They describe how stretched or calm price is, not which way it will move next.
How are Bollinger Bands calculated?
+First take the middle band as an N-period simple moving average of closing prices, with N usually 20. Then compute the standard deviation of price over the same N periods. The upper band is the middle band plus K times that standard deviation, and the lower band is the middle band minus K times it, with K usually 2. So the bands are the average plus or minus two standard deviations of price, and their distance apart is a live reading of current volatility.
Why are the bands set at two standard deviations?
+For a roughly normal distribution about two standard deviations spans around 95 percent of observations, which is where the idea that price mostly stays inside the bands comes from. But market prices are not normally distributed: they have fat tails and cluster in trends, so in practice a 20 and 2 setting has been noted to contain closer to 88 to 89 percent of price action. Treat the 95 percent as a rule of thumb, not a law, which is exactly why a move outside the bands is notable rather than forbidden.
What is a Bollinger Band squeeze?
+A squeeze is when volatility falls so far that the upper and lower bands contract to a narrow neck. Because low-volatility periods tend to be followed by high-volatility ones, a squeeze historically precedes an expansion, a large move. The crucial limit is that the squeeze tells you a move is likely, not its direction: the bands can break upward or downward from the same coil. It is the most useful and the most misused Bollinger concept.
Is touching the upper Bollinger Band a sell signal?
+No. John Bollinger states directly that a tag of the upper band is not, in and of itself, a sell signal, and a tag of the lower band is not a buy signal. A touch says price is high or low relative to its recent range, which in a range can mark a turn but in a strong trend simply confirms momentum. Price can ride the upper band upward, or the lower band downward, for many bars, so fading the touch means fighting the trend.
What does it mean when price walks the band?
+Walking the band is when price clings to the upper band through a strong uptrend, or the lower band through a downtrend, tagging it again and again without reversing. It is the direct refutation of the beginner reflex to sell every upper-band touch. A band walk signals a persistent, powerful trend, not exhaustion. This is the same trap as reading an indicator as overbought and selling into strength, which is why the bands are context, not a reversal trigger.
What are %B and bandwidth?
+They are the two companion readings that turn the visual bands into numbers. %B locates price within the bands: it equals price minus the lower band, divided by the upper band minus the lower band, so it reads 1 at the upper band, 0 at the lower band and 0.5 at the middle, and can go above 1 or below 0. Bandwidth measures the width of the envelope, the upper band minus the lower band divided by the middle band, and it is what falls to an extreme low during a squeeze.
Do Bollinger Bands predict direction?
+No. Bollinger Bands are built from past prices, so they summarise the current state of volatility and relative price, they do not forecast where price goes. A squeeze does not say up or down; a band touch does not say reverse. John Bollinger frames the bands as a framework that helps identify setups where the odds may be in your favour, to be read alongside trend and structure, not as a source of standalone buy and sell signals.
What settings do most traders use for Bollinger Bands?
+The original and most widely used default is a 20-period simple moving average with the bands at 2 standard deviations, as set by John Bollinger. He is explicit that these are defaults, not sacred numbers: a shorter lookback reacts faster but produces more noise, a longer one is smoother but slower, and some traders widen the multiple. This guide is educational and does not recommend any specific setting, timeframe or instrument.
Where the facts come from
Sources
- John Bollinger, the 22 rules of Bollinger Bands. The originator's own rules: Rule 6 states tags of the bands are not signals, so an upper-band tag is not a sell and a lower-band tag is not a buy; Rule 9 sets the defaults of 20 periods and 2 standard deviations; Rules 15 and 18 define %B and BandWidth; Rule 22 frames the bands as a framework, not continuous advice. bollingerbands.com
- Bollinger Bands construction and containment. The middle band as a 20-day SMA, the upper and lower bands at plus and minus two standard deviations, the note that the 20 and 2 setting contains roughly 88 to 89 percent of price action, and the description of walking the bands and the Bollinger Band Squeeze. stockcharts.com
- %B and the two-standard-deviation intuition. The %B formula, price minus lower band over upper band minus lower band, and the statistical basis for the two-standard-deviation default. fidelity.com
Educational note. This guide explains an indicator and its mechanics. It is not a recommendation to trade or invest, not a suggestion of any setting or strategy, and it is not investment advice. Bharath Shiksha is an educational publisher, not a SEBI-registered investment adviser or research analyst.