Bollinger Bands Complete Guide
Beyond "Sell at the Upper, Buy at the Lower"

Bollinger Bands is a volatility indicator invented by John Bollinger in the 1980s, and nearly every trader on the planet has seen it. But 90% of users only know the beginner playbook: "sell when price touches the upper band, buy when it touches the lower band" — and that approach gets repeatedly slapped down in strong trends.

This guide covers the math (why ±2 standard deviations), four real-world applications, the "Squeeze" breakout strategy, the Walk the Bands phenomenon Bollinger himself defined, and complete Pine Script code.

1. The Math Behind Bollinger Bands

Three lines:

Middle Band = SMA(close, 20)         // 20-period simple moving average
Std Dev     = stdev(close, 20)       // standard deviation of the last 20 closes
Upper Band  = Middle + 2 × Std Dev
Lower Band  = Middle - 2 × Std Dev

Key concept: standard deviation.

Why ±2 standard deviations?

Statistical principle: if price follows a normal distribution,95.4% of the time it will sit inside ±2σ. In plain English: out of 100 candles, roughly 95 will stay inside the bands and 5 will touch or pierce through.

But real financial assets aren't normally distributed — they have "fat tails": extreme events happen more often than a normal distribution predicts. So in practice, price touches the bands more frequently than 5% of the time. That deviation is the trap Bollinger Bands set for you, and the root cause of the "Walk the Bands" phenomenon we'll get to later.

2. Beginner Application: Mean Reversion (Most Common, Most Likely to Blow Up)

The intuition:

• Price hits the upper band → overextended, short the bounce
• Price hits the lower band → overextended, buy the bounce

//@version=5
strategy("BB Mean Reversion (Teaching Version)", overlay=true)

length = input.int(20, "Period")
mult = input.float(2.0, "Std Dev Multiplier")

basis = ta.sma(close, length)
dev = mult * ta.stdev(close, length)
upper = basis + dev
lower = basis - dev

// Price touches lower band → buy
if (ta.crossunder(low, lower))
    strategy.entry("Long", strategy.long)

// Price touches upper band → sell
if (ta.crossover(high, upper))
    strategy.entry("Short", strategy.short)

plot(basis, "Middle", color=color.orange)
plot(upper, "Upper", color=color.red)
plot(lower, "Lower", color=color.green)

3. Intermediate Application: Squeeze Breakout Strategy

This is where Bollinger Bands really shine. When the bands contract (upper and lower draw close together), volatility is dropping, the market is storing energy → the breakout direction often unleashes a big move.

How do you define a Squeeze?

Two common definitions:

A. Bollinger Bands Width (BBW)

BBW = (Upper - Lower) / Middle

BBW drops to its lowest value over the past X periods = Squeeze. Bollinger himself uses a 6-month low as the threshold.

B. Bollinger Squeeze (vs Keltner Channel)

When the Bollinger Bands sit entirely inside the Keltner Channel, that's a Squeeze. This is John Carter's version (from his book Mastering the Trade).

//@version=5
strategy("BB Squeeze Breakout", overlay=true)

length = input.int(20, "Period")
mult = input.float(2.0, "BB Multiplier")
keltnerMult = input.float(1.5, "Keltner Multiplier")

basis = ta.sma(close, length)
dev = mult * ta.stdev(close, length)
bbUpper = basis + dev
bbLower = basis - dev

atr = ta.atr(length)
keltnerUpper = basis + keltnerMult * atr
keltnerLower = basis - keltnerMult * atr

// Squeeze condition: BB inside Keltner
squeeze = bbUpper < keltnerUpper and bbLower > keltnerLower

// Squeeze ends + price breaks above upper band → go long
if (not squeeze and squeeze[1] and close > bbUpper)
    strategy.entry("Long", strategy.long)

// Squeeze ends + price breaks below lower band → go short
if (not squeeze and squeeze[1] and close < bbLower)
    strategy.entry("Short", strategy.short)

plot(basis, "Middle", color=color.orange)
plot(bbUpper, "BB Upper", color=color.red)
plot(bbLower, "BB Lower", color=color.green)
bgcolor(squeeze ? color.new(color.purple, 90) : na, title="Squeeze")

4. Advanced Application: The %B Indicator

Bollinger later added a companion indicator called %Bthat quantifies where price sits inside the bands:

%B = (close - Lower) / (Upper - Lower)

%B = 0     → price just touched the lower band
%B = 0.5   → price is at the middle band
%B = 1     → price just touched the upper band
%B > 1     → price broke above the upper band (strong trend)
%B < 0     → price broke below the lower band (strong trend)

With %B you can run:

• %B divergence: price makes a new high but %B doesn't → trend momentum is fading
• %B + RSI: %B > 0.95 and RSI > 70 → double-confirmed overbought
• %B crossing above 0.5: price moves from the lower half to the upper half — often a trend-starting signal

5. Pro Application: BB + RSI + Trend Filter

The combination Bollinger himself recommended most. Full workflow:

1. Use the SMA 200 to gauge larger trend direction (above SMA 200 = bullish structure)
2. Price pulls back to the BB middle band (not the lower band) → trend pullback entry
3. RSI is simultaneously > 50 (bullish momentum still intact)
4. Enter long

//@version=5
strategy("BB Middle Pullback + Trend Filter", overlay=true)

basis = ta.sma(close, 20)
dev = 2.0 * ta.stdev(close, 20)
upper = basis + dev
lower = basis - dev

sma200 = ta.sma(close, 200)
rsi = ta.rsi(close, 14)

// Bullish structure: price > SMA 200 + RSI > 50
bullishStructure = close > sma200 and rsi > 50

// Pullback to middle: low touches middle band but close reclaims above it
pullback = low <= basis and close > basis

if (bullishStructure and pullback)
    strategy.entry("Long", strategy.long)

// Close below middle + RSI < 50 → exit
if (close < basis and rsi < 50)
    strategy.close("Long")

plot(basis, "Middle", color=color.orange)
plot(upper, "Upper", color=color.red)
plot(lower, "Lower", color=color.green)
plot(sma200, "SMA 200", color=color.purple)

6. Reading BB Across Different Timeframes

7. Common Mistakes

❌ Treating BB as "absolute boundaries"

Price touching the upper band ≠ automatic sell. In a bull market, price can "Walk the Bands" — touching them on a dozen consecutive candles. You need to read the larger trend structure to decide whether fading the move is even appropriate.

❌ Cranking the parameters too short

Some people set length to 5 and mult to 1.0 because they "want it more sensitive" — the result is bands that are too narrow, signals flooding in, and a win rate collapsing to 30%. Stick with the 20 / 2.0 defaults.

❌ Entering during a Squeeze

A Squeeze is the dormant phase — the market isn't moving. Enter during this period and you'll get chopped up until you stop out. Wait for the Squeeze to end and a clear breakout to develop before you pull the trigger.

❌ Looking at BB without checking volume

If price breaks the upper band but volume doesn't expand, the odds of a fakeout are high. Real breakouts are almost always accompanied by a volume surge.

8. Highlights from Bollinger's Own "19 Rules"

John Bollinger wrote Bollinger on Bollinger Bands, which lists 19 rules. Here are the most critical ones:

• Rule 4: Price touching the bands is not, by itself, a signal — it requires confirmation from other indicators
• Rule 6: During a Squeeze the market stores energy; the breakout direction marks the start of a major move
• Rule 11: Walking the Bands is normal trending behavior — don't fight it
• Rule 14: BB is a "relative" tool, not an "absolute" one — the upper band is just "relatively high" and the lower band is just "relatively low"