> ## Documentation Index
> Fetch the complete documentation index at: https://docs.darvas.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Bollinger Bands

> Full working Bollinger Bands indicator with width band shading using Series and LineFill.

## What this builds

* Upper and lower Bollinger Bands (SMA +/- 2 standard deviations)
* Basis (middle) line
* Band shading using `fill()` on the plot handles
* Bandwidth oscillator in a sub-pane

## Full script

```js theme={null}
// overlay = true
const length = input.int("Length", 20, { min: 2, max: 500 });
const mult   = input.float("StdDev Mult", 2.0, { min: 0.1, max: 10, step: 0.1 });
const src    = input.source("Source", "close");

const upperPlot  = plot("BB Upper",  null, { color: "#3b82f6FF", linewidth: 1 });
const middlePlot = plot("BB Basis",  null, { color: "#3b82f680", linewidth: 1 });
const lowerPlot  = plot("BB Lower",  null, { color: "#3b82f6FF", linewidth: 1 });

fill(upperPlot, lowerPlot, { color: "#3b82f615" });

onBar(() => {
  const bb = ta.bb(src, length, mult);
  if (na(bb.upper)) return;

  plot("BB Upper",  bb.upper);
  plot("BB Basis",  bb.middle);
  plot("BB Lower",  bb.lower);
});
```

## With %B and bandwidth in sub-pane

```js theme={null}
// overlay = true
const length = input.int("Length", 20, { min: 2 });
const mult   = input.float("Mult", 2.0, { min: 0.1 });
const src    = input.source("Source", "close");

const upperP = plot("BB Upper", null, { color: "#3b82f6FF" });
const lowerP = plot("BB Lower", null, { color: "#3b82f6FF" });
plot("BB Middle", null, { color: "#3b82f650" });

fill(upperP, lowerP, { color: "#3b82f615" });

// %B and bandwidth require their own sub-pane plots
plot("BB %B",    null, { color: "#f59e0bFF" });
plot("BB Width", null, { color: "#a78bfaFF" });

hline(1, { color: "#ef4444FF", style: "dashed" });
hline(0, { color: "#22c55eFF", style: "dashed" });

onBar(() => {
  const bb = ta.bb(src, length, mult);
  if (na(bb.upper)) return;

  plot("BB Upper",  bb.upper);
  plot("BB Middle", bb.middle);
  plot("BB Lower",  bb.lower);

  // %B: where is price within the band?
  const width   = bb.upper - bb.lower;
  const pctB    = width > 0 ? (src() - bb.lower) / width : 0.5;
  const bbWidth = width / bb.middle * 100;

  plot("BB %B",    pctB);
  plot("BB Width", bbWidth);
});
```

## Key notes

* `ta.bb` returns `NaN` during the warmup period (first `length - 1` bars). Guard with `if (na(bb.upper)) return`.
* `fill(upperP, lowerP, { color })` shades between the two plot handles - pass the return values from `plot()` directly.
* The `%B` value is 1.0 at the upper band, 0.0 at the lower band, and 0.5 at the middle. Values outside \[0, 1] indicate price is beyond the bands.

## Related pages

<CardGroup cols={2}>
  <Card title="Volatility" href="/ta/volatility">
    ta.bb and ta.keltner reference.
  </Card>

  <Card title="hline, fill, bgcolor" href="/plotting/hline-fill-bgcolor">
    fill() syntax and budget.
  </Card>

  <Card title="NaN handling" href="/essentials/nan-handling">
    Why na(bb.upper) check is needed.
  </Card>
</CardGroup>