Bollinger Bandwidth normalizes the Bollinger band width by the middle band, giving a single 0+ value that's comparable across price regimes.

Formula

bandwidth = (upper - lower) / middle

Params

period - same as Bollinger (default 20).
std dev - same as Bollinger (default 2.0).

Output

Single column named after your indicator (e.g. Bollinger bandwidth).

Usage

Squeeze detection: low bandwidth = compressed volatility. Strategies wait for bandwidth to compress to a percentile low, then enter on the directional breakout.
Volatility regime filter: high bandwidth means trending / panic markets. Some strategies skip trades when bandwidth is above a threshold; others require it for breakouts.

Bandwidth vs raw width

The raw Bollinger width is in price units and not comparable across symbols or price levels. Bandwidth is dimensionless and directly comparable: a bandwidth of 0.04 on BTC and on ES means roughly the same relative volatility.

Pitfalls

The middle band can be zero or near-zero in degenerate test data, which blows up the division. Real markets don't hit this case; synthetic test rows can.