# How Bollinger Bands Indices Indicator Works

Bollinger Bands stock indices indicator calculations uses standard deviation to draw the bands, the default value used is 2.

## Bollinger Bands Stock Indexes Calculation

**The middle Bollinger band line** is a simple moving average

**The upper Bollinger band line is**: Middle line + Standard Deviation

**The lower Bollinger band line is**: Middle line - Standard Deviation

Bollinger bands stock indices indicator considers the best default moving average to calculate the Bollinger bands to be 20 periods moving average and the bands are then overlaid on the stock indices chart stock indexes price action.

Standard Deviation is a statistics concept. It originates from the notion of normal distribution. One standard deviation away from the mean either plus or minus, will enclose 67.5 % of all stock indexes price action movement. **Two** standard deviations away from the mean either plus or minus, will enclose 95 % of all stock indexes price action movement.

This is why the** Bollinger Bands stock indices indicator uses the standard deviation of 2 which will enclose 95 % of all stock indexes price action.** Only 5 % of stock indices chart stock indexes price action will be outside the 3 indices trading bollinger bands, this is why indices traders open or close stock indexes trades when stock indexes price hits one of the outer Bollinger Bands.

The Bollinger Bands stock indices indicator main function is to measure stock indexes price action volatility. What the Bollinger bands upper and lower limits try to do is to confine stock indexes price action of up to 95 percent of the possible closing stock indices prices.

Bollinger Bands stock indices indicator compares the current closing stock indexes price with the moving average of the closing indices price. The difference between these two stock indices prices is the volatility of the current stock indexes price compared to the moving average. The stock indexes price volatility will increase or decrease the standard deviation of the bollinger bands stock indices indicator.