Bollinger Bands are a trend indicator that detects the volatility and dynamics of the price movement in Forex market. It was developed in the 80's by John Bollinger to trade in the stock market. It is a part of the standard kit of the MetaTrader 4 trading platform. The indicator is used to measure volatility and to forecast the current trend turnarounds or continuation.
Bollinger Bands indicator comprises three directed lines. The central line is the simple moving average (SMA) with the period of 20. The lower line is the central line minus two standard deviations. The upper line of the Bollinger Bands is the same SMA(20) plus two standard deviations. Below you can see how the indicator attached to the daily chart of the EUR/USD currency pair:
The central line of the Bollinger Bands indicator is calculated using the moving average formula using the closing prices of the latest 20 bars:


——————  
N 
where CL — central line,
Close_{i} — close price of the candle number i,
N — period of the BB.
Before proceeding to the formula of the upper and lower bands, we have to calculate the standard deviation:
SD = √ 

where SD — standard deviation,
Close_{i} — close price of the candle number i,
CL — average value for the N periods (calculated as the central line earlier),
N — period of the BB.
In other words, you need to subtract the average price from every close price, sum up the squares of the differences, divide by the number of periods, and find the square root from the result to get the standard deviation. The standard settings for the number of candles in Bollinger Bands is 20.
The lower band is corrected using two standard deviations according to this formula:
LL = CL – 2 × SD,
where LL — lower line,
SD — standard deviation.
The upper line is calculated in a similar manner:
UL = CL + 2 × SD,
where UL — the upper line of the Bollinger Bands.
Because the indicator is made up of three lines, the results of the calculation will contain three different numbers for a given bar:
We can use the the EUR/USD quotes for the beginning of 2016 to illustrate the calculation process. The closing prices for the first 20 days of the year are presented below:
i  Close 

1  1.0832 
2  1.0746 
3  1.0780 
4  1.0929 
5  1.0925 
6  1.0858 
7  1.0857 
8  1.0876 
9  1.0863 
10  1.0913 
11  1.0894 
12  1.0907 
13  1.0888 
14  1.0873 
15  1.0797 
16  1.0848 
17  1.0868 
18  1.0892 
19  1.0939 
20  1.0830 
21  1.0887 
22  1.0918 
23  1.1102 
24  1.1207 
25  1.1158 
The calculation of the 21st candle of the Bollinger Bands' central line looks like this:
CL_{21} = (1.0832 + 1.0746 + 1.0780 + 1.0929 + 1.0925 + 1.0858 + 1.0857 + 1.0876 + 1.0863 + 1.0913 + 1.0894 + 1.0907 + 1.0888 + 1.0873 + 1.0797 + 1.0848 + 1.0868 + 1.0892 + 1.0939 + 1.0830) / 20
CL_{21} = 1.0866
The central line values for the next candles are calculated in the same way. The easiest way to do it is to use the Microsoft Office Excel or LibreOffice Calc tables. The results are presented below:
i  Close  CL 

21  1.0887  1.0866 
22  1.0918  1.0869 
23  1.1102  1.0877 
24  1.1207  1.0893 
25  1.1158  1.0907 
The next stage is the standard deviation calculation. Doing it manually would be cumbersome, so we will employ the same software to get the following results:
i  Close  CL  SD 

21  1.0887  1.0866  0.0050 
22  1.0918  1.0869  0.0050 
23  1.1102  1.0877  0.0042 
24  1.1207  1.0893  0.0060 
25  1.1158  1.0907  0.0093 
Let's compute the Bollinger Bands' upper line for the candles #2125:
UL_{21} = 1.0866 + 2 × 0.0050 = 1.0967;
UL_{22} = 1.0869 + 2 × 0.0050 = 1.0968;
UL_{23} = 1.0877 + 2 × 0.0042 = 1.0961;
UL_{24} = 1.0893 + 2 × 0.0060 = 1.1014;
UL_{25} = 1.0907 + 2 × 0.0093 = 1.1092.
The values of the lower line:
LL_{21} = 1.0866  2 × 0.0050 = 1.0765;
LL_{22} = 1.0869  2 × 0.0050 = 1.0769;
LL_{23} = 1.0877  2 × 0.0042 = 1.0793;
LL_{24} = 1.0893  2 × 0.0060 = 1.0772;
LL_{25} = 1.0907  2 × 0.0093 = 1.0722.
You can use the following assumptions when trading Forex using the Bollinger Bands indicator: