Exponential Moving Average
is a variation on Simple Moving Average. Instead of giving equal weight to all days in the period, the EMA gives more weight to recent days in the period.
First determine the weighting multiplier or percentage as 2 / (Period + 1).
So for a 10 day period the multiplier is .1818 or 18.18%. This is the weighting to be applied to the most recent bar. (As opposed to a 10 day SMA which will use 10% for all bars.)
To provide a base for the EMA, next calculate a 10 day SMA for the immediately preceeding bars.
The first EMA is then calculated as SMA + ((Close - SMA) * Weighting).
And subsequent bars as Previous EMA + ((Close - Previous EMA) * Weighting), and so on.
Average Other Values
Although the default input is Close Price, you can vary this to any other item. For example, you could specify Volume as the Custom Input to create Exponential Moving Average Volume.
Sufficient Data for Accuracy
EMA is one of several indicators that include an element of prior data. As such a 50 day EMA based on 50 days of underlying data will be significantly different to a 50 day EMA based on 500 days of data. This site will always include enough data to ensure 'accuracy'.