This is usually the opposite of Conversion Rate (click to read more) and is used when one is more interested in the percentage of source events that did not lead to a desired destination event and is calculated so:
1 - (Number of Dest. Events ÷ Number of Source Events) = Abandonment Rate
Abandonment Rate must be multiplied by 100 to be represented in percentage and is usually much more sensible source event occurs more often than the destination event.
As an example to determine the Shopping Cart Abandonment Rate for a month during which 8,000 Shopping Carts were created by visitors and 2,000 orders were placed, one would use the following formula:
1 - ( 2,000 ÷ 8,000)= 1 - (0.25) = 0.75 = 75%
This means that 75 out of 100 carts are not converted into orders (which is not necessarily an abnormally high number).