The POISSON.DIST function returns the Poisson distribution. A common application of the Poisson distribution is predicting the number of events over a specific time, such as the number of cars arriving at a toll plaza in 1 minute.
 This function was introduced in Excel 2010 and so is not available in earlier versions.
 The POISSON.DIST function replaces the POISSON function included in earlier versions of Excel.
Syntax
=POISSON.DIST(x,mean,cumulative)
Arguments
Argument  Description  

x  The number of events – must be ≥ 0  
mean  The expected number of events – must be ≥ 0  
cumulative  A logical value that determines the form of the probability distribution returned

Examples
A  B  C  D  

1  Data  Description  
2  3  
3  10  
4  
5  Formula  Result  Notes  
6  =POISSON.DIST(A2,A3,TRUE)  0.010336  Cumulative Poisson probability with the arguments specified in A2 and A3  
7  =POISSON.DIST(A2,A3,FALSE)  0.007567  Poisson probability mass function with the arguments specified in A2 and A3 
Common Function Error(s)
Problem  What went wrong 

#VALUE!  Occurs if either of the supplied x or mean arguments are nonnumeric 
#NUM!  Occurs if either of the supplied x or mean arguments are < 0 
The Poisson distribution is a discrete probability function that can be used to find the probability of a number of events occurring in a specified time period.
The probability mass function shows the probability of exactly x occurrences, and is calculated as:
where is the expected number of occurrences within the specified time period.
The cumulative distribution functions shows the probability of at most x occurrences, and is calculated as:
See Wikipedia for more information on poisson distribution.