This article describes the formula syntax and usage of the EXPONDIST function in Microsoft Excel.
Returns the exponential distribution. Use EXPONDIST to model the time between events, such as how long an automated bank teller takes to deliver cash. For example, you can use EXPONDIST to determine the probability that the process takes at most 1 minute.
Important: This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. Although this function is still available for backward compatibility, you should consider using the new functions from now on, because this function may not be available in future versions of Excel.
For more information about the new function, see EXPON.DIST function.
Syntax
EXPONDIST(x,lambda,cumulative)
The EXPONDIST function syntax has the following arguments:

X Required. The value of the function.

Lambda Required. The parameter value.

Cumulative Required. A logical value that indicates which form of the exponential function to provide. If cumulative is TRUE, EXPONDIST returns the cumulative distribution function; if FALSE, it returns the probability density function.
Remarks

If x or lambda is nonnumeric, EXPONDIST returns the #VALUE! error value.

If x < 0, EXPONDIST returns the #NUM! error value.

If lambda ≤ 0, EXPONDIST returns the #NUM! error value.

The equation for the probability density function is:

The equation for the cumulative distribution function is:
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Data 
Description 

0.2 
Value of the function 

10 
Parameter value 

Formula 
Description 
Result 
=EXPONDIST(A2,A3,TRUE) 
Cumulative exponential distribution function 
0.86466472 
=EXPONDIST(0.2,10,FALSE) 
Probability exponential distribution function 
1.35335283 