The Excel EXPONDIST Function

Exponential Distribution

The exponential distribution is a continuous probability distribution, which is often used to model the time between events.

The probability density function for the Exponential Distribution is given by the formula:

Exponential Probability Density Function Equation

and the Cumulative Exponential Distribution is given by the formula:

Cumulative Exponential Distribution Function Equation

where x is the independant variable and λ is the parameter of the distribution.

Basic Description

The Excel Expondist function returns the value of the exponential distribution for a give value of x. The user can specify whether the probability density function or the cumulative distribution function is used.

Note that in recent versions of Excel (Excel 2010 and later), the Expondist function has been renamed the Expon.Dist function. However, the Expondist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.

The syntax of the Expondist function is:

EXPONDIST( x, lambda, cumulative )

Where the function arguments are:

x-The value at which the exponential distribution is to be evaluated (must be ≥ 0).
lambda-The parameter of the distribution (must be > 0).
cumulative-

A logical argument that specifies the type of distribution to be calculated. This can be either:

TRUE-Calculate the cumulative distribution function
FALSE-Calculate the probability density function

Expondist Function Examples

Example 1 - Probability Density Function

Plot of the Exponential Distribution Probability Density Function with the Parameter lambda set to 0.5, 1 and 2
Probability Density Exponential Distribution with λ = 0.5, 1 and 2

The above chart on the right shows the Exponential Distribution probability density function with the parameter λ set to 0.5, 1, and 2.

If you want to calculate the value of the probability density function with the parameter λ set to 1, at the value x = 0.5, this can be done, using the Excel Expondist function, as follows:

=EXPONDIST( 0.5, 1, FALSE )

This gives the result 0.60653066.


Example 2 - Cumulative Distribution Function

Plot of the Cumulative Exponential Distribution with the Parameter lambda set to 0.5, 1 and 2
Cumulative Exponential Distribution with λ = 0.5, 1 and 2

The above chart on the right shows the Cumulative Exponential Distribution Functions with the parameter λ set to 0.5, 1 and 2.

If you want to calculate the value of the cumulative function with the parameter λ set to 1, at the value x = 0.5, this can be done using the Excel Expondist function, as follows:

=EXPONDIST( 0.5, 1, TRUE )

This gives the result 0.39346934.


Further examples of the Excel Expondist function are provided on the Microsoft Office website.


Expondist Function Errors

If you get an error from the Excel Expondist function this is likely to be one of the following:

Common Errors
#NUM!-

Occurs if either:

  • The supplied value of x is negative
or
  • The supplied lambda is ≤ 0.
#VALUE!-Occurs if any of the supplied arguments are non-numeric.