The LOGEST function calculates an exponential curve of best fit, through a supplied set of x and y values.
Syntax
=LOGEST(known_y’s,[known_x’s],[const],[stats])
Arguments
Argument  Description  

known_y’s  The set of yvalues you already know in the relationship y = b*m^x


[known_x’s]  Optional. An optional set of xvalues that you may already know in the relationship y = b*m^x


[const]  Optional. A logical value specifying whether to force the constant b to equal 1


[stats]  Optional. A logical value specifying whether to return additional regression statistics
The additional regression statistics are as follows:

Note: Because LOGEST returns an array, it must be entered as an array formula. If the function is not entered as an array formula, only the first ‘m’ value in the calculated array of statistical information is returned.
Examples
A  B  C  

1  Known (Xs)  Known (Ys)  
2  12  15  
3  17  18  
4  14  27  
5  13  16  
6  
7  Result  Array Formula entered in cells A8:B8  
8  1.03506  11.40767  {=LOGEST(B2:B5,A2:A5,TRUE,FALSE)} 
Common Function Error(s)
Problem  What went wrong  

#VALUE!  Occurs if either:


#REF!  Occurs if the array of [known_x’s] is not the same length as the array of known_y’s 
The LOGEST function calculates the exponential curve that best fits a supplied set of y and xvalues. If there is a single range of xvalues, the calculated exponential curve satisfies the equation:
where,
 x is the independent variable
 y is the dependent variable
 m is a constant base for the x value
 b is a constant which is the value of y when x = 0
If there are multiple ranges of xvalues, the calculated exponential curve satisfies the following equation:
where,
 the x’s are the independent variable ranges
 y is the dependent variable
 the m’s are constant bases for the x values
 b is a constant