Turns out that the factorial function, FACT(), only works for integers. Luckily, you can still get the factorial of a positive non-integer value by using the Gamma function. By definition, x! = G(x+1) = xG(x). In Excel, you have access to the GAMMALN() function which returns LN(G(x)), so to calculate x! for values greater than zero, you can use

=x*EXP(GAMMALN(x))

Ref: Factorial at Wikipedia.com

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