The number 1 isn't a prime number. I can't see how to adjust the formula
other than with an additional IF statement
=IF(C8=1,"not prime",IF(OR(C8=2,C8=3),"prime",
IF(AND((MOD(C8,ROW(INDIRECT("2:"&C8-1)))<>0)),"prime","not prime")))
or perhaps a UDF
Function IsPrime(Number As Single) As Boolean
Dim x As Long
If Number < 2 Or (Number <> 2 And Number Mod 2 = 0) _
Or Number <> Int(Number) Then Exit Function
For x = 3 To Sqr(Number) Step 2
If Number Mod x = 0 Then Exit Function
Next
IsPrime = True
End Function
Mike