ISPRIME

Public Function ISPRIME(ByVal lNumber As Long) As Boolean 
Dim i As Long 

    ISPRIME = True 
    For i = 2 To Application.RoundDown(Math.Sqr(lNumber), 0) 
        If (lNumber Mod i = 0) Then 
            ISPRIME = False 
            Exit For 
        End If 
    Next i 
End Function 

As an excel formula
=NOT(OR(MOD(A1,SEQUENCE(ROUNDDOWN(SQRT(A1),0)-1,,2))=0))