IsPrime

Option Explicit

Private Sub cmdIsPrime_Click()
If txtNumber.Text = "1" Then MsgBox _
"1 cannot be tested.", vbOKOnly, "Error": Exit Sub
MsgBox IsPrime(txtNumber.Text)
End Sub

Public Function IsPrime(Number) As Boolean
Dim CountUp As Long
CountUp = 2
Do Until CountUp = Number
If (InStr(1, CStr((Number / CountUp)), ".")) = 0 Then
IsPrime = False: Exit Function
Else
IsPrime = True
End If
CountUp = CountUp + 1
Loop
End Function