Option Explicit
Private Sub Ok_Click()
Dim weight As String
Dim height As String
Dim bmi As String
Dim lngMyRow As Long
weight = txtweigh.Text
height = txtheight.Text
bmi = (weight) / height ^ 2
'Assumes the output will be across columns A, B & C of the ActiveSheet. Change to suit.
On Error Resume Next 'Account for there being no data on the ActiveSheet
lngMyRow = Range("A:C").Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row + 1
If lngMyRow = 0 Then lngMyRow = 2 'Default row number if there's no data on the ActiveSheet. Change to suit.
On Error GoTo 0
Range("A" & lngMyRow).Value = Val(weight)
Range("B" & lngMyRow).Value = Val(height)
Range("C" & lngMyRow).Value = Val((weight) / height ^ 2)
End Sub