Here is a correction that allows you to easily adjust.

Sub AddFormulas()
Dim i As Long, numClasses As Long
Dim numStudents As Long
Dim StepValue As Long

NumClasses = 17 ' now 17
numStudents = 100 ' now 100
StepValue = NumClasses + 2
For i = numStudents * StepValue + 2 To 4 Step -StepValue
Range("B" & i).Formula = _
"=SUM(AB" & i - 1 & ":AB" & i - _
NumClasses & ")"
Next i

End Sub

--
Regards,
Tom Ogilvy





"David" wrote in message
...
Tom Ogilvy wrote

Misread - I thought the formulas were in contiguous cells in column B
- like what you showed.

That's ok. I'm not always as clear as I ought to be. I went with Bob's
less
elegant, but concise and effective loop. Thanks for jumping in, anyway.

--
David