"Rob van Gelder" wrote...
Long datatype overflowed for me - using Double instead:

Sub Test()
Dim i As Long, dbl As Double

For i = 500000 To 1000000
dbl = dbl + i
Next
MsgBox dbl
End Sub
....

Brute force. Better to use Gauss's formula.

MsgBox 1000000 * 1000001 / 2 - 499999 * 500000 / 2

which recognizes that

Sum(500000..1000000) = Sum(1..1000000) - Sum(1..499999)

Amazing what a little math does for programming.