Overflow means the number is too large for the data type.

As an example, open the Immediate Window (Ctrl+G), and enter:
   ? 200 * 200
It overflows, because the default data type in VBA is Integer (16-bit),
which runs out at 32767. 40000 is too big, so we must convert one of the
numbers to a Long Integer (32-bit) so it does not overflow:
   ? CLng(200) * 200

Use CDbl() to convert to a double, CCur() to convert to Currency, CVDate()
to convert to date.