There are different methods of measuring V-belts that depend on the type of tool being used. To measure a V-belt, the belt first needs to be marked with a start/finish measurement point. A flexible cloth-like tape measure can be held at the start/finish point and wound all the way around.
If you have a rigid tape measure, lay the tape out on a bench, and with the start/finish position at the free end of the measure, roll the belt along the tape to the start/finish point and note the length. The width of the V-belt then should be measured. With these two measurements, a replacement belt can be found.
When looking for a replacement belt, you may notice that some companies sell by the width and the internal diameter, and some go by the width and the external diameter. Most manufacturers are able to convert the measurements to supply the correct belt.