Use the roll out method:
No,
I don't mean hang yerself like Minion suggested just yet.
Put a white mark in the inside of the belt, chalk magic marker etc.
line up the mark with one you put on the floor, or a crack, or edge of a bench etc.
and carefully roll out the belt till the mark reappears, your tape measurer will
will tell you the belt length exactly.
Try it twice just to make sure.
Now belts are identified in several ways, but I will not go into that just now.
---just google it.
