T&T: alternator belts

[Larry N. Brown]

Can't remember if I posted the problems that developed vis-avis my new series
6 Balmar aternator and the resolution of them but the upshot is that the
manufacturer recommends the Gates green stripe belt. When I installed the
alternator, I just used the same belt that was on the alternator- a Yanmar
belt.

When I got a new belt, it was very slightly smaller than the original. There
was not enough diameter to fit it around the alternator pulley. A slightly
larger belt will allow it to rub on a protuberance from the engine. Unless I
fabricate a different mount, I'll need precisely the original size belt.

Question: how can one reliably measure a belt size to order a replacement?

Regards,

Larry and Teri
M/V Cigano, 47' Prairie Sundeck Cruiser
Lying: Bear Point Marina
           Orange Beach, AL