I just went through the headache of belt hunting, here's what I found out. For a 3/8ths pulley get a piece of 3/8ths rope and put around both pulleys to get the length. Here's where it got stupid. Went to Napa and the 3/8ths belt in the right length was actually a 1/2in. wide. Went to Auto Zone and guy looked up 3/8ths belt got home because it said on package in the part number and yup, 1/2in belt. Went to O'reily's same thing, watched the guy enter into computer and ordered in got home and another 1/2in belt.
By now pissed off and went onto Oreily's site and started digging, now the really stupid part, you can check the site for any of them to save you the headache. The 3/8ths belts listed on their site if you look are 1/2in wide(13mm). The 7/16s belt are actually 3/8ths(10mm) wide. Somehow the wider belt(7/16) is narrower than the (3/8ths)belt. I stumped all the employees when I told them and they tried to make sense of it and couldn't. Hopefully this can help you