If you didnt know, the "PK" number on a belt can be used to determine its size. It's almost always on the belt.
For example belt size 6PK2360. There are three forms of information in it. 6-->PK-->2360. Ribs, profile, length. So it breaks down as:
Ribs-6
Profile PK- automotive
length -2360mm
You'll want a 6PKxxxx belt. For example if you currently have a 6PK2360 and it's too tight look for the next size up. Which might be 6PK2365 (5mm longer than your original)or 6PK2370 (10mm longer than your original).
Its just easier searching for a belt using the standardized manufacturing size numbers rather than trying to crosss reference the length in inches. Sort of like tires.
Edit* attached picture
Below is a picture of my 8 rib supercharger belt and my 6 rib accessory drive belt from two different companies. There's lots of manufacturing numbers on them that don't mean anything to us as far as actual size. But the "PK" numbers tell us the exact size just like looking at a tire sidewall.