There are two main o-ring sizing types. Standard was developed for use in the USA, while the international community uses metric for their o-ring dimensions.
O-Ring Groove Design Standard AS568B • Seal & Design, Inc
Seal & Design Manufacturer & Distributor of Custom Sealing Products
O-Ring Groove Design - Daemar Inc
How to Measure an O Ring (with only basic tools)
How to Choose an O-ring How to Choose O-ring Size
O Rings 270 Pieces Seal Gasket Washer O Ring Assortment Set Kit 18 Sizes, Replacement O-Ring Kit Grommets Heavy Duty Professional for A/C Automotive, Mechanic,Tools & Home Repairs with Case - COPACHI
O-Ring Groove Design Global O-Ring and Seal
Browse all O-Ring Groove Design Guides - Marco Rubber
What is an O Ring - Selection, Material and Storage : Barnwell