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
Ring Seal - an overview
New Gland Calculator Tool at
O-Ring Calculator Ceetak Sealing Solutions
Seals Eastern - O-ring Rod Seal Design Guideline
O-Rings – Orange Seal
Groove Design: O-Ring Seals - Minnesota Rubber & Plastics
O-Ring Groove Design Standard AS568B • Seal & Design, Inc
ORing eHandbook
O-Rings – Orange Seal