The size standard for O-rings mainly follows several key specifications that define the inner diameter, outer diameter and cross section diameter (line diameter) of O-rings, ensuring interchangeability and universality on a global scale. The following is an overview of O-ring size standards:

GB/T 34452.1-2005: This is China's current new national standard, which adopts the way of integrating with international standards, mainly expressed by the inner diameter of the O-ring multiplied by the section diameter, such as "O-ring 7.5×1.8-G-N", where "7.5" represents the inner diameter of 7.5 mm, "1.8" is the section diameter, GB/T 34452.1-2005 is the standard number, The latter "G-N" may refer to material grades and other characteristics.

Old National standard GB1235-76: This is an old standard that has ceased to be used, and has used the outer diameter D times the wire diameter d2, such as "O-ring 20*2.4, Ⅱ-2 GB1235-76", where "20" represents the outer diameter of 20 mm, "2.4" is the wire diameter, and "Ⅱ-2" may refer to the material type, GB1235-76 is the number of the standard.

ISO 3601 is a set of O-ring standards developed by the International Organization for Standardization (ISO). ISO 3601 is divided into several parts, among which ISO 3601-1:2012 provides dimensions, tolerances, test methods and identification rules for O-rings. This set of standards defines two O-ring size series, Class A and Class B. Class A dimensions are compatible with the US standard AS 568B, with the same size code, and are suitable for most standard applications. Class B sizes are designed to provide metric size O-rings for easy installation in metric size grooves, with the first digit of the size code representing the sectional size group (A-E) and the last four digits representing an approximation of the inner diameter (in one-tenth of A millimeter).

AS 568: This is a set of O-ring standards published by the Society of Automotive Engineers (SAE) and widely used in North America and the rest of the world. The AS 568B standard specifies the inner diameter, section diameter and dimensional tolerance of O-rings, and the dimensions are identified by "dash number" (dash number), such as "-010" to "-580".

In addition to the two main standards mentioned above, there are some country or regional specific standards, such as the UK's BS 1806 (which has been replaced by ISO 3601, but is still used as a reference in some cases), and the Japanese JIS standard. However, in international trade and global production, ISO 3601 and AS 568B are the two most frequently cited standards.

When selecting O-rings, the specific needs of the application should be considered, including the working environment (such as temperature, pressure, media), the required sealing performance, and installation space, etc., to determine the applicable standard and specific size. In addition, the choice of O-ring materials (such as NBR, Viton, Silicone, etc.) is also determined by the application environment and media compatibility.