About the ISO 3601-1 standard for O-rings

The ISO 3601-1 standard for O-rings is produced by the International Organization for Standardization. The ISO 3601-1 specification describes 709 O-rings in two different groups: inch and metric.

The inch group describes 349 O-rings and matches the AS 568 A sizing very closely (the -9## O-rings are not included). Each size is designated with a dash number and can be described using two values: the inside diameter and diameter of the cross-section. The ISO 3601-1 inch standard includes all of the 5 main cross section groups in AS 568 A standard. The inch group is intended for general use.

The metric group describes 360 O-rings. Each size is designated by a letter (A - E) indicating the cross-section. The last four digits indicate the inside diameter rounded to one-tenth millimeter. The metric group is intended for aerospace use.

In contrast to the AS 568 A inch system, the nominal and physical dimensions of metric O-rings match precisely. That is, a 1 mm O-ring will have a cross-section of 1 mm. The space that is being filled must be about 10% to 15% smaller than the cross-section of the O-ring in order for O-ring to seal properly.