About the DIN 3771 standard for O-rings

The DIN 3771 standard for O-rings is produced by the Duetsches Institut für Normung (German Institute for Standards). The DIN 3771 specification describes 258 O-rings. Each size is designated by the ID × cross-section. Optionally, this designation may be followed by a letter indicating the quality level (N - normal quality; S - special quality. Subsequent to the letter indicating the quality level, the rubber polymer and IRHD hardness may be specified.

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.