About the BS 4518 standard for O-rings

The BS 4518 standard for O-rings is produced by the British Standards Institution. The BS 4518 specification describes 203 O-rings. Each size is designated by a four digit number indicating the O-ring ID in tenths of millimeters followed by a hyphen and two digits indicating the O-ring cross-section, also in tenths of a millimeter.

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.