About the NF T 47-501 standard for O-rings

The NF T 47-501 standard for O-rings is produced by the Association Française de Normalisation (French Standards Institute). The NF T 47-501 specification describes 447 O-rings and is very similar to the ISO 3601-1 standard. 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.

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.