ISO 4397:2011.

Title:
Fluid power connectors and associated components - Nominal outside diameters of tubes and nominal hose sizes, 4 pgs.

Scope:
This International Standard specifies information on the nominal outside diameters of rigid or semi-rigid tubing and nominal hose sizes to be used with hydraulic and pneumatic fluid power connectors and associated components:

a) nominal outside diameters for rigid or semi-rigid tubes, irrespective of material composition, used in fluid power
and
b) a list of nominal sizes for hoses made of rubber or plastic used in fluid power.

Benefits:
ISO 4397 helps designers of fluid power connectors and conductors by providing a selected number of preferred nominal sizes. This makes it easier for manufacturers to produce components for use at a limited number of sizes, which reduces costs for users.