Tubing is generally measured by the OD Outside Diameter because it is used with push-to-connect fittings. Polyurethane (in general), nylon, and polyethylene tubing all commonly use a push to connect fitting.
Softer tubes that use barbed fittings or compression fittings are measured by ID Inside Diameter. PVC tubing is a prime example of a softer tube that would use barbed fittings or two piece compression fittings to make the connection.
Posted in: Pneumatic Tubing FAQs