Hose sizes, with few exceptions, are designated by the inside diameter, (ID), which is the size of a round rod that the hose can be slipped onto. The outside diameter (OD) of hose is only important when selecting hose clamps.

In reality, a "half-inch" pipe doesn't actually measure a half inch either inside or outside. In our catalog, we generally list the size designation and often the inside diameter. Use this diagram, together with careful measuring, to help pick out what you need.