Although plumbing is now metricated, it is still fairly common to hear the term “half inch pipe” especially amongst older people, or in the USA where they remain stuck in the past with imperial measures.
Oddly, there is nothing about a ½” pipe that is ½”, be it copper, iron, or PVC. The outside diameter of copper is 5/8″, and the inner diameter is either 0.527″, 0.545″, or 0.569″, depending on the series. So called ½” iron pipe is 0.840″ outside diameter and 0.622″ inside. PVC is the same outside as cast iron, but it is 0.608″, 0.528″, or 0.480″ inside.
If no dimension is ½”, why call it ½” pipe? It appears that way back in time, (copper pipe was introduced in the 1930′s) copper pipe was indeed standardized at the nominal ½” diameter inside with a 1/16″ wall, making it 1/8″ bigger on the outside. As the metallurgy improved, allowing makers to use thinner metal (and thereby increase their profits), they increased the inner diameter rather than decrease the outer diameter simply to allow the pipe to fit existing manufactured fittings.
As the manufacturers began to make bigger and bigger pipe, they found that 1/16″ wall thickness was insufficient. Thus, the inner diameter of larger pipes is smaller than the nominal size, while smaller pipes are usually larger.
Wrought iron pipe has a similar history. When PVC was developed, it inherited it’s outside diameter from iron pipe.
