Polychloride vinyl fittings, sized similarly to pipes of the same material, vary by diameter and wall thickness. The difference between the outside and inside diameters are generally standardized into one of two dimensionless measurements: Schedule 40 and Schedule 80. The former indicates pipes and fittings with a wall thickness between 0.068 and 0.687 inches, while the latter refers to material wall thickness between 0.095 and 1.218 inches.
Standardized PVC pipe and fitting sizes of Schedule 40 and 80 material are referred by a nominal pipe size. This is a referent that is a loose approximation of the average between the internal and external pipe diameter; a higher schedule number refers to a thicker wall. Nominal pipe sizes increase by 1/8 inch through the 1/2-inch size, and then step up by 1/4 inch through the 1 1/2-inch measurement. Three 1/2-size increases after this segue to 1-inch, 2-inch and 4-inch size increments. Pipe and fitting sizes typically range from 1/8 inch to 24 inches for each schedule type.
Schedule 80 PVC pipes and fittings are capable of withstanding a water pressure one and a half times that of the thinner-walled Schedule 40 materials. Generally, smaller diameter pipes and fittings have higher pressure ratings than larger diameters.