How to measure pipe size

Pipe is named by a nominal size that is not the number a tape measure gives you. This page turns the nominal size into the numbers you can actually measure. Pick copper Type L or PVC Schedule 40 and the nominal size, and it returns the outside diameter, the circumference a tape wrapped around the pipe reads, the wall thickness, the inside diameter and the water held in a run of the length you enter. Measure the circumference on site and match it to the figure here, because the outside diameter is what fittings are made to.

Copper is nominal plus an eighth

Copper water tube under ASTM B88 has an outside diameter one eighth of an inch larger than its nominal size, whatever the type: half-inch copper is 0.625 inches outside, three-quarter is 0.875, one-inch is 1.125. Types K, L and M share the outside diameter and differ only in wall thickness, which is why a fitting fits all three. The page carries Type L, the common wall for residential water lines.

PVC follows the iron pipe sizes, so the numbers are odd

Schedule 40 PVC under ASTM D1785 uses the same outside diameters as steel pipe: half-inch is 0.840 inches outside, three-quarter is 1.050, one-inch is 1.315, two-inch is 2.375. That is why a half-inch PVC fitting will never go on half-inch copper, and why measuring the outside diameter tells you the material as well as the size. The wall is much thicker than copper's, so the inside diameter is close to the nominal figure.

Wrap a tape, divide by pi, or read the figure here

On a pipe in a wall you cannot get callipers on, wrap a tape or a strip of paper around it and read the circumference. The page prints the circumference for every size so you match the reading rather than divide by 3.14 on a ladder. A half-inch copper line reads a hair under two inches around; a half-inch PVC line reads about 2.6 inches.