How To Measure Pipe Size. The dimension in column 3 will be your nominal pipe thread size. Now you can take your dimensions and run to your local hardware store to purchase the correct size fitting.
Thread dimensions are considered a nominal size and don’t match standard units of measurement. The nominal size of copper pipe, whether supply lines or dwv, is always 1/8 inch less than the od measurement. Measure circumference using a flexible tape and divide the outside circumference by 3.1415.
Unwrap the string and use a tape measure to find its length.
The diameter of a pipe is the distance between one wall of the pipe to the other wall of the pipe going through the center of the pipe. As you can see, the actual measured size is slightly more than 3/4″. This gives you the pipe's outside diameter. The diameter of a pipe is the distance between one wall of the pipe to the other wall of the pipe going through the center of the pipe.