Pipe Sizes

Pipe sizes have their own system, called nominal sizes. Customers say the kitchen drain line measures just shy of 2 inches but is called 1 1/2 inches, and 7/8-inch line has a 1/2-inch valve. Pipe sizes explained are as follows.

I have often wondered myself. When you deal with all the different brands of faucets and their unique parts, you will know there is a monster out there.

As for the sizes of pipe being not what they measure, it is because they are identified as nominal sizes. NPS is Nominal Pipe Size. Nominal is defined as ‘not exact’ or ‘trade name’. I have been told that the nominal size of pipe is the inside diameter (id) of the smallest fitting of that size. As a rule, the fitting with the smallest id is a street elbow, whether it is a 45 or 90 degree, also called 1/8 or 1/4 bend respectively.

If you were to take a 1 1/2-inch 90 degree street elbow of what you think is 2 inch pipe and measure the inside diameter you will find it is 1 1/2-inches inside. The pipe has a slightly larger id measuring in at almost 1 5/8-inces.
This rule applies to steel and PVC pipe. The OD of 1/2-inch galvanized pipe is about 7/8-inch and so is 1/2-inch PVC. 3/4-inch pipe measures 1-1/16 inches, 1-inch measures 1 5/16 inches and 1 1/4 inches measures 1 5/8 inches.

These same rules apply to rigid copper plumbing. The outside diameter of 1/2″ copper measures 5/8″. 3/4″ copper measures 7/8″ and 1″ measures 1 1/8″. There is another plastic used in plumbing called CPVC. Its measurements emulate that of copper tubing, which is smaller than galvanized pipe of the same size.

Here are the exceptions to the rule.

Soft copper tubing that comes in rolls is usually referred by its outside diameter. It is used to supply humidifiers, icemakers, dishwashers and sink and toilet supply lines. 1/4″ copper tubing for the icemaker or humidifier measures 1/4″ outside diameter. A 3/8″ line for a sink supply or a dishwasher measures 3/8″ OD. However, if it is called 1/4L or 3/8L tubing, that means it is an inside measurement and the tubing is actually 3/8″ and 1/2″ OD correspondingly.

Last item of their conspiracy is tubular drain plumbing. These parts connect sink drains to traps, disposals and sinks together and come in a multitude of shapes and lengths that can baffle any handyman. These parts are available in chrome over brass, rough brass and PVC. They all measure by the outside diameter and are available in two basic sizes. 1 1/4″ for wash basins and 1 1/2″ for everything else.

Tubular parts usually seal with jam nuts and washers and come oversized on one end so one part can slip into another. The assembly can be so easy it is not often an amateur will use twice as many parts as needed.

Amateurs take note: see a professional when doing sink plumbing for the first time, and save time and money.