How Can I Tell What Size Pipe I Have?!

Need to order a system but don't know how to measure pipe size?

It is useful to know the size of your incoming pipes.  For instance, say you decide you want to install an iron filter system for your house.  They come in different pipe sizes, such as ¾”, 1”, etc., which generally refers to the pipe diameter.

It is easy to check the size of your pipes.  First, check on the pipe itself. Often it will be labeled or written on the side or a valve.  If not, you can measure copper pipe size, and other water pipe sizes use the String Method.

The String Method

The string method (which measures the pipe's circumference) is probably the best way to determine your pipe diameter size.  The circumference is the distance it takes to go around the pipe once.

Remove any insulation from the pipe.   Using a piece of string about 6” long (or a cloth tape measure), wrap the string around the pipe once and measure to the nearest 1/8 of an inch…

Once you have found the circumference, use the chart below to find your pipe or tube size.

(Click here if you want to learn more about measuring pipe size.)

Also, you can watch a how-to measure pipe size and pipe diameter video on our Clean Water Store YouTube channel:

Pipe goes by nominal sizes rather than actual measurements which can be confusing.

Copper 3/4-inch pipe, for instance, doesn’t measure 3/4-inch.  Its outside diameter (OD) is 0.875 inch and inner diameter (ID) is 0.811 inches for Type M pipe (a designation that relates to wall thickness).

Galvanized steel 3/4-inch pipe is 1.050 inch OD and 0.824 inch ID for standard Schedule 40 pipe (also a wall thickness designation).

To make things a little more confusing sizes for tubings, such as the small-diameter plastic or copper tubing used to hook up icemakers are the actual outside diameter and hose sizes indicate actual inside diameter.

One other way to find out the size is to look for manufacturer’s markings printed on plastic, copper, and cast iron pipe.

What pipe thread sizes are available or common in the U.S.?

Two common pipe thread sizes are available, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).

The tapered threads are for joining and sealing, the straight threads are only for joining. Dry-seal thread (NPTF) allows for joining without sealants.

Female NPT threads may be designated as “FPT”
Male NPT threads may be designated as “MPT.”

Three less common threads exist, the Garden Hose Thread (GHT),
Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT).
The NPT and NPTF threads are interchangeable with sealants such as PTFE tape.
None of the other thread standards are interchangeable.
Definition of pipe thread acronyms
NPT National Pipe Thread
FPT Female (National) Pipe Thread
MPT Male (National) Pipe Thread
PTF SAE short tape pipe thread
NPTF American National taper pipe thread for Dryseal
pressure-tight joints
NPSM American National straight pipe thread for mechanical joints
NPSI American National straight intermediate pipe thread