How do you measure pipe fittings?
Wrap a string around the pipe (if it's a male thread) and mark where the string touches. Determine the length between the end of the string and the touch point, which gives you its circumference. To get diameter, divide the circumference by pi (3.14159). You may need to convert decimal to fraction.
OD and Nominal Pipe Size
Tubing is measured by the OUTSIDE DIAMETER (O.D.), specified in inches (e.g., 1.250) or fraction of an inch (eg. 1-1/4″). Pipe is usually measured by NOMINAL PIPE SIZE (NPS).
NPT fittings are slightly tapered so the “nominal” diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads. This is a bit confusing, but NPT threads are made to the ANSI B1.
In short: Tube is measured by outside diameter, pipe is measured by inside diameter. There is often confusion as to which size die the customer actually needs - Pipe Size or Tubing Size. Keep in mind that pipe size refers to a nominal - not actual - inside pipe diameter. Schedule refers to the pipe's wall thickness.
When measuring a female pipe thread (FPT), measure the OD of the pipe itself and subtract ¼ inch to find the correct pipe thread size. Perhaps even easier than this is to purchase a measuring tool called a caliper that will measure OD and inside diameter (ID).
They're sized by the nominal size of the pipe, or the inner diameter. If you buy a 1/2 inch thread and measure the diameter, it will measure over 1 inch because it's designed to have a nominal pipe fitting. The female fitting should be sized just as large as the male fitting to get the right fit.
| PIPE SCHEDULES WALL THICKNESS (Inches) | ||
|---|---|---|
| Nominal | O.D. Inches | 80s & E.H. |
| 1/8 | .405 | .095 |
| 1/4 | .540 | .119 |
| 3/8 | .675 | .126 |
A 1” NPT MALE CONNECTION HAS AN OUTSIDE DIAMETER (OD) OF ABOUT 1.3”. AN 8” PIPE HAS AN OD OF ABOUT 8.6”. THE MEASUREMENTS ARE “TRADE SIZES.” THAT MEANS THEY ARE UNIFORM SIZES ACCORDING TO THE DIAMETER OF THE PIPE OR FITTING THAT THEY CAN BE USED WITH.
| Nominal Pipe Size | Outside Diameter of Pipe | Wrench Makeup Length for Internal Thread |
|---|---|---|
| 1/8 | 0.405 | 0.1111 |
| 1/4 | 0.540 | 0.1667 |
| 3/8 | 0.675 | 0.1667 |
| 1/2 | 0.840 | 0.2143 |
How To Correctly Measure Pipe Threads - YouTube
What is the OD of 3/8 pipe?
| NPS | OUTSIDE DIAMETER (IN) | WEIGHT (LB/FT) |
|---|---|---|
| 3/8 | 0.675" | 0.57 lb/ft |
| 1/2 | 0.840" | 0.85 lb/ft |
| 3/4 | 1.050" | 1.13 lb/ft |
| 1 | 1.315" | 1.68 lb/ft |
In the plumbing trade, the size of copper tubing is measured by its outside diameter in millimeters. Common sizes are 15 mm and 22 mm.
Wall Thickness
The OD for 1/2" PVC pipe is 0.840" and the OD for 3" PVC pipe is 3.500".
How to Identify Thread Pitch and Size | Tech Tips | Swagelok [2020]
Darren's Fitting Allowance original - YouTube
Short answer: the stated size is the total length of the pipe nipple including the threaded sections. This does appear to be consistent among manufacturers (at least in the US / North American market). Also an important detail is that the length of the threaded sections is standard for each pipe diameter.
Tubing is measured using the size of the inside diameter (I.D.) and outside diameter (O.D.). The measurement between the inside and outside diameter of a plastic tube is referred to as “the wall” and is used to measure tube strength: the thicker the wall, the stronger the tube.
PVC Pipe is identified by the Inside Diameter (ID) of the pipe. This measurement is known as 'Nominal' which means in-name-only, or a referenced name.
In the plumbing trade, the size of copper tubing is measured by its outside diameter in millimeters. Common sizes are 15 mm and 22 mm.
All you need to do is wrap a string or a flexible tape measure around the outside of the pipe. Now, take this measurement and divide it by pi (pi = 3.1415). This calculation is the plumbing pipe's Outside Diameter that you can use to look up the Nominal Size.