Determining the sizes of pipes you need for your project can be a bit of a challenge, especially if you have not previously learned or researched how to measure pipe thread size or pipe fittings. It is understandable to assume the size of the pipe refers to its outer diameter. However, as with lumber, we have to consider “nominal” versus “actual” sizes.
Pipe fittings must have inside diameters that are large enough to fit over the outside diameter of a pipe. Usually, a half-inch fitting will have an outside diameter of around 1.25 inches.
Here’s a quick overview of what you should know about nominal versus actual diameter when measuring pipes and pipe fittings.
Making the conversion
The best way to determine the nominal pipe size you need is to measure the outside diameter and convert to nominal.
There are several ways you can measure the outside diameter of the pipe. If you’re able to simply measure a sawed-off end, then that’s easy enough. But if the pipe is still in position, you’ll need to get a little creative.
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.
If it’s a female thread, you’ll instead measure the inside diameter of the pipe or pipe fitting using a tape measure or ruler, and use that measurement to determine the nominal diameter. You can find charts online that give you an easy way to transfer decimals to nominal diameters and vice versa, as well as the number of threads per inch (in case that information is important for your project).
Keep in mind that piping and tubing are measured differently. The methods you use to measure your pipe fittings cannot be used for tubing, as that is measured (and also named) based on the tube’s actual diameter.
We mention this because PEX (cross-linked polyethylene tubing) is becoming rapidly more common in a wide range of settings, and it’s measured and named differently than standard piping. Keep this in mind if you need to replace PEX tubing.
Note that you will also need to select the thread type in addition to choosing the proper nominal diameter for your piping. National pipe thread (NPT) is perhaps the most common, and features tapered threads for joining and sealing pipes. But there are other pipe threads as well, which may have different benefits for different types of joints and jobs. Make sure you’re choosing the right type for the application you have in mind.
Interested in learning more about how to measure pipe fittings and pipe thread size to ensure you’re picking the correct pipe fitting for your upcoming job? We encourage you to contact the team at Royal Brass Incorporated, where we are specialists in hoses, fittings and much more. We look forward to assisting you soon.
Categorised in: Pipe Fittings