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Pipes & Pipe Fittings Guide


When it comes to finding the right plumbing parts, there is a lot to consider. Pipe fittings and connectors are an essential part of plumbing systems, helping join and lengthen pipes or change the direction of water flow. Installing the incorrect fittings can result in various issues, ranging from corrosion and downtime to possible injury. Learn how to choose the right pipe fittings for your plumbing needs with our guide.

Different Types of Pipe Fittings

Pipe fittings are used to attach pipes together, and the type of fitting is determined by its function. Since every system is different, the fittings and accessories you need may vary in material, size or usage requirements. Consider your needs to help select the correct fitting. Are you extending a pipe, changing directions or trying to reduce water flow?

Explore the most common pipe fittings and their uses:

Type Use Example


Extends or changes connection type at the end of a pipe


Joins pipes of various sizes


Covers the end of a pipe


Straight fitting used to make permanent pipe connection


Changes direction of flow


Fits inside pipes to extend the length


Fits inside to seal pipes


Changes pipe size 


Combines fluid flow from multiple branches


Helps easily connect and disconnect pipes


Stops or regulates the flow 

How to Choose the Right Fittings

There is a wide selection of pipe tubing and pipe fitting types and finding the right ones for your pipes is essential to an efficient plumbing system. The most important consideration when selecting tubing and fittings is fixture compatibility. Without this, your plumbing system won’t operate properly.

Here’s what to consider when choosing pipe tubing and fittings:

Pipe vs. Tubing 

Although people might use the terms “pipe” and “tubing” interchangeably, they have different purposes and are not measured the same way. Pipes serve as a vessel to transfer fluids and gases in large plumbing applications. Tubes, on the other hand, can be square, round or rectangular and are typically used in structural applications. Selecting the right type of tubing helps ensure optimal and cost-effective plumbing performance. 

Tubing can be made from hard or soft materials. However, tubing usage falls into three main categories:  

  • Fluid transport: tubes that transport fluids from one place to another
  • Structural applications: tubes designed to use in buildings and structures with mechanical properties
  • Electrical sheathing: tubes that are designed around electrical wires or applications to protect against abrasion   

Thread Types

Pipe fittings are threaded on both ends, with female threads on the inside of a fitting. Male threads are on the outside of a fitting and screw into the female threads. A common thread type used in North America is National Pipe Thread (NPT), featuring tapered threads used for joining and sealing pipes. Fittings come in male pipe threading (MPT or MNPT) and female pipe threading (FPT or FNPT). Another common thread type is British Standard Pipe (BSP), the standard used in most countries for plumbing and pipefitting. Other thread-type standards vary based on the country and industry.


The material of fittings depends on several factors, including temperature conditions, pressure ratings and cost. Pipes come in a wide variety of materials, as do fittings. However, both fittings and tubing components are typically matched with the material of the pipe itself. The most common materials for fittings are metals and plastics, including brass, copper, steel, black iron, polyvinyl chloride (PVC), high-density polyethylene and more.

For tubing, the material plays a huge role in determining the right kind for your application. Hard, metal pipe tubing is used when pipes need strength and hardness. Copper, aluminum and steel are the most commonly used metals for tubing. These materials are frequently used in plumbing and heating applications for their corrosion-resistant durability. Soft tubing is a more flexible alternative in plumbing systems. The most commonly used plastics are nylon, polyethylene, polypropylene, polyurethane and PVC. Soft tubing offers corrosion resistance, strength and helps reduce pressure leaks.


Finding what size pipes are needed for a project can be confusing. However, pipe size refers to the nominal diameter (DN) or average inside diameter of the pipe measurement. Pipes can be easily interchanged and connected to the same nominal diameter pipes. The nominal diameter can be calculated using standard tables and schedules based on the pipe dimensions. 

Fitting size is determined by the inside diameter (ID) of the pipe it’s connecting to and is measured in inches or millimeters. ID measures the size of the empty portion of the cylinder, and OD sizes the thickness of the tubing wall.

The tubing size is somewhat similar. The dimensions for tube sizing are determined by the OD, ID and wall thickness and measured in inches or millimeters. Nominal sizes for tubes are based on the outside diameter.

Taking time to measure carefully will help ensure your sizing is correct. Selecting the proper tubing and pipe fitting types ensures your plumbing system can operate at peak performance.

Frequently Asked Questions

Q: What does Nominal Pipe Size (NPS) mean?

A: Nominal pipe size is a pipe measurement system that refers to the North American pipe classification that provides an approximate pipe size in inches based on the bore or internal pipe diameter.

Q: Is the Nominal Diameter (ND) the same as the Nominal Pipe Size (NPS)?

A: No. Nominal diameter refers to the outside diameter of pipes measured in millimeters and is used in metric pipe systems as a secondary classification system.

Q: How is pipe size measured?

A: There are a few ways to measure pipe size depending on the type of pipe. According to Plumb University, the two most important measurements are the outside diameter (OD) and nominal pipe size (NPS). The first step is to check on the pipe since many pipes, like PVC, are labeled with the type, nominal pipe size and wall thickness. Many plumbing lines also have a control valve that displays the pipe size on the side. If you can easily access an open end of a pipe, you can use a ruler or tape measure to directly measure the diameter across the end of the pipe. A pair of calipers can help measure the diameter of an installed pipe. It’s important to note that the diameter can vary depending on the material and thickness of the pipe.

Q: How do I measure the diameter of a pipe?

A: Before you can find the diameter of a pipe, you need to determine whether it has male or female threads or contains no threads. The measurement process is different for male and female threads. Male threads contain small groves around the outside of the pipe that can be seen from the outer edge, whereas female threads are inside and can be harder to spot. You can use a flexible tape measure or caliper to find the inner diameter, or the distance across the middle, at the end of a pipe where there is a cross-section for a pipe with female threads. For male threads or non-threaded pipe, measure the outer diameter. Once you find the diameter, refer to a standard schedule chart based on the type of pipe to find the nominal pipe size. Follow the steps below to help find the diameter of a male or female threaded pipe.

Male Threads

1. Measure the Outside Diameter (OD) of your pipe or pipe fitting by using a flexible tape measure or placing a string around the pipe and marking the point where the string overlaps.  

2. Use a ruler or tape measure to find the circumference by measuring the length between the mark and the tip of the string.

3. Divide the circumference by 3.14159.

4. Refer to a nominal diameter chart according to the type of pipe to find the correct size.  

Female Threads

1. Use a ruler or a caliper to find the Inside Diameter (ID) of a pipe with female threads.

2. Find the inside diameter by measuring the distance across the hollow or center part of the pipe, excluding the walls.   

3. Refer to the appropriate nominal diameter chart based on the type of pipe to find the correct size.



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The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.