Maya Ellison
Measuring the diameter of a plastic pipe is a simple task, but it's important to do it correctly to avoid wasting time and money on the wrong materials. The first step is to identify the type of pipe you're dealing with, as different pipes have different sizing systems. For example, PVC, CPVC, and PEX pipes are usually labelled with the size on the side, which refers to the outside diameter. Copper, iron, and steel pipes, on the other hand, are not labelled and will need to be measured manually. When measuring, it's important to use the correct unit of measurement (inches or millimetres) and to measure the outside diameter, as this is what determines the nominal pipe size that you'll find in stores.
How to Measure Plastic Pipe Size
| Characteristics | Values |
|---|---|
| Measurement | Use a soft measuring tape, a caliper, and a calculator. |
| Diameter | Measure the outside diameter of the pipe, not the inside diameter. |
| Units | Use the metric system (mm) or the inch/imperial system (inches). |
| Nominal Pipe Size | The nominal pipe size is the size sold in hardware stores. It may not reflect the actual diameter of the pipe. |
| Pipe Type | PVC, CPVC, and PEX pipes are usually labeled with the size on the side. Copper, iron, and steel pipes are not typically labeled. |
| Male and Female Pipes | Male pipes have grooves on the outside or are thinner than female pipes. Female pipes have grooves on the inside or have the same inside diameter as the male pipe's outside diameter. |
| Pipe Wall Thickness | The pipe wall thickness can vary, affecting the inside diameter while the outside diameter remains the same. |
| Sizing Charts | Use sizing charts to determine the correct pipe size, especially when ordering. |
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What You'll Learn
Measuring the outside diameter
To measure the outside diameter, you can use a flexible measuring tape, a ruler, or a caliper. Place the measuring tape around the circumference of the pipe and then divide the result by pi (approximately 3.14159) to get the outside diameter. For example, if the circumference is 12.57 inches (319 mm), then the outside diameter is 4 inches (100 mm). If you don't have a measuring tape, you can use a piece of string by wrapping it around the circumference of the pipe, marking the point where it completes a circle, and then measuring the string against a ruler.
It is important to note that the size of a pipe is not always the outside diameter. The actual size of a pipe is the nominal diameter, which depends on the type of pipe. For example, a 1/2-inch copper pipe has an outside diameter of 5/8 of an inch. Therefore, after measuring the outside diameter, you may need to use a conversion chart to get the nominal pipe size, which is the size sold in hardware stores.
Additionally, some pipes, especially PVC, CPVC, or PEX pipes, may have the size written on the side, which refers to the outside diameter. Before measuring, it is a good idea to inspect the outside of the pipe for any size markings, as this can save time and effort.
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Using a sizing chart
Measuring the size of a plastic pipe can be a little confusing, especially because pipe sizes should be treated more like names than measurements. The size of a pipe is its nominal diameter, which depends on the type of pipe. For male pipes, you need to measure the outside diameter, and for female pipes, you need to measure the inside diameter.
Before measuring the pipe, it is important to know what kind of pipe you are dealing with. Male pipes have grooves on the outside, which help them fit into female pipes, or they are simply thinner than the female end into which they fit. Female pipes have grooves on the inside, which help them grip male pipes, or the inside diameter is the same as the male pipe's outside diameter.
Many pipes, especially PVC and PEX, have size and material information printed on them, which is the quickest way to identify pipe size. If the pipe is not labelled, you can measure the outer diameter with a caliper and then check online for a pipe sizing chart. Compare the caliper measurement with the sizing chart to know exactly which size you are working with. You can also use a tape measure across the pipe's width to determine the size.
It is important to remember that PVC pipe uses a different sizing system than other types of pipe. For example, the cream-coloured CPVC pipe found in some homes uses Copper Tube Sizing (CTS). A 2" PVC fitting will not fit a 2" piece of copper tube size pipe, but it will always fit a 2" nominal size PVC pipe.
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Metric vs. imperial systems
When it comes to measuring plastic pipe sizes in the UK, there are two systems in use: the Metric system and the Imperial system. The Metric system is the more modern of the two, commonly used in Europe and gaining popularity in the UK. It uses measurements such as millimetres, centimetres, and metres. In the Metric system, pipe sizes are always referred to by their outer diameter (O.D.). So, for example, a 20mm pipe refers to the outer diameter of the pipe.
On the other hand, the Imperial system is much older and well-established in the UK and the USA. It uses measurements such as inches, feet, and yards. In this system, sizes are referred to using the nominal bore (N.B.) of the pipe, which often does not accurately reflect the inner diameter. For instance, a 1/2" N.B. U-PVC pipe may have an inner diameter of 18mm, despite the nominal size suggesting a smaller dimension.
The main difference between the two systems lies in their size designation and manufacturing standards. While both systems offer the same functionality and performance, as they use the same PVC raw material, they are not easily interchangeable due to differences in measurements and referencing. The Imperial system, with its use of nominal bore, can be confusing when comparing pipe sizes, as the inner diameter may change with different pressure ratings, but the name remains the same.
It is important to note that while adaptors and converters are available to connect pipes from the two systems, it is not a common practice. Therefore, when specifying pipe sizes and fittings, it is crucial to understand which system is being used and to refer to the actual dimensions, especially with the Imperial system, to avoid any sizing issues.
To measure a pipe's size, it is recommended to use a soft measuring tape to measure the outside circumference. Then, divide the measurement by pi to find the nominal pipe size, which is the standard size used for purchasing pipes in stores. Alternatively, some pipes, especially those made of PVC, CPVC, or PEX, may have the size labelled on the side, indicating the outside diameter.
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Nominal pipe size
The nominal pipe size identifies the size of the diameter of the pipe's hole with a non-dimensional number. The pipe is then identified by its diameter and another non-dimensional number for the pipe's wall thickness, referred to as the "schedule". The schedule number on a pipe increases as the pipe's wall size increases, and the wall thickness changes based on a Nominal Pipe Size chart. The most common pipe schedules are schedule 40 pipe dimensions and schedule 80 pipe dimensions. The outside diameter of a pipe can be measured with a soft tape measure, and this measurement can then be used to find the nominal pipe size using a conversion chart.
It is important to note that the nominal pipe size and the outside diameter (OD) values are not always equal, which can create confusion. For NPS 1⁄8 to 12, the NPS and OD values are different. For NPS 14 and up, the NPS and OD values are equal. In other words, an NPS 14 pipe has an OD of 14 inches (360 mm). The nominal bore (NB) term is also frequently used interchangeably with DN.
The nominal pipe size is important for pipe sizing and ensuring that pipes and fittings are compatible. For example, a 2" PVC fitting will not fit on a 2" piece of copper tube size pipe, but it will always fit on a 2" nominal size PVC pipe.
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Male and female pipes
When measuring a pipe, the outside diameter corresponds with a nominal pipe size. For example, a 1/2-inch copper pipe has an outside diameter of 5/8 of an inch. The nominal pipe size is the size sold in hardware stores.
Male pipes are used in drain pipes, on things like backflow preventers and water meters, and on connections to fixtures. Female pipes are used in drain pipes, and can have grooves on the inside to help them grip a male pipe.
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Frequently asked questions
The easiest way to measure the size of a plastic pipe is to look for the size written on the side of the pipe. If it's not there, you will need to measure the outside diameter of the pipe. You can do this with a soft measuring tape, then divide your measurement by pi. You will then need to use a conversion chart to get the nominal pipe size.
The nominal pipe size is the size sold in hardware stores. For example, a 1/2-inch copper pipe has an outside diameter of 5/8 of an inch.
The metric system labels pipe diameters in millimetres, while the inch/imperial system labels pipe diameters in inches. The metric system is used in Europe and many other parts of the world, including the UK. The inch/imperial system is used in North America, Asia, and the UK.
You can measure the inside diameter of a pipe by wrapping a piece of string around the inside of the pipe, marking the string, and then measuring the length of the string.
No, the inside diameter of a plastic pipe is not relevant to the size of the pipe. The outside diameter is the important measurement because the pipe needs to fit into the socket.
