Iron Pipe Size, Nominal Pipe Size, and Copper Pipe Size: Major Differences

In this blog post, you’ll read:Choosing the right pipe size for your plumbing needs can be challenging with terms like Iron Pipe Size (IPS), Copper Pipe Size (CPS), and Nominal Pipe Size (NPS). Each has distinct applications and measurements. This guide clarifies these terms, their uses, and provides standard size charts. For reliable components, trust Plumberstar, a leading stainless steel pipe fittings manufacturer.
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Are you confused about the suitable selection of specific pipe sizes and fittings that match your system? In selecting the right pipe size, one must understand Iron Pipe Size, Copper Pipe Size, or Nominal Pipe Size.

In recent years, the evolution of iron pipe size into new systems like copper tubes has confused lay people about their differences. In this blog, we will discuss in detail What nominal pipe size means and how it varies with different piping tubes. Moreover, you will get authentic answers to choosing the right size for a plumbing unit.

For your convenience, we have collected a detailed ductile iron pipe size chart that defines every pipe size and its relevant terminologies. Experts at Plumber Star are always available to entertain your further queries.

Firstly, we will start with the most basic iron pipe size!

What is Iron Pipe Size (IPS)?

You can call Iron Pipe Size (IPS) a standard system for measuring the diameter of the pipe for gas, water, and any other fluid system. This term is common with carbon steel, red brass, 304 & 316 stainless steel, and PVC pipes. You get the measurement of the bore’s inside diameter in inches without considering the external diameter.

Now, we will talk about nominal pipe size.

What is Nominal Pipe Size (NPS)?

The nominal pipe size is similar to that of the iron pipe. However, this term is popular for pipes of different materials, including iron or steel. Other options are copper plastic, etc. It also represents the inside diameter of the pipes in inches.

With the nominal copper tube size increase, the O.D. inches also increase. Remember that the diameter of such pipes depends on their material and structure. Lastly, we will tell you about modernized copper pipe size.

What is Copper Pipe Size (CPS)?

This term is the most recent pipe size that different drain pipe manufacturers are now practicing, representing the pipe’s outside diameter. This measurement is available in inches and millimeters; most water systems have such fittings. Copper, CPVC, and PEX are common materials for these pipes.

Important Terms You Need to Know

  • IPS = Iron Pipe Size
  • CTS = Copper Tube Size
  • NPS = Nominal Pipe Size
  • D = Outside Diameter
  • D = Inside Diameter
  • SDR= Standard Dimension Ratio
  • CTS = Copper Tube Sizes

With basic knowledge about different pipe sizes, you should also be aware of their general characteristics. Scroll down to learn which one is best for your system!

Difference Between IPS, NPS, and CPS

NPS is a general size term applicable to different pipe materials; however, IPS is specific to iron and steel. On the other hand, the term CPS is relevant to copper pipes. Another difference between these size categories is the diameter of the pipe.

IPS and NPS demonstrate the inner diameter; IPS was the only basic measurement parameter in the past. After a while, NPS took the place of the IPS. A nominal outside dimension and schedule showcase the exact dimensions of the IPS.

Currently,  many plumbing experts and ductile iron pipe manufacturers use the term IPS for more clarity regarding the measurements of the pipeline inside diameter. CPS is different from these two terms and showcases the outside diameter.

Pipe size and pipe schedule explained | What are NPS, DN, IPS, and NB? | How to read pipe schedule

from Jeferson Costa Chemical Proce

Common Application

Where are different-size terminologies commonly utilized? As IPS is the most basic and old-size measurement, its application is widespread, especially in gas piping and industrial and commercial distribution systems.

NPS is common in various industrial systems, such as heating, ventilation, and air conditioning. Likewise, CPS is also common in heating, ventilation, air conditioning systems, and other simple and complex plumbing setups.

Along with wall thickness and schedules, a cast iron pipe size chart is the best way to evaluate the sizes representing inner or outside diameters. So, let’s move to it.

Nominal Pipe Size Chart

Below is a standard iron pipe size chart for your complete guidance. To understand the nominal pipe size chart in inches, you need to clear your mind about the following things:

  • D stands for Outside Diameter
  • IPS and NPS have the same values

Let’s check out the different diameters in various cast iron pipe size ranges:

Nominal O.D inches
1/8 0.405
1/4 0.540
3/8 0.675
1/2 0.840
~ ~
3/4 1.050
1 1.315
1 ¼


1 ½ 1.900
2 2.375
2 ½ 2.875
3 3.500
3 ½ 4.000
4 4.500
4 ½ 5.000
5 5.563
6 6.625
7 7.625
8 8.625
10 10.750
12 12.750
14 14.000
16 16.000
18 18.000


Now let’s have a look at the copper tube size chart that mostly stainless steel product manufacturers follow:

Nominal O.D inches
~ ~
1/4 0.375
3/8 0.500
1/2 0.625
5/8 0.750
3/4 0.875
1 1.125
1 ¼


1 ½ 1.625
2 2.125
2 ½ 2.625
3 3.125
3 ½ 3.625
4 4.125
~ ~
5 5.125
6 6.125
~ ~
8 8.125
10 10.125
12 12.125
14 ~
16 ~
18 ~


The above-mentioned detailed copper tube and the black iron pipe size chart depict the difference in the diameters of the pipes under these categories.

Pipe Wall Thickness


Aluminum Dome Floor Strainer thickness
Aluminum Dome Floor Strainer thickness

When considering IPS, you need to focus on various wall thicknesses on the base of the schedules. These schedules may vary from 5 to 160 in value. With a high value, the thickness increases. Most plumbing systems have scheduled 80 or 40-sized pipes.

Higher thickness means the pipes are capable of bearing more pressure. However, the high thickness reduces the inside diameter, affecting the fluid flow. The nominal diameter remains the same and doesn’t change with the thickness.

Standard Dimension Ratio

There is another way of measuring wall thickness with the standard dimension ratio. This ratio represents the comparison between wall thickness and the outside diameter. Its value normally lies between 7.4 to 13.5. A higher value indicates thin wall thickness compared to the outside diameter and vice versa.

This ratio helps determine the pressure rating of pipelines, resulting in the right wall thickness selection for different systems like irrigation, gas, water distribution, etc.

Bottom Line

After reading this article, we hope you get answers to all the questions regarding nominal pipe size or copper tube size. Still, choosing a suitable Iron pipe size for your piping system is a tricky decision. You can get help from experts to avoid any major plumbing issues if fittings are of the wrong sizes.

Remember to choose only a reliable stainless steel pipe fittings manufacturer like Plumberstar for various setups. Here, you will get quality plumbing components that remain functional for years without major repair requirements.

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