#389 – How to Select a Pipe Cover Plate: Understanding IPS Dimensions

When choosing pipe wall plates, don’t confuse Iron Pipe Size and actual size

To get the right-sized pipe wall cover plate, match the inside diameter of the plate to the outside diameter of the pipe. When you do this, be sure not to get confused by the difference between nominal pipe sizes and actual pipe sizes. That way, your wall cover plate will perfectly match your pipe.

When a pipe passes through a wall, a hole has to be cut to accommodate it. An oversized gap is unsightly and also usually necessary. Pipe wall plates cover these holes, providing a clean, finished appearance. They offer the same benefit to pipes that escutcheons do for fire sprinklers.

But things don’t look so clean or finished when the wall plate is the wrong size for the pipe. Most often, people end up with plates that are too big for their pipes and look just as messy as the original hole.

When purchasing all kinds of pipe and fittings, including pipe wall plates, it pays to know the difference between nominal size and actual size. This article explains how Iron Pipe Size (IPS) measurements work, what kinds of components use them, and how to figure out the actual size of a measure given in IPS.

Good craftsmanship is in the details. For a clean finish where pipe penetrates walls, use a properly sized wall plate. Our selection of wall plates is available in white or chrome and features a wide range of sizes bound to meet your needs.

What is IPS? Understanding iron pipe size

When shopping for pipe wall plates or any other kind of pipe accessory, you might come across the acronym IPS, which stands for Iron Pipe Size. IPS is a sizing system that is used for all kinds of pipe, not just iron pipe. PVC, iron, stainless steel, and copper pipe can all be measured in IPS.

Bear in mind that IPS measurements are not the actual size of the pipe. The IPS number for a pipe or fitting is always smaller than the actual size. Let’s dive into this more.

IPS is not actual size       

Many people run into trouble with IPS because it’s a nominal measurement. In other words, a 2-inch IPS pipe is 2 inches in name only. The actual outside diameter of 2-inch IPS pips is 2.375 inches. Likely, no part of a 2-inch IPS pipe has a dimension of 2 inches.

This may sound silly. However, important innovations bought us nominal sizes like IPS, so it is used because of history and convenience. It’s easier to say and remember “2 inches IPS” than “2.375 inches.” More on this later.

Fortunately, IPS sizes are consistent—a 2-inch IPS pipe always has an outer diameter of 2.375”. To convert between IPS and the outer diameter of a pipe, you’ll have to consult a table. Here is a handy table of IPS sizes and outside diameters:

IPS Outer Diameter (Inches)
1/2″ 0.84
3/4″ 1.05
1″ 1.315
1-1/4″ 1.66
1-1/2″ 1.9
2″ 2.375
2-1/2″ 2.875
3″ 3.5
3-1/2″ 4
4″ 4.5
5″ 5.563
6″ 6.625
8″ 8.625

If you confuse IPS and actual size, you’ll end up with wall plates that are too big!

What would happen if you wanted to buy plumbing supplies—say, a wall plate to cover a wall penetration—but you didn’t know the difference between IPS and actual size? Maybe you measure your pipe and find that the outer diameter is 1.9 inches. So, you go with a 2-inch IPS wall plate. That should work, right?


Again, the actual outer diameter of a pipe is always bigger than the IPS size by roughly 10%. Mixing up IPS and actual size will result in components that don’t fit together.

ips size and plate size
When selecting pipe wall plates, it pays to know the difference between IPS and actual size.

How to find the perfect wall plate

Selecting a wall plate to fit around your pipe and cover the hole is easy. You need two measurements: the size of the hole you need to cover and the size of the pipe.

First, measure the size of the hole in the wall. There’s no trick to this. If the hole is 4 inches across, look for a plate that says “4-inch hole.” There are no nominal sizes for holes in walls.

Second, measure the outer diameter of the pipe. Then, don’t forget to convert this measurement to IPS. If you skip this step, you might end up with a wall plate that fits too loosely and looks just as ugly as the original hole.

Finally, find a wall plate that matches the dimension of the hole and the IPS size of the pipe. This plate will snuggly fit your pipe and neatly cover the gap in the wall.

pipe wall plates
For a clean, attractive finish where pipes penetrate walls, use a wall plate to cover the hole. Shop our selection of wall plates that fit pipes from 1/2-inch to 10-inches IPS!

Understanding nominal sizes like IPS, NPS, and more

Iron pipe size is one of many nominal measurements used in the building and fire protection trades. Many people are familiar with 2-by lumber sizes. A 2-by-4 is nominally 2 inches by 4 inches, but really 1.5 inches by 3.5 inches.

Some industries where nominal measurements are used include:

  • Electric power—grid or mains power has a nominal voltage of 120 V, but the actual voltage is permitted to vary slightly
  • Batteries—Different battery chemistries have characteristic nominal voltages. Actual voltage can vary
  • Lumber—Wood board sizes do not match their nominal measurements
  • Pipe size—Pipes can be sized based on Nominal Pipe Size (NPS) or Iron Pipe Size (IPS)
  • Pipe thickness—The wall thickness of pipes uses the pipe schedule system
  • Pipe threads—Threaded pipes are sized based on the National Pipe Thread (NPT) system

Here at QRFS, fire protection and pipes are some of our favorite subjects. So, let’s dive into the history and application of nominal systems for pipe size, pipe thickness, and pipe threads.

Standard sizes for pipes and threads—IPS, NPS, and NPT

IPS and NPS are both nominal pipe size systems that don’t match the actual pipe size. But why don’t we just classify things based on the outer diameter of the pipe?

Pipe sizes are the way they are because of the history of industrialization. Before mass production, components like wrought iron pipes were made by hand. Pipes were manufactured by welding together two halves, and different iron mills used their own measurements. Items from separate manufacturers would never fit together!

Back in 1862, amid the US Civil War, the enterprising Robert Briggs introduced the world’s first set of pipe standards. Briggs was superintendent of the Pascal Ironworks in Philadelphia and convinced the iron mills near him to adopt his sizes. This resulted in interchangeable pipes and fittings—something we take for granted in the modern world. IPS descends from the “Briggs Standard,” and NPS superseded IPS.

Once upon a time, IPS roughly referred to the internal diameter of the pipe. At the time, this was true, though there were only three standard pipe wall thicknesses: STD (“standard”), XS (“extra-strong”), and XXS (“extra-extra-strong”). Nowadays, there are too many pipe thicknesses for this to make sense.

For its part, NPS does more than just describe the outer diameter. It also helps ensure standardization in threaded pipes. The thread sizes on tapered threaded pipe are measured with a system called National Pipe Thread (NPT). Because pipe threading ultimately depends on outer diameter, NPT sizes match up with NPS sizes. Thus, male and female ends of any 2-inch NPS threaded pipe will always fit.

The pipe schedule system

In engineering and the trades, knowing the outer diameter of a pipe is barely half the battle. To understand how the pipe will handle pressure or how big the internal diameter is, you need to know the wall thickness. In the past, STD, XS, and XXS were the measurements used for wall thickness. Now, we use pipe schedule.

A pipe’s wall thickness is denoted by its schedule. For example, schedule 40 is a standard size. The higher the schedule number, the thicker the walls. But there is no single wall thickness for schedule 40 pipe.

Instead, schedule is a dimensionless measurement, and actual wall thickness depends on the combination of nominal size and pipe schedule.

For reference, check out this table of standard nominal pipe sizes and pipe schedules:

NPS Outside Diameter Wall Thickness Inside Diameter Wall Thickness Inside Diameter
Schedule 40 Schedule 40
1/8 0.405 0.068 0.269 0.095 0.215
1/4 0.54 0.088 0.364 0.119 0.302
3/8 0.675 0.091 0.493 0.126 0.423
1/2 0.84 0.109 0.622 0.147 0.546
3/4 1.05 0.113 0.824 0.154 0.742
1 1.315 0.133 1.049 0.179 0.957
1-1/4 1.66 0.14 1.38 0.191 1.278
1-1/2 1.9 0.145 1.61 0.2 1.5
2 2.375 0.154 2.067 0.218 1.939
2-1/2 2.875 0.203 2.469 0.276 2.323
3 3.5 0.216 3.068 0.3 2.9
3-1/2 4 0.226 3.548 0.318 3.364
4 4.5 0.237 4.026 0.337 3.826

Let’s recap. First, nominal size determines outer diameter (even if 2-inches nominal does not mean 2-inches actual diameter). Second, pipe wall thickness is described by schedule. The bigger the schedule, the thicker the walls. However, schedule is not an absolute measurement—it depends on outer diameter.

Understanding nominal sizes like IPS helps ensure all parts fit properly

Standardization is one of the hallmarks of the industrial era. This includes the standardization of parts and equipment. For example, pipes are sized based on a few nominal measurements. Nominal measurements like Iron Pipe Size may not match any actual dimension of a pipe. This can be annoying, but once you understand how this works, you can easily navigate the specifications of pipes and other components.

It’s certainly useful when you need to purchase wall plates for pipes. When pipes pass through walls, there are frequently unsightly holes cut for the penetration. Similar to how escutcheons clean up the gap around a fire sprinkler, wall plates make sure these penetrations are neat and attractive.

For a clean finish, it’s essential to get wall plates that exactly match the size of your pipes. If you confuse IPS and actual size, you’re going to end up with a plate that doesn’t work. To find the exact right wall plate for your needs, shop our wide selection that includes plates sized for pipes as small as 1/2-inch or as big as 10-inches IPS.

If you have questions or need help placing an order, call us at 888.361.6662 or email support@qrfs.com.

This blog was originally posted at blog.qrfs.com. Check us out at Facebook.com/QuickResponseFireSupply or on Twitter @QuickResponseFS.

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