Are Plumbing Pipes Measured Inside or Outside (3 Easy Ways to Measure)

Have you ever wondered how plumbing pipes are measured? Whether it’s for a DIY project or just out of curiosity, understanding how pipes are measured can be quite helpful.

In this article, we will explore the difference between inside and outside diameter measurements, as well as provide you with three easy ways to measure pipe diameters from the outside. Let’s dive in!

How are plumbing pipes measured?

Plumbing pipes are typically measured using two systems: nominal pipe size (NPS) and actual outside diameter (OD).

The NPS system, common in North America, assigns a numerical designation to represent the approximate inside diameter of the pipe.

However, it doesn’t directly correspond to the actual dimensions.

On the other hand, the OD system is based on the actual measurements of the pipe’s outside diameter and is commonly used in countries outside of North America.

The OD is typically measured in millimeters or inches and varies depending on the material and manufacturing standards.

Pipe thickness, specified by a schedule number, can also affect overall dimensions.

Hence, considering regional standards and project requirements is crucial when selecting plumbing pipes or fittings.

What is the difference between inside and outside diameter measurements?

The inside diameter (ID) and outside diameter (OD) measurements refer to different aspects of a pipe’s dimensions:

Inside diameter (ID)

The inside diameter of a pipe is the measurement of the empty space within the pipe.

It represents the diameter of the opening through which fluids or materials flow.

The ID measurement is essential when determining the flow capacity and compatibility with fittings or valves that need to connect to the inner surface of the pipe.

Outside diameter (OD)

The outside diameter of a pipe, as the name suggests, refers to the measurement of the outer surface of the pipe.

It represents the total diameter of the pipe, including the thickness of the pipe wall.

The OD measurement is crucial for determining the size and compatibility of fittings, as they need to fit over the outside of the pipe.

3 easy ways to measure pipe diameters from the outside

1. Vernier caliper

Use a vernier caliper, which is a precision measuring tool, to measure the outside diameter of the pipe.

Open the jaws of the caliper and place them around the pipe, ensuring they are in contact with the outer edges.

Close the jaws until they snugly fit around the pipe, and read the measurement displayed on the caliper scale.

2. Tape measure

Wrap a flexible tape measure around the outside of the pipe, making sure it follows the curvature of the pipe’s surface.

Hold the tape measure firmly in place, ensuring it is not twisted or overlapping.

Read the measurement at the point where the zero mark aligns with the other end of the tape measure.

3. Pipe diameter

Tape: Use a specialized pipe diameter tape that is designed specifically for measuring pipe diameters.

These tapes have measurements marked on them in a circular format.

Wrap the tape around the outside of the pipe, aligning the starting point with the end of the tape. Read the measurement at the point where it aligns with the starting point.

How do you measure the outside diameter of a pipe?

Measuring the outside diameter of a pipe involves using various methods.

One common approach is to use a vernier caliper or digital caliper.

By placing the caliper jaws around the pipe, ensuring contact with the outer edges, and gently closing them, the measurement displayed on the caliper scale or digital screen corresponds to the outside diameter.

Alternatively, a tape measure can be wrapped around the pipe’s outer surface, following its curvature.

By reading the measurement where the zero mark aligns with the other end of the tape, the circumference of the pipe can be obtained.

Dividing this circumference by π (approximately 3.14159) provides the outside diameter measurement.

Finally, a specialized pipe diameter tape, designed for pipe measurements, can be wrapped around the pipe, and the measurement at the starting point indicates the outside diameter.

How do you measure the circumference of a pipe?

Measuring the circumference of a pipe can be achieved using a few methods.

The simplest way is to wrap a flexible tape measure around the pipe, ensuring it follows the contour of the pipe’s outer surface.

Holding the tape measure firmly in place, the measurement is read at the point where the zero mark aligns with the other end of the tape.

This measurement directly represents the circumference of the pipe. Another approach is to use a string or a flexible material to encircle the pipe, making sure it lies flat against the pipe’s outer surface.

Then, mark the point where the string overlaps, straighten it, and measure the length using a ruler or tape measure.

This measured length corresponds to the circumference of the pipe.

What is the nominal pipe size?

Nominal Pipe Size (NPS) is a standard designation used to refer to the approximate inside diameter (ID) of a pipe rather than its actual dimensions.

It is a numerical value assigned to pipes for ease of identification and compatibility purposes.

The NPS system is widely used in North America and is based on historical conventions.

The NPS does not directly correspond to the actual measurements of the pipe. Common NPS sizes for residential plumbing include 1/2″, 3/4″, 1″, 1 1/4″, 1 1/2″, 2″, and so on.

Note that the actual outside diameter (OD) and wall thickness of the pipe can vary depending on the material and manufacturing standards.

What is the tolerance range for pipe measurements?

The tolerance range for pipe measurements can vary depending on various factors such as the specific pipe material, manufacturing standards, and intended application.

The tolerance range defines the acceptable deviation from the specified dimensions of the pipe. It ensures that pipes meet certain quality and performance standards.

In general, pipe measurements have tolerances for both the outside diameter (OD) and the wall thickness.

The specific tolerance values are typically defined by industry standards or specifications.

For example, for copper pipes, the OD tolerance might range from ±0.003 inches to ±0.008 inches, while the wall thickness tolerance could be around ±10% of the specified value.

Why does measuring a pipe’s inside and outside matter?

  • Compatibility and fit: The inside diameter (ID) measurement of a pipe determines the available space for fluid flow or the insertion of fittings, valves, or other components. Knowing the ID ensures that the pipe is compatible with the intended application and that fittings or accessories can properly fit inside the pipe.
  • Flow capacity: The inside diameter directly affects the flow capacity of a pipe. A larger ID allows for greater fluid flow rates, while a smaller ID restricts flow. Accurate measurement of the ID helps determine the pipe’s suitability for a specific flow rate requirement.
  • Fitting selection: The outside diameter (OD) measurement is crucial when selecting and fitting pipes together or connecting them to fittings. Matching the OD of pipes and fittings ensures a proper and secure connection, preventing leaks and maintaining the structural integrity of the plumbing system.
  • Structural stability: The OD measurement is also essential for determining the wall thickness of the pipe. The wall thickness, along with the material strength, is critical for ensuring the structural stability and pressure-bearing capacity of the pipe. Accurate measurement of the OD helps in selecting pipes that meet the required specifications for a given application.

What can you do when pipe sizes don’t fit?

  • Use an adapter or reducer: If the pipe sizes are close but not an exact match, you can use an adapter or reducer fitting. These fittings are designed to connect pipes of different sizes. An adapter fitting allows for a transition from a larger pipe size to a smaller one, while a reducer fitting does the opposite. Ensure that the adapter or reducer fitting is compatible with the material and type of pipes you are connecting.
  • Use a bushing or coupling: A bushing is a fitting that can be used to connect pipes with different sizes. It is inserted into the larger pipe and reduces the opening to match the smaller pipe size. A coupling, on the other hand, is used to join two pipes of the same size but may have different connection methods (e.g., threaded to non-threaded). These fittings help bridge the gap between pipe sizes.
  • Modify or fabricate pipe sections: In some cases, it may be necessary to modify or fabricate pipe sections to achieve a proper fit. This could involve cutting and rejoining pipes using appropriate techniques like welding, soldering, or solvent cementing. However, it’s important to ensure that the modifications are done properly and adhere to local plumbing codes and regulations. This option may require the expertise of a professional plumber.
  • Consider flexible connectors: Flexible connectors, such as flexible hoses or expansion joints, can be used when there is a slight difference in pipe sizes or when some flexibility is needed due to movement or vibration. These connectors provide some adaptability and can accommodate small variations in pipe sizes.
  • Consult a professional plumber: If you encounter difficulties in fitting pipes together, it is recommended to seek advice from a professional plumber. They have the expertise and knowledge to evaluate the situation, recommend suitable solutions, and ensure proper pipe connections that meet plumbing standards and regulations.

What is a pipe wall thickness, schedule (SCH), and gauge?

Pipe wall thickness

The pipe wall thickness refers to the measurement of the thickness of a pipe’s wall. It is the distance between the inner and outer surfaces of the pipe.

The wall thickness can vary depending on the pipe size, material, and pressure rating requirements.

Thicker walls provide increased strength and durability, while thinner walls are typically used for less demanding applications.

Schedule (SCH)

The schedule, often denoted as SCH, is a numerical designation that specifies the wall thickness of a pipe.

It is used primarily for steel pipes. The schedule number represents the ratio of the pipe’s wall thickness to its outside diameter.

Common schedule numbers include SCH 5, SCH 10, SCH 40, SCH 80, etc. As the schedule number increases, the wall thickness increases, resulting in a stronger and more robust pipe.


Gauge is a measurement unit used to determine the thickness of materials, including pipes.

However, it is primarily used for sheet metal rather than pipes.

The gauge measurement represents the thickness of the material in a standardized manner.

In the context of pipes, gauge is not as commonly used as schedule or actual wall thickness measurements.

How do you measure pipe wall thickness?

  • Calipers: Use a pair of calipers, such as vernier calipers or digital calipers, to measure the pipe’s outer diameter (OD) and inner diameter (ID). Once you have both measurements, subtract the ID from the OD to obtain the wall thickness. Ensure that the caliper jaws are in contact with the pipe’s surfaces for accurate measurements.
  • Ultrasonic thickness gauge: An ultrasonic thickness gauge is a specialized tool designed specifically for measuring the thickness of various materials, including pipes. It uses ultrasonic waves to determine the wall thickness without requiring physical contact with the pipe. Simply place the gauge on the pipe’s surface, and it will provide an accurate measurement of the wall thickness.
  • Micrometer: A micrometer can also be used to measure pipe wall thickness, especially for smaller pipes or pipes with thinner walls. Open the micrometer jaws and position them on opposite sides of the pipe, ensuring they are in contact with the inner and outer surfaces. Gently tighten the micrometer until the jaws touch the surfaces, and read the measurement displayed on the micrometer scale.
  • Cut and measure: In some cases, when precise measurements are necessary, you may need to cut a section of the pipe to directly measure the wall thickness. Once the pipe is cut, use a ruler, caliper, or micrometer to measure the thickness of the pipe’s wall directly.

What is the difference between male and female threads?

Male and female threads refer to the two types of threads used in plumbing and various other applications for connecting pipes, fittings, and other components.

The difference lies in their physical characteristics and how they interact during the joining process:

  • Male threads: A male thread is characterized by an external thread on the outside of a cylindrical or tapered surface. It is designed to fit into a corresponding female thread. The male thread has the appearance of a raised spiral pattern wrapping around the external surface of a pipe or fitting. When two components with male and female threads are joined, the male thread is inserted into the female thread and tightened to create a secure connection.
  • Female threads: A female thread, on the other hand, refers to an internal thread that is located inside a hollow cylinder or fitting. It is designed to receive and mate with a male thread. The female thread has the appearance of a recessed spiral pattern inside a pipe or fitting. It provides the space and grip necessary for the male thread to be inserted and secured.

The male and female threads work together to form a threaded connection.

The male thread is inserted into the female thread and rotated to tighten the joint, creating a seal and preventing leaks.

This threaded connection is commonly used in plumbing, piping systems, and various mechanical applications where a secure and watertight connection is required.

Other interesting articles:

Author: Logan

I help people connect with businesses