No, drain pipes, specifically plastic drain pipes, are generally measured by their outside diameter (OD), not their inside diameter (ID).
This means that when you see a specification for a "3-inch" PVC pipe, the 3 inches refers to the measurement of the pipe's outside diameter. This is crucial for ensuring proper fitting and compatibility when connecting pipes and fittings.
Why Outside Diameter?
Measuring by the outside diameter provides a consistent reference point for manufacturers and installers. The inside diameter can vary slightly depending on the pipe's wall thickness, which can differ based on the material and pressure rating. Using the OD ensures that fittings will properly connect to the pipe, regardless of minor variations in the ID.
Important Considerations:
- Verify Material: While plastic pipes generally use OD measurements, other materials like copper pipes may use nominal sizes that relate to the ID but aren't exact. Always confirm the sizing convention for the specific material you are working with.
- Measure Carefully: Accurately measure the pipe's outside diameter to identify the correct size. Using calipers or a measuring tape can help.
- Check Markings: Look for markings on the pipe itself, which often indicate the size and material specifications.
- Consult with Experts: If you're unsure about the correct size or material, consult with a plumbing professional or supplier.
Example
Imagine you're replacing a section of PVC drain pipe labeled as "2-inch." To ensure you get the correct replacement, measure the outside diameter of the existing pipe. If the outside diameter is approximately 2.375 inches (a common dimension for 2" PVC pipe), then you know you need to purchase a 2-inch PVC pipe.
