Plastic Pipe Size Guide: What Diameter Do You Need?

How to Figure Out Pipe Size from Markings

When you're trying to identify or replace a plastic pipe, one of the easiest ways to determine its size is by reading the markings printed directly on the pipe. These markings can tell you the nominal diameter, material type, pressure rating, and specs, which all affect what fittings or replacement pipes you’ll need.

Here are the key parts of a typical pipe marking that relate to sizing:

• Nominal Size - The labeled pipe size (e.g. 1/2", 2"). This helps you match to standard fittings - but it may not match the actual physical measurement.

• Material Type (PVC, CPVC, PEX) - Different materials follow different sizing systems. For example, PVC uses IPS (Iron Pipe Size), while CPVC and PEX often use CTS (Copper Tube Size).

• SDR or Schedule - Affects wall thickness, and therefore the inner diameter (ID) of the pipe, even when the nominal size is the same.

• ASTM Standard - Tells you what sizing system and application the pipe follows (e.g. ASTM D2241 = SDR pressure pipe, ASTM D1785 = Schedule 40/80 PVC).

Real Example: Charlotte Pipe Marking

Let’s decode this actual pipe marking:

Charlotte Pipe 20020 2" PVC 1120 SDR-21 PR 200 PSI @ 23°C ASTM D2241

Here's what it tells you about the pipe and it’s size:

• Charlotte Pipe: The manufacturer (find Charlotte Pipe products)

• 20020: Internal product/catalog number for a specific 2" SDR-21 pressure pipe

• 2": Nominal pipe size

• PVC 1120: Material type - PVC made from 1120 resin, which meets specific strength and durability requirements

• SDR-21: Standard Dimension Ratio - defines wall thickness relative to pipe diameter

• PR 200 PSI @ 23°C: Pressure Rating - safe for 200 PSI at 23°C (73.4°F). Must be derated at higher temps.

• ASTM D2241: Complies with the ASTM standard for SDR-rated pressure pipes

How to Measure Pipe Size When Markings Are Missing

If the markings on a pipe are faded, painted over, or completely gone, you can still figure out the size with a simple measurement. Use calipers or tape measure to measure the outside diameter (OD) of the pipe. Then, compare that number to a standard size chart for the pipe’s material (PVC, CPVC, or PEX). Keep in mind that the nominal size doesn't match the OD exactly. For example, a 1-inch PVC pipe actually has an OD of 1.315 inches.

What Pipe Size Should I Use for Common Projects?

Choosing the right pipe size depends on what you're trying to do. Using a pipe that’s too small can restrict flow or cause backups, while using one that’s too large might be overkill - or simply not fit your existing setup. Here's a quick reference for recommended pipe sizes based on common household and light commercial plumbing projects: