Understanding Pipe Sizes

Plumbing pipe sizes are perhaps the most confusing in construction. The "nominal" size typically refers to an approximate inside diameter, but what you measure with calipers is the outside diameter (OD).

Data Sources

All plumbing pipe dimensions and specifications are sourced from:
• ASTM B88: Seamless Copper Tube for Water, Gas, and Sanitation
• ASTM D2241: PVC (Polyvinyl Chloride) Plastic Pipe, Schedules 40, 80, and 120
• NSF/ANSI Standards: Certification standards for potable water systems
• IPS (Iron Pipe Size): Historical standards referenced for PVC and other materials

Copper Tubing (CTS)

Copper tubing follows a consistent rule: the OD is always 1/8" larger than the nominal size. This makes copper sizing relatively predictable.

• Type L: Standard residential water lines (thicker walls)
• Type M: Heating systems, lower pressure (thinner walls)
• Type K: Underground burial (thickest walls)

PVC Pipe (IPS)

PVC uses Iron Pipe Size (IPS) standards inherited from steel pipe. The OD varies by size and doesn't follow a simple formula.

• Schedule 40: Standard white PVC for drain, waste, vent (DWV)
• Schedule 80: Gray PVC with thicker walls for higher pressure

Hole Sizing Rule

When drilling holes for pipes, add 1/8" to 1/4" to the OD for clearance. If the pipe needs to move (thermal expansion), add more clearance and use a grommet.