Short definition
Pipe schedule is a wall-thickness rating system for pipe. Higher schedule means thicker wall, higher pressure rating, and a smaller flow ID for the same nominal pipe size. Standard schedules: 40 (residential standard), 80 (extra strong), 120, and 160. For PVC and CPVC, residential cold water and DWV are typically Schedule 40; Schedule 80 is required wherever the pipe is going to be threaded.
What it is
Two pipes of the same nominal size — say, 1-inch — can have different actual wall thicknesses depending on schedule. Schedule 40 has the standard wall; Schedule 80 has a thicker wall, which means a smaller inside diameter, a higher pressure rating, and the strength to be threaded without removing too much wall material.
The threading rule matters in plumbing practice. Schedule 40 PVC must not be threaded — threading removes too much wall and creates a stress concentration that fails. Threaded PVC connections require Schedule 80 (or thicker) on the threaded end. Modern installations often handle this with a Sch 40 run that transitions to a Sch 80 male adapter for any threaded interface.
For steel pipe (galvanized, black iron), residential gas distribution is typically Schedule 40. Heavier schedules show up only in high-pressure or large-diameter installations.
Why it matters to a homeowner
You’ll see schedule called out on:
- Drain line quotes — “Sch 40 PVC drain” is the residential default.
- Sprinkler manifold installs — Sch 40 for runs, Sch 80 for any threaded transition (the brass valve at the manifold inlet, for example).
- Hardware store buys — threaded PVC fittings have to be Sch 80 (or thicker) on the threaded end.
Common variants and what schedule is not
- Schedule 40 vs. Schedule 80 — same OD, different wall thickness. Sch 80 has a smaller ID, lower flow capacity, higher pressure rating. Color isn’t a reliable indicator; read the printed label.
- Schedule rating vs. SDR — SDR (Standard Dimension Ratio) is a different convention used for some plastics (HDPE service line, some CPVC) where the ratio of OD to wall thickness is held constant.