All pipe schedules define a minimum wall thickness. Schedule 40 is the most recognized and can be traced back to 1862 from the Pascal Iron Works. The original wall thicknesses included pipe sizes up to 4". Since then everything else appears to be patchwork additions.
12" schedule 40 pipe is .406" wall or a .375" wall, depending on who you ask. Actually, 12" schedule 40s is .375" and 12" true schedule 40 is .406". But 12" standard wall is .375". Confused yet?
So are many of our callers and even some of our suppliers. If you ask a steel pipe mill for 12” schedule 40, you will get a .406” wall. Ask a stainless steel pipe mill and you will get .375” wall. Ask a PVC pipe mill and you will get .406” wall.
Hayward pipe has published a Metal Pipe ID-OD Chart. The chart clarifies the differences and duplication's between pipe schedules. This chart includes only pipes commonly defined by pipe schedule.
Since the wall thickness influences the thermal attributes, weight, cost, ID and pressure/temperature limits it is always prudent to specify the actual wall thickness, not just the schedule, when ordering pipe, fittings or valves.