Pipe Fitting Schedule Numbers Explained: Wall Thickness Guide

What Does Schedule Mean?

Pipe schedule numbers indicate the wall thickness of a pipe and its matching fittings. Common schedules include 10, 40, 80, 160, and XXS. The schedule number is not a direct thickness measurement, but a ratio related to pressure design. Higher schedules mean thicker walls and higher pressure capacity.

Schedule 40 vs Schedule 80

Schedule 40 is the standard wall thickness for general-purpose piping in low to moderate pressure systems. Schedule 80 has approximately twice the wall thickness of Schedule 40 for the same nominal size, allowing it to handle higher pressures or more corrosion allowance. For the same NPS, the outside diameter stays constant while the inside diameter shrinks with thicker walls.

Why Schedule Matters for Fittings

When you order an elbow, tee, or reducer, the schedule must match the pipe you are welding it to. A mismatched schedule creates a wall thickness discontinuity at the weld, which can cause stress concentration and improper fit-up. Always specify both the NPS and the schedule on the purchase order.

Pressure Rating Relationship

The pressure rating of a fitting increases with schedule and decreases with temperature. At room temperature, a Schedule 80 fitting can safely carry roughly twice the pressure of a Schedule 40 fitting of the same material and size. At elevated temperatures, both ratings drop.

How to Choose the Right Schedule

Base your schedule on the design pressure, temperature, fluid corrosivity, and the required corrosion allowance. Thicker walls also provide more material for external mechanical damage protection. Consult ASME B31.3 or the applicable piping code for the exact wall thickness calculations.