How To Calculate A Pipe Schedule Number

When you're checking your plumbing system or building objects like desks or boats, it's important to get the right shape and size of the pipes you plan to use. The schedule number tells you how thick a pipe is. Learn more about it so you can make better decisions about the shape and size of pipes.

The ​schedule number definition​ (SCH) is the thickness of the walls of a pipe. The value itself has no dimensions or units so it's represented by a number alone.

Engineers measure the ratio of design pressure to allowable stress of pipes to estimate schedule numbers. The SCH is about 1000 times this ratio, and this gives you a schedule number formula for future calculations. Higher SCH values increase the wall size of a pipe, and the nominal pipe size (NPS), the approximate inside diameter of the pipe, changes as well.

From this nominal diameter meaning, you can check how SCH changes the inside diameter of a pipe, but not its outside diameter. The inside diameter measures the diameter between the internal walls of the pipe while the outside diameter is between the points on the exterior of the pipe.

The usage codes for pipe systems dictate different amounts of thickness for different projects and purposes. Many codes such as B 31.3, B 31.1 and IBR give equations to calculate the minimum allowed wall thickness based on the pressure of the material within the pipe.

Engineers also take into account the stress and temperature at which the pipes operate in determining wall thickness. The most common standards are B 36.10 Welded and Seamless Wrought Steel Pipe and B 36.19 Stainless Steel Pipe by the American Society of Mechanical Engineers.

Under these codes, SCH values of 40 with NPS of 10 or lower are known as Standard (STD). An SCH of 80 with NPS up to 8 are Extra-Strong (XS). SCH of 160 with NPS of 1/8 to 6 are Double Extra Strong (XXS).

Using the design codes from the American National Standards Institute or the ones from the American Society of Mechanical Engineers, you can write another schedule number formula to determine the pressure a pipe allows as

\(P=\frac{2SE(t_m-A)}{D_0-2y(t_m-A)}\)

for maximum allowable stress ​SE​ (in psi), wall thickness ​_t_m​ (in inches), additional thickness of the type of pipe ​A​ (in inches), coefficient of material and temperature ​y​ and outside diameter ​D₀_​ (in inches).

Check the manufacturing tolerance for the thickness of the walls of the pipe. Check the properties such as the sturdiness and durability of pipes when they're under the allowable stress to make sure they're functioning properly.

This equation is based off ​Barlow's formula​, which is internal pressure

\(P=\frac{2S_yt}{d_0}\)

for yield strength ​_S_y​ (in psi or MPa), wall thickness in ​t​ (in inches or millimeters) and the outside diameter ​d₀_​ (in inches or millimeter). You must use Barlow's formula for internal pressure at minimum yield, a specification of a strength by the pipe's design, or for the ultimate burst pressure.

The ultimate burst pressure is is the pressure at the maximum tensile strength a pipe can withstand. You can use these formula to measure the maximum allowable pressure on the pipe.

Online charts for comparing schedule number alongside other properties such as diameters and allowable stress. The Engineering Toolbox offers one for carbon steel pipes.

Other charts like the one from Engineers Edge compare across different codes of pipe design. You can compare these charts to determine the pressure or force of liquid that a certain pipe can allow.