Pipe fitting dimensions are in either metric or Standard English. Because pipe fitting covers Pipe Fitting Dimensions several aspects, only the most common pipe fitting sizes can be given here.

How are pipe fittings measured?

Pipe fittings are measured by their diameter, wall thickness (known as “schedule”), and shape or configuration. (Fittings are also defined by their material grade and whether they are welded or seamless.)

Diameter refers to outside diameter of a pipe or fitting.

The North American standard is known as Nominal Pipe Size (NPS). The International Standard is known as Diameter Nominal (DN). Pipes and fittings are actually made in similar sizes around the world: they are just labeled differently.

From ? in to 12 inch “Nominal Pipe Size”, outside diameters are slightly larger than indicated size; inside diameters get smaller as schedules grow.

From 14 in and larger “Nominal Pipe Size”, outside diameters are exactly as indicated size; inside diameters get smaller as schedules grow.
As with other North American standards (inch, foot, yard, mile, …), many pipe standards (diameters up to 12 inch and wall thickness) are based on historical precedents (a toolmaker’s dies during US Civil War) rather than a “scientific” method.

Schedule Numbers

The schedule numbers are used by the ANSI (American National Standards Institute) to denote wall thickness. The schedule numbers encompass all pipe dimensions beginning at NPS 1/8” up NPS 36”. Note that this configuration is only for fittings that match with a particular ANSI schedule number.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.

• Schedule, often shortened as sch, is a North American standard that refers to wall thickness of a pipe or pipe fitting. Higher schedules mean thicker walls that can resist higher pressures.
• Pipe standards define these wall thicknesses: SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS and XXS. (S following a number is for stainless steel. Sizes without an S are for carbon steel.)
• Higher schedules are heavier, require more material and are therefore more costly to make and install.

What does “schedule” mean for pipe fittings?

Schedule, often shortened as SCH, is a North American standard that refers to wall thickness of a pipe or pipe fitting.

What is schedule 40, SCH80?

Higher schedules mean thicker walls that can resist higher pressures.

Pipe standards define these wall thicknesses: SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS and XXS.
(S following a number is for stainless steel. Sizes without an S are for carbon steel.)

Higher schedules are heavier, require more material and are therefore more costly to make and install.

Why are fittings sometimes thicker and heavier than pipes to which they are connected?

Fittings are sometimes thicker than their connecting pipes to meet performance requirements or due to manufacturing reasons.
Due to fitting geometry, stress is very different when compared to a pipe. Using extra material is often necessary to compensate for such additional stress, especially for tees and tight curve elbows.
Fitting manufacturers may not always stock plates or pipes for all metal grades or sizes. When responding to an order, manufacturers always use the right metal or alloy, but sometimes made with next-higher available plate or pipe size while still respecting specified inside diameters.