UNDERSTANDING PIPE SIZES

Pipe sizes can be confusing. If you ask for a 4" Pipe, the actual outside diameter is 4.50". Why? It comes down to the history of "Nominal Pipe Size" (NPS) and how modern manufacturing standards evolved.

Standard Pipe Chart for reference.

---

1. The "14 Inch" Rule

The relationship between the name of the pipe (NPS) and the physical size (OD) changes at the 14-inch mark.

2. Understanding "Schedules"

Pipe sizes have two non-dimensional numbers: a Size (NPS) and a Schedule (SCH).

• The OD is constant. A 4" pipe will always be 4.500" wide on the outside, regardless of how thick it is.
• The Wall Thickness changes. As the Schedule number goes up (Sch 40 -> Sch 80), the wall gets thicker.
• The ID shrinks. Because the outside is fixed, the extra thickness is added to the inside, reducing the bore size.

3. Sch 40 vs. "Standard"

A common misconception is that "Schedule 40" and "Standard Wall" (STD) are always the same. They are not.

• Up to 10": Sch 40 and STD are identical.
• 12" and Larger: They diverge. For 12" pipe, Sch 40 is 0.406" wall, while Standard remains 0.375".

Always check the pipe chart if you are dealing with large diameter pipe.

4. IPS, XS, and XXS

You may see older specifications refer to Iron Pipe Size (IPS) terms. These correspond to modern schedules as follows: