Stainless Steel Pipe Sizing Guide: NPS, OD, and Wall Thickness Explained

Understanding Pipe Sizing Standards

Pipe sizing in the United States follows conventions that can be confusing, especially for engineers new to piping design. This guide explains the key standards and how to specify stainless steel pipe correctly.

Nominal Pipe Size (NPS)

Nominal Pipe Size is a dimensionless designator that identifies pipe by a number (e.g., NPS 2, NPS 4). It is **not** the actual measured diameter of the pipe.

Key points:

For NPS 1/8 through NPS 12, the NPS number is **not** the outside diameter

For NPS 14 and above, the NPS number **equals** the outside diameter in inches

The outside diameter is standardized for each NPS — it doesn't change with wall thickness

Outside Diameter (OD)

Common NPS sizes and their actual outside diameters:

Wall Thickness Schedules

The wall thickness of pipe is specified by "schedule" numbers. Common schedules include:

**Schedule 5S** — thin wall, used where pressure is low

**Schedule 10S** — standard for stainless steel in many applications

**Schedule 40S** — heavier wall, higher pressure rating

**Schedule 80S** — thick wall for high-pressure service

The "S" suffix indicates stainless steel schedules per ASME standards.

**Important:** For a given NPS, the outside diameter stays the same regardless of schedule. The wall thickness increases inward, reducing the inside diameter.

Tubing vs. Pipe

Stainless steel tubing is specified differently from pipe:

Tubing is ordered by actual OD and wall thickness (e.g., 1.000" OD x 0.065" wall), not by NPS and schedule.

How to Specify What You Need

When requesting a quote, provide:

**Material grade** (304, 304L, 316, or 316L)

**Size** (NPS + Schedule for pipe, or OD + Wall for tubing)

**Length** (random lengths, or cut to specific length)

**Quantity** (number of pieces or total footage)

**Specification** (ASTM A312 for pipe, ASTM A269 for tubing)

**End finish** (plain end, beveled, threaded)

MAYWELL Inventory and Manufacturing

We stock common sizes of 304 and 316 stainless steel tubing and pipe, and can manufacture custom sizes to your specifications. Contact us with your requirements for a prompt quotation.