PVF Reference Library · Measure & Identify

How to Measure Pipe Size (NPS vs Actual OD)

Nominal Pipe Size (NPS) is a name, not a measurement, so a 1-inch pipe does not measure 1 inch. This sheet explains why and shows how to confirm a pipe's size in the field.

Ref PV-0066ASME B36.10M

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

NPS names steel and many other pipes. For NPS 1/8 through 12, the outside diameter (OD) is fixed for each size and does not match the nominal number. For NPS 14 and larger, the OD equals the NPS in inches. The OD stays the same for every schedule of a given size; a heavier schedule makes the wall thicker and the inside smaller. For example, NPS 1 has an OD of 1.315 inches and NPS 2 has an OD of 2.375 inches. Full dimensions are in the Pipe Schedule & Dimension Chart (NPS 1/8 to 12). Copper tube and tubing use different sizing systems.

How to use it

Read the pipe marking first. It often gives NPS, schedule and specification.
Measure the OD with calipers across the pipe, square to the axis.
Or measure the circumference with a tape or string. OD = circumference ÷ π (3.1416).
Match the OD to the chart to find the NPS.
Find the schedule by measuring wall thickness at a cut end, or with an ultrasonic gauge, and compare with the chart.
Check the material. Copper, PVC, tubing and metric pipe follow other systems.

Measuring methods

Method	Tool	Good for	Watch for
Read marking	Eyes	Any marked pipe	Paint or insulation hiding it
Caliper OD	Calipers	Small and medium pipe	Hold square; avoid weld seams
Circumference	Tape or pi tape	Large pipe, installed pipe	Tape must wrap square
Wall at cut end	Calipers	Finding schedule	Burrs and bevels read wrong
Ultrasonic thickness	UT gauge	Installed pipe	Needs calibration and couplant
Thread gauge	Thread gauge	Threaded pipe	Thread matches NPS, not OD alone

Key points

Same NPS means same OD, whatever the schedule. Butt-weld fittings must also match the wall schedule.
Sizes from NPS 14 up have an OD equal to the NPS.
DN is the metric designation for the same sizes. See NPS to DN Conversion Chart.
Copper tube size names (Type K, L, M) do not match NPS ODs. Do not mix them.
PVC Schedule 40 and 80 pipe uses the same ODs as steel pipe of the same NPS.

Common mistakes

Measuring the ID and calling it the size.
Calling a 1.315-inch OD pipe "1-1/4 inch."
Assuming copper and steel of the same nominal size share an OD.
Measuring OD over a weld seam or a fitting.

Governing standards: ASME B36.10M (welded and seamless wrought steel pipe); ASME B36.19M (stainless steel pipe); ASTM D1785 (PVC Schedule 40, 80 and 120).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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