Pipe Size Chart: Nominal vs Actual Sizes

The real outside and inside diameter of every common pipe size, for steel and PVC, copper and PEX, the threads that go with them, and how to identify a pipe with a tape measure or a piece of string.

Updated October 13, 2026

Cross-sections of 1/2, 3/4, 1 and 2 inch Schedule 40 pipe drawn to scale, each labeled with its nominal size, its actual outside diameter and its inside diameter

Pipe sizes are nominal: names, not measurements. A “1/2 inch” steel or PVC pipe measures 0.840 inch across the outside, and a “1 inch” pipe measures 1.315 inches. The outside diameter stays the same for each size so fittings always fit; the wall thickness, set by the schedule, changes the inside diameter.

Steel and PVC pipe size chart (Schedule 40 and 80)

Nominal size	Outside diameter	OD (mm)	ID, Sch 40	ID, Sch 80	Circumference
1/8 in	0.405 in	10.3	0.269 in	0.215 in	1 9/32 in
1/4 in	0.540 in	13.7	0.364 in	0.302 in	1 11/16 in
3/8 in	0.675 in	17.1	0.493 in	0.423 in	2 1/8 in
1/2 in	0.840 in	21.3	0.622 in	0.546 in	2 5/8 in
3/4 in	1.050 in	26.7	0.824 in	0.742 in	3 5/16 in
1 in	1.315 in	33.4	1.049 in	0.957 in	4 1/8 in
1 1/4 in	1.660 in	42.2	1.380 in	1.278 in	5 7/32 in
1 1/2 in	1.900 in	48.3	1.610 in	1.500 in	5 31/32 in
2 in	2.375 in	60.3	2.067 in	1.939 in	7 15/32 in
2 1/2 in	2.875 in	73.0	2.469 in	2.323 in	9 1/32 in
3 in	3.500 in	88.9	3.068 in	2.900 in	11 in
4 in	4.500 in	114.3	4.026 in	3.826 in	14 1/8 in
6 in	6.625 in	168.3	6.065 in	5.761 in	20 13/16 in
8 in	8.625 in	219.1	7.981 in	7.625 in	27 3/32 in
10 in	10.750 in	273.1	10.020 in	9.562 in	33 25/32 in
12 in	12.750 in	323.8	11.938 in	11.374 in	40 1/16 in

Outside diameters and wall thicknesses are from ASME B36.10, the standard for steel pipe; Schedule 40 and 80 PVC uses the same outside diameters. From 14 inches up, the nominal size equals the outside diameter.

Which schedule? Schedule 40 is the everyday pipe for water, drains (DWV) and gas lines. Schedule 80, often dark gray in PVC, has thicker walls for higher pressure and industrial use.

Copper tube and PEX sizes

Copper tube and PEX use copper tube size (CTS): the outside diameter is always the nominal size plus 1/8 inch. So they don’t fit steel or PVC fittings without an adapter.

Nominal size	Outside diameter	OD (mm)
1/4 in	3/8 in (0.375)	9.5
3/8 in	1/2 in (0.500)	12.7
1/2 in	5/8 in (0.625)	15.9
3/4 in	7/8 in (0.875)	22.2
1 in	1 1/8 in (1.125)	28.6
1 1/4 in	1 3/8 in (1.375)	34.9
1 1/2 in	1 5/8 in (1.625)	41.3
2 in	2 1/8 in (2.125)	54.0

Copper comes in three wall thicknesses with the same outside diameter: Type K (thickest, underground), Type L (standard for water lines) and Type M (thinnest, mostly for low-pressure heating). Fittings fit all three.

Pipe thread sizes (NPT)

US pipe threads are NPT (National Pipe Taper): the threads taper slightly so the joint tightens as it’s screwed together. They’re measured in threads per inch, like bolts, but they don’t match bolt threads.

Pipe size	Threads per inch
1/8 in	27
1/4 and 3/8 in	18
1/2 and 3/4 in	14
1 to 2 in	11 1/2
2 1/2 in and up	8

A 1/2 in NPT male thread measures about 0.840 in across, the pipe’s outside diameter, which is why people often mistake it for a 3/4 in fitting. Garden hoses use a different thread (GHT, 11 1/2 per inch on a 1 1/16 in diameter), so they don’t fit NPT fittings either.

How to identify a pipe

Measure around it with a string or a flexible tape, then find the circumference in the chart. This works even when you can’t reach the end of the pipe. If you don’t have a flexible tape, mark the string and measure it against the tape measure or a ruler.
Or measure the outside diameter with calipers or across the end with a ruler, and find the closest OD. About 7/8 in is 1/2 in steel or PVC; exactly 5/8 in is 1/2 in copper.
Look at the material. Copper and PEX use the copper tube column; steel, black iron, galvanized and PVC use the first chart.

To convert a measurement, use inches to mm or mm to inches.

More DIY charts: lumber sizes · rebar sizes · thread pitch · wire gauge.

Frequently asked questions

What is the actual size of 1/2 inch pipe?

1/2 inch steel or PVC pipe is 0.840 in (21.3 mm) across the outside. Schedule 40 has an inside diameter of 0.622 in. Half-inch copper and PEX are different: 0.625 in (5/8 in) outside.

Why is pipe size different from the actual size?

Pipe sizes are nominal. Around 1900, the size roughly matched the inside diameter of the thick-walled iron pipe of the time. When walls got thinner, makers kept the outside diameter the same so fittings still fit, and the inside grew instead. The name stuck even though no measurement matches it.

How do I find the size of a pipe?

Measure around it with a string or flexible tape, then look up the circumference: about 2 5/8 in is 1/2 in pipe, 3 5/16 in is 3/4 in pipe, and 4 1/8 in is 1 in pipe. Or measure the outside diameter and compare it with the chart.

Is Schedule 40 and Schedule 80 the same outside diameter?

Yes. For a given size, every schedule has the same outside diameter so they use the same fittings. Schedule 80 has a thicker wall, so its inside diameter is smaller and it handles more pressure.

Are PVC and steel pipe the same size?

Yes. Schedule 40 and 80 PVC pipe has the same outside diameters as steel pipe of the same nominal size. Copper tube and PEX use a different system, where the outside diameter is the nominal size plus 1/8 inch.