Pipe sizes

22 pipe sizes with the NPS label, the DN equivalent, and the diameter the pipe actually is.

A half-inch pipe is 0.84 inches across

NPS is a label, not a measurement. NPS 1/2 pipe has an outside diameter of 21.3 mm — 0.839 inches, nearly double the name, and a bore of 15.8 mm in schedule 40. The number matches neither.

The reason is historical and the same shape as a 2×4. NPS came from wrought-iron pipe whose much thicker walls gave roughly a half-inch bore at that outside diameter. When walls got thinner the outside diameter had to stay fixed, because threads and fittings were already standardised on it — so the label froze while the pipe behind it changed.

Above NPS 14 the number becomes real

This is the part almost nobody knows. From NPS 14 upward, the NPS number is the outside diameter in inches, exactly. NPS 14 pipe is 14.000 inches across; NPS 12, one size down, is 12.748. The scale changes from a name to a measurement between those two sizes and nothing on a chart marks the transition.

DN is just as nominal

The metric designation is DN, and it is a translation of the same labels rather than a fix for them. DN 15 is NPS 1/2 and is still 21.3 mm across, not 15. Where a drawing gives DN, it is naming the same pipe by a different convention — not measuring it.

The full chart

NPS	DN	OD (mm)	OD (in)	Sch 40 wall	Sch 40 bore
1/8	6	10.3	0.406	1.73 mm	6.8 mm
1/4	8	13.7	0.539	2.24 mm	9.2 mm
3/8	10	17.1	0.673	2.31 mm	12.5 mm
1/2	15	21.3	0.839	2.77 mm	15.8 mm
3/4	20	26.7	1.051	2.87 mm	21.0 mm
1	25	33.4	1.315	3.38 mm	26.6 mm
1-1/4	32	42.2	1.661	3.56 mm	35.1 mm
1-1/2	40	48.3	1.902	3.68 mm	40.9 mm
2	50	60.3	2.374	3.91 mm	52.5 mm
2-1/2	65	73	2.874	5.16 mm	62.7 mm
3	80	88.9	3.500	5.49 mm	77.9 mm
4	100	114.3	4.500	6.02 mm	102.3 mm
5	125	141.3	5.563	6.55 mm	128.2 mm
6	150	168.3	6.626	7.11 mm	154.1 mm
8	200	219.1	8.626	8.18 mm	202.7 mm
10	250	273	10.748	9.27 mm	254.5 mm
12	300	323.8	12.748	10.31 mm	303.2 mm
14	350	355.6	14.000	11.13 mm	333.3 mm
16	400	406.4	16.000	12.7 mm	381.0 mm
18	450	457.2	18.000	14.27 mm	428.7 mm
20	500	508	20.000	15.09 mm	477.8 mm
24	600	610	24.016	17.48 mm	575.0 mm

Bold rows are the sizes where NPS equals the outside diameter.

Outside diameter is what fittings are made for

A schedule number sets the wall thickness, and therefore the bore, for a given NPS. The outside diameter does not change with it. That is the whole point of the system: a fitting cut for NPS 2 threads onto schedule 10, 40 or 80 pipe alike, because they are all 60.3 mm on the outside and differ only inside.

So when a size is being checked against a fitting, measure the outside. When it is being checked against a flow rate, the bore is the number, and the schedule has to be known before it can be looked up.

Metric threads · Lumber sizes — the same nominal-versus-actual problem in timber. Unit converters