What does pipe schedule mean?

It is the series that sets the wall of a pipe in each size: schedule 10, 40, 80, 160. The higher the number, the thicker the wall for the same outside diameter. STD, XS and XXS are the older weights -- standard, extra strong and double extra strong --, and the S schedules, 5S to 80S, are those of stainless pipe.

Is 2 inch pipe 2 inches across?

No: the size is nominal. A 2 inch pipe (DN 50) has a 2.375 inch -- 60.3 mm -- outside diameter in any schedule. From 14 inch up the outside diameter is the size itself: a 14 inch pipe is 14 inches across.

Are schedule 40 and STD the same?

Up to 10 inch, yes: the same wall. From 12 inch up STD stays at 3/8 inch (0.375) and schedule 40 gets thicker -- 0.406 inch at 12 inch. The same goes for schedule 80 and XS up to 8 inch.

Pipe schedule chart: OD and wall thickness (ASME B36.10M and B36.19M)

Steel pipe is ordered by nominal size -- NPS, in inches, or DN -- and by schedule, which sets the wall. The outside diameter is the same in every schedule of a size, and the wall grows with the number. The chart gives the wall in mm; the page of each size and each schedule also gives the inside diameter and the weight.

Outside diameter and wall thickness in mm, by size and schedule
Size	DN	OD	5	5S	10	10S	20	30	40	STD	40S	60	80	XS	80S	100	120	140	160	XXS
1/8 inch	6	10.3			1.24	1.24		1.45	1.73	1.73	1.73		2.41	2.41	2.41				3.15	4.83
1/4 inch	8	13.7			1.65	1.65		1.85	2.24	2.24	2.24		3.02	3.02	3.02				3.68	6.05
3/8 inch	10	17.1			1.65	1.65		1.85	2.31	2.31	2.31		3.20	3.20	3.20				4.01	6.40
1/2 inch	15	21.3	1.65	1.65	2.11	2.11		2.41	2.77	2.77	2.77		3.73	3.73	3.73				4.78	7.47
3/4 inch	20	26.7	1.65	1.65	2.11	2.11		2.41	2.87	2.87	2.87		3.91	3.91	3.91				5.56	7.82
1 inch	25	33.4	1.65	1.65	2.77	2.77		2.90	3.38	3.38	3.38		4.55	4.55	4.55				6.35	9.09
1-1/4 inch	32	42.2	1.65	1.65	2.77	2.77		2.97	3.56	3.56	3.56		4.85	4.85	4.85				6.35	9.70
1-1/2 inch	40	48.3	1.65	1.65	2.77	2.77		3.18	3.68	3.68	3.68		5.08	5.08	5.08				7.14	10.15
2 inch	50	60.3	1.65	1.65	2.77	2.77		3.18	3.91	3.91	3.91		5.54	5.54	5.54				8.74	11.07
2-1/2 inch	65	73.0	2.11	2.11	3.05	3.05		4.78	5.16	5.16	5.16		7.01	7.01	7.01				9.53	14.02
3 inch	80	88.9	2.11	2.11	3.05	3.05		4.78	5.49	5.49	5.49		7.62	7.62	7.62				11.13	15.24
3-1/2 inch	90	101.6	2.11	2.11	3.05	3.05		4.78	5.74	5.74	5.74		8.08	8.08	8.08
4 inch	100	114.3	2.11	2.11	3.05	3.05		4.78	6.02	6.02	6.02		8.56	8.56	8.56		11.13		13.49	17.12
5 inch	125	141.3	2.77	2.77	3.40	3.40			6.55	6.55	6.55		9.53	9.53	9.53		12.70		15.88	19.05
6 inch	150	168.3	2.77	2.77	3.40	3.40			7.11	7.11	7.11		10.97	10.97	10.97		14.27		18.26	21.95
8 inch	200	219.1	2.77	2.77	3.76	3.76	6.35	7.04	8.18	8.18	8.18	10.31	12.70	12.70	12.70	15.09	18.26	20.62	23.01	22.23
10 inch	250	273.0	3.40	3.40	4.19	4.19	6.35	7.80	9.27	9.27	9.27	12.70	15.09	12.70	12.70	18.26	21.44	25.40	28.58	25.40
12 inch	300	323.8	3.96	3.96	4.57	4.57	6.35	8.38	10.31	9.53	9.53	14.27	17.48	12.70	12.70	21.44	25.40	28.58	33.32	25.40
14 inch	350	355.6	3.96	3.96	6.35	4.78	7.92	9.53	11.13	9.53	9.53	15.09	19.05	12.70	12.70	23.83	27.79	31.75	35.71
16 inch	400	406.4	4.19	4.19	6.35	4.78	7.92	9.53	12.70	9.53	9.53	16.66	21.44	12.70	12.70	26.19	30.96	36.53	40.49
18 inch	450	457	4.19	4.19	6.35	4.78	7.92	11.13	14.27	9.53	9.53	19.05	23.83	12.70	12.70	29.36	34.93	39.67	45.24
20 inch	500	508	4.78	4.78	6.35	5.54	9.53	12.70	15.09	9.53	9.53	20.62	26.19	12.70	12.70	32.54	38.10	44.45	50.01
22 inch	550	559	4.78	4.78	6.35	5.54	9.53	12.70		9.53		22.23	28.58	12.70		34.93	41.28	47.63	53.98
24 inch	600	610	5.54	5.54	6.35	6.35	9.53	14.27	17.48	9.53	9.53	24.61	30.96	12.70	12.70	38.89	46.02	52.37	59.54
26 inch	650	660			7.92		12.70			9.53				12.70
28 inch	700	711			7.92		12.70	15.88		9.53				12.70
30 inch	750	762	6.35	6.35	7.92	7.92	12.70	15.88		9.53				12.70
32 inch	800	813			7.92		12.70	15.88	17.48	9.53				12.70
34 inch	850	864			7.92		12.70	15.88	17.48	9.53				12.70
36 inch	900	914			7.92		12.70	15.88	19.05	9.53				12.70

The figures are those of ASME B36.10M (carbon steel pipe: the numbered schedules and the STD, XS and XXS weights) and B36.19M (stainless pipe: the S schedules), 2018 edition. Up to 10 inch, schedule 40 is STD; up to 8 inch, schedule 80 is XS.

What about schedule 40 PVC pipe?

ASTM D1785 schedule 40 and 80 PVC pipe has the same outside diameter as steel pipe in every size, and the same schedule 40 wall from 1/8 to 12 inch, 16 and 18 inch -- elsewhere the difference is a thousandth of an inch. Two caveats: the PVC wall is a minimum while the steel wall is nominal, and the weights in these charts are for steel.

By schedule

Questions

What does pipe schedule mean?: It is the series that sets the wall of a pipe in each size: schedule 10, 40, 80, 160. The higher the number, the thicker the wall for the same outside diameter. STD, XS and XXS are the older weights -- standard, extra strong and double extra strong --, and the S schedules, 5S to 80S, are those of stainless pipe.
Is 2 inch pipe 2 inches across?: No: the size is nominal. A 2 inch pipe (DN 50) has a 2.375 inch -- 60.3 mm -- outside diameter in any schedule. From 14 inch up the outside diameter is the size itself: a 14 inch pipe is 14 inches across.
Are schedule 40 and STD the same?: Up to 10 inch, yes: the same wall. From 12 inch up STD stays at 3/8 inch (0.375) and schedule 40 gets thicker -- 0.406 inch at 12 inch. The same goes for schedule 80 and XS up to 8 inch.