What is the difference between schedule 40 and schedule 80 pipe?

The wall, and nothing else about the outside. At the same nominal size both have the same outside diameter, so they take the same fittings and look identical from outside. Schedule 80 has the thicker wall, which means a smaller bore and more weight. At NPS 2 both measure 2.375 in across; schedule 40 is 0.154 in thick and schedule 80 is 0.218 in.

Is schedule 80 always the same amount thicker than schedule 40?

No, and this is the most commonly repeated error about the two. Below NPS 4 the difference stays in a narrow band, 35% to 42%. From NPS 6 up it steps into a different range entirely, 54% to 77%, reaching 77% at NPS 24. It is not a smooth progression either — the two are separate wall tables, not one series scaled — so any rule of thumb that puts a single percentage on the pair is wrong at one end of the range or the other.

How much smaller is the bore of schedule 80?

At NPS 2 the bore drops from 2.067 in to 1.939 in, a loss of 0.128 in. At NPS 24 it drops from 22.624 in to 21.562 in, a loss of 1.062 in. The outside diameter is unchanged in both cases.

How much heavier is schedule 80 than schedule 40?

Between 26% and 73%, depending on the size. At NPS 2 it is 3.66 lb/ft against 5.03 lb/ft. Every mass here is computed from the standard's own formula rather than copied from a published column.

Do schedule 40 and schedule 80 use the same fittings?

They share an outside diameter at every one of the 23 sizes that carry both, so anything that locates on the outside — a socket weld, a slip-on flange, a clamp — fits either. What changes is the bore, so a butt weld between the two leaves a step inside the joint.

Does every pipe size come in both schedule 40 and schedule 80?

No. ASME B36.10M-2015 lists both at 23 sizes. Schedule 40 appears without schedule 80 at NPS 32, 34, 36. Schedule 80 appears without schedule 40 at NPS 22. Outside that range the standard names other walls instead.

Schedule 40 vs Schedule 80

The same pipe from outside and a different pipe inside. Both share an OD at every size, so the schedule changes only the wall, the bore and the weight — and it does not change them by a fixed proportion. Below NPS 4 schedule 80 runs 35%–42% thicker; from NPS 6 up it runs 54%–77% thicker.

What Changes and What Does Not

A schedule number specifies wall thickness. It says nothing about the outside diameter, which is fixed by the nominal size alone and is identical across every schedule at that size. This is checkable against the data rather than asserted: at all 23 sizes carrying both schedules, ASME B36.10M-2015 publishes one outside diameter and two walls.

Outside diameter: Unchanged. At NPS 2 both are 2.375 in (60.30 mm).
Wall thickness: Thicker on schedule 80 everywhere the two meet. At NPS 2, 0.154 in against 0.218 in.
Bore: Smaller on schedule 80, by twice the wall difference. At NPS 2 the bore falls from 2.067 in to 1.939 in. Bore is derived here as OD minus twice the wall, not copied.
Weight: Higher on schedule 80. At NPS 2, 3.66 lb/ft against 5.03 lb/ft.

The Gap Is Not a Constant Percentage

Schedule 80 is often described as though it were a fixed step up from schedule 40. It is not. The two are separate wall tables rather than one series scaled, and the distance between them is not even a smooth function of size — it is 40% at NPS 1/8, dips to 35% at NPS 1, and only then climbs.

What does hold is a split between the small sizes and the large ones. Below NPS 4 the gap stays inside a band of 35% to 42%. From NPS 6 upward it sits in a band that does not overlap it at all, 54% to 77%, topping out at 77% at NPS 24. A single rule of thumb misprices one end of that or the other.

The same is true of weight, which follows from the wall. Schedule 80 is 26% heavier at the narrowest point and 73% heavier at the widest. For the mass formula these figures come from, and why it is computed rather than quoted, see pipe weight per foot.

Every Size Compared

Schedule 40 against schedule 80, ASME B36.10M-2015 — 23 sizes carry both. The outside diameter column is shared because there is only ever one.
NPS	OD (in)	Schedule 40		Schedule 80		Wall difference (in)	80 thicker by
NPS	OD (in)	wall (in)	bore (in)	wall (in)	bore (in)	Wall difference (in)	80 thicker by
⅛	0.405	0.068	0.269	0.095	0.215	0.027	40%
¼	0.540	0.088	0.364	0.119	0.302	0.031	35%
⅜	0.675	0.091	0.493	0.126	0.423	0.035	38%
½	0.840	0.109	0.622	0.147	0.546	0.038	35%
¾	1.050	0.113	0.824	0.154	0.742	0.041	36%
1	1.315	0.133	1.049	0.179	0.957	0.046	35%
1 ¼	1.660	0.140	1.380	0.191	1.278	0.051	36%
1 ½	1.900	0.145	1.610	0.200	1.500	0.055	38%
2	2.375	0.154	2.067	0.218	1.939	0.064	42%
2 ½	2.875	0.203	2.469	0.276	2.323	0.073	36%
3	3.500	0.216	3.068	0.300	2.900	0.084	39%
3 ½	4.000	0.226	3.548	0.318	3.364	0.092	41%
4	4.500	0.237	4.026	0.337	3.826	0.100	42%
5	5.563	0.258	5.047	0.375	4.813	0.117	45%
6	6.625	0.280	6.065	0.432	5.761	0.152	54%
8	8.625	0.322	7.981	0.500	7.625	0.178	55%
10	10.750	0.365	10.020	0.594	9.562	0.229	63%
12	12.750	0.406	11.938	0.688	11.374	0.282	69%
14	14.000	0.438	13.124	0.750	12.500	0.312	71%
16	16.000	0.500	15.000	0.844	14.312	0.344	69%
18	18.000	0.562	16.876	0.938	16.124	0.376	67%
20	20.000	0.594	18.812	1.031	17.938	0.437	74%
22	22.000	—	—	1.125	19.750	—	—
24	24.000	0.688	22.624	1.219	21.562	0.531	77%
32	32.000	0.688	30.624	—	—	—	—
34	34.000	0.688	32.624	—	—	—	—
36	36.000	0.750	34.500	—	—	—	—

Showing ASME B36.10M-2015. 766 of 772 rows verified against the standard itself; every figure is checked against the standard’s own mass formula. How this data was verified.

Where Only One of the Two Exists

The pairing is not universal. ASME B36.10M-2015 publishes both schedules at 23 sizes, from NPS 1/8 to NPS 24. Above that, schedule 40 continues alone at NPS 32, 34, 36, where the standard names no schedule 80 wall at all. Schedule 80 appears without a schedule 40 counterpart at NPS 22. A chart that prints a value in every cell of both columns has filled in something the standard does not publish.

The naming has a second trap in it. Schedule 40 and STD are the same wall only up to a point, and so are schedule 80 and XS — above their break sizes the older names freeze while the numbered schedules keep climbing. That divergence is set out in STD vs schedule 40. For what the schedule number specifies in the first place, see how the pipe schedule system works, and for the full table at one schedule, schedule 40 pipe dimensions.

Frequently Asked Questions

What is the difference between schedule 40 and schedule 80 pipe?: The wall, and nothing else about the outside. At the same nominal size both have the same outside diameter, so they take the same fittings and look identical from outside. Schedule 80 has the thicker wall, which means a smaller bore and more weight. At NPS 2 both measure 2.375 in across; schedule 40 is 0.154 in thick and schedule 80 is 0.218 in.
Is schedule 80 always the same amount thicker than schedule 40?: No, and this is the most commonly repeated error about the two. Below NPS 4 the difference stays in a narrow band, 35% to 42%. From NPS 6 up it steps into a different range entirely, 54% to 77%, reaching 77% at NPS 24. It is not a smooth progression either — the two are separate wall tables, not one series scaled — so any rule of thumb that puts a single percentage on the pair is wrong at one end of the range or the other.
How much smaller is the bore of schedule 80?: At NPS 2 the bore drops from 2.067 in to 1.939 in, a loss of 0.128 in. At NPS 24 it drops from 22.624 in to 21.562 in, a loss of 1.062 in. The outside diameter is unchanged in both cases.
How much heavier is schedule 80 than schedule 40?: Between 26% and 73%, depending on the size. At NPS 2 it is 3.66 lb/ft against 5.03 lb/ft. Every mass here is computed from the standard's own formula rather than copied from a published column.
Do schedule 40 and schedule 80 use the same fittings?: They share an outside diameter at every one of the 23 sizes that carry both, so anything that locates on the outside — a socket weld, a slip-on flange, a clamp — fits either. What changes is the bore, so a butt weld between the two leaves a step inside the joint.
Does every pipe size come in both schedule 40 and schedule 80?: No. ASME B36.10M-2015 lists both at 23 sizes. Schedule 40 appears without schedule 80 at NPS 32, 34, 36. Schedule 80 appears without schedule 40 at NPS 22. Outside that range the standard names other walls instead.