Schedule 40 vs Schedule 80 Pipe
Same outside diameter, thicker wall, smaller bore. Every size that publishes both schedules, with the wall, bore, weight and flow area penalty computed for each.
- Sizes with both
- 27 (NPS 1/8 – NPS 24)
- Typical weight penalty
- +45% for Sch 80
- Typical bore loss
- -13% flow area
- Same OD?
- Yes — always
Short answer. Schedule 80 is the thicker wall. Both schedules share the same outside diameter, so Schedule 80 buys pressure capacity by eating into the bore: across the 27 sizes that publish both, it adds 45% weight on average and gives up 13% of the flow area. Use Schedule 40 for ordinary service and Schedule 80 where pressure, corrosion allowance or mechanical abuse demands the metal.
The outside diameter never changes
This is the single fact that makes schedules work. NPS 4 pipe is 4.500 in (114.3 mm) on the outside in every schedule from 5 to 160. Schedule is a wall thickness series, so moving up a schedule pushes the wall inward: the fittings, flanges and supports that clamp the outside are unaffected, and the bore shrinks.
At NPS 4 the wall goes from 0.237 in (6.0 mm) to 0.337 in (8.6 mm) — 42% more metal — and the bore falls from 4.026 in (102.3 mm) to 3.826 in (97.2 mm). Flow area drops from 12.73 in² to 11.50 in², a loss of 10%. Pumping the same volume through it costs more head.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | OD | Sch 40 wall | Sch 80 wall | Sch 40 bore | Sch 80 bore | Sch 40 weight | Sch 80 weight | Weight change | Flow area change |
|---|---|---|---|---|---|---|---|---|---|
| NPS 1/8 | 0.40510.29 mm | 0.0681.73 mm | 0.0952.41 mm | 0.2696.83 mm | 0.2155.46 mm | 0.240.36 kg/m | 0.310.47 kg/m | +29% | -36% |
| NPS 1/4 | 0.54013.72 mm | 0.0882.24 mm | 0.1193.02 mm | 0.3649.25 mm | 0.3027.67 mm | 0.420.63 kg/m | 0.540.80 kg/m | +26% | -31% |
| NPS 3/8 | 0.67517.14 mm | 0.0912.31 mm | 0.1263.20 mm | 0.49312.52 mm | 0.42310.74 mm | 0.570.84 kg/m | 0.741.10 kg/m | +30% | -26% |
| NPS 1/2 | 0.84021.34 mm | 0.1092.77 mm | 0.1473.73 mm | 0.62215.80 mm | 0.54613.87 mm | 0.851.27 kg/m | 1.091.62 kg/m | +28% | -23% |
| NPS 3/4 | 1.05026.67 mm | 0.1132.87 mm | 0.1543.91 mm | 0.82420.93 mm | 0.74218.85 mm | 1.131.68 kg/m | 1.472.19 kg/m | +30% | -19% |
| NPS 1 | 1.31533.40 mm | 0.1333.38 mm | 0.1794.55 mm | 1.04926.64 mm | 0.95724.31 mm | 1.682.50 kg/m | 2.173.23 kg/m | +29% | -17% |
| NPS 1 1/4 | 1.66042.16 mm | 0.1403.56 mm | 0.1914.85 mm | 1.38035.05 mm | 1.27832.46 mm | 2.273.38 kg/m | 3.004.46 kg/m | +32% | -14% |
| NPS 1 1/2 | 1.90048.26 mm | 0.1453.68 mm | 0.2005.08 mm | 1.61040.89 mm | 1.50038.10 mm | 2.724.04 kg/m | 3.635.40 kg/m | +34% | -13% |
| NPS 2 | 2.37560.32 mm | 0.1543.91 mm | 0.2185.54 mm | 2.06752.50 mm | 1.93949.25 mm | 3.655.44 kg/m | 5.027.47 kg/m | +37% | -12% |
| NPS 2 1/2 | 2.87573.02 mm | 0.2035.16 mm | 0.2767.01 mm | 2.46962.71 mm | 2.32359.00 mm | 5.798.62 kg/m | 7.6611.40 kg/m | +32% | -11% |
| NPS 3 | 3.50088.90 mm | 0.2165.49 mm | 0.3007.62 mm | 3.06877.93 mm | 2.90073.66 mm | 7.5811.27 kg/m | 10.2515.26 kg/m | +35% | -11% |
| NPS 3 1/2 | 4.000101.60 mm | 0.2265.74 mm | 0.3188.08 mm | 3.54890.12 mm | 3.36485.45 mm | 9.1113.56 kg/m | 12.5118.61 kg/m | +37% | -10% |
| NPS 4 | 4.500114.30 mm | 0.2376.02 mm | 0.3378.56 mm | 4.026102.26 mm | 3.82697.18 mm | 10.7916.06 kg/m | 14.9822.30 kg/m | +39% | -10% |
| NPS 4 1/2 | 5.000127.00 mm | 0.2476.27 mm | 0.3559.02 mm | 4.506114.45 mm | 4.290108.97 mm | 12.5418.66 kg/m | 17.6126.21 kg/m | +40% | -9% |
| NPS 5 | 5.563141.30 mm | 0.2586.55 mm | 0.3759.52 mm | 5.047128.19 mm | 4.813122.25 mm | 14.6221.75 kg/m | 20.7830.92 kg/m | +42% | -9% |
| NPS 6 | 6.625168.27 mm | 0.2807.11 mm | 0.43210.97 mm | 6.065154.05 mm | 5.761146.33 mm | 18.9728.24 kg/m | 28.5742.52 kg/m | +51% | -10% |
| NPS 7 | 7.625193.67 mm | 0.3017.65 mm | 0.50012.70 mm | 7.023178.38 mm | 6.625168.27 mm | 23.5435.04 kg/m | 38.0556.62 kg/m | +62% | -11% |
| NPS 8 | 8.625219.07 mm | 0.3228.18 mm | 0.50012.70 mm | 7.981202.72 mm | 7.625193.67 mm | 28.5542.49 kg/m | 43.3964.57 kg/m | +52% | -9% |
| NPS 9 | 9.625244.47 mm | 0.3428.69 mm | 0.50012.70 mm | 8.941227.10 mm | 8.625219.07 mm | 33.9150.46 kg/m | 48.7372.52 kg/m | +44% | -7% |
| NPS 10 | 10.750273.05 mm | 0.3659.27 mm | 0.59415.09 mm | 10.020254.51 mm | 9.562242.87 mm | 40.4860.25 kg/m | 64.4395.88 kg/m | +59% | -9% |
| NPS 11 | 11.750298.45 mm | 0.3759.52 mm | 0.50012.70 mm | 11.000279.40 mm | 10.750273.05 mm | 45.5667.80 kg/m | 60.0889.40 kg/m | +32% | -4% |
| NPS 12 | 12.750323.85 mm | 0.40610.31 mm | 0.68817.48 mm | 11.938303.23 mm | 11.374288.90 mm | 53.5379.65 kg/m | 88.63131.90 kg/m | +66% | -9% |
| NPS 14 | 14.000355.60 mm | 0.43811.13 mm | 0.75019.05 mm | 13.124333.35 mm | 12.500317.50 mm | 63.4494.41 kg/m | 106.13157.95 kg/m | +67% | -9% |
| NPS 16 | 16.000406.40 mm | 0.50012.70 mm | 0.84421.44 mm | 15.000381.00 mm | 14.312363.52 mm | 82.77123.18 kg/m | 136.62203.31 kg/m | +65% | -9% |
| NPS 18 | 18.000457.20 mm | 0.56214.27 mm | 0.93823.83 mm | 16.876428.65 mm | 16.124409.55 mm | 104.67155.76 kg/m | 170.93254.37 kg/m | +63% | -9% |
| NPS 20 | 20.000508.00 mm | 0.59415.09 mm | 1.03126.19 mm | 18.812477.82 mm | 17.938455.63 mm | 123.11183.21 kg/m | 208.87310.84 kg/m | +70% | -9% |
| NPS 24 | 24.000609.60 mm | 0.68817.48 mm | 1.21930.96 mm | 22.624574.65 mm | 21.562547.67 mm | 171.30254.92 kg/m | 296.59441.37 kg/m | +73% | -9% |
Bore, weight and flow area are derived from the published outside diameter and wall thickness. Weight is bare steel pipe: w = 10.6802 × t × (OD − t).
Pressure capacity, roughly
Barlow's formula puts the burst-limited pressure in proportion to wall thickness over diameter, so at the same allowable stress Schedule 80 carries close to the wall-thickness ratio more pressure. At NPS 4 that ratio is 1.42, so the Schedule 80 pipe is good for roughly 42% more internal pressure than the Schedule 40 pipe in the same material. The exact number needs the design code — see the wall thickness calculation guide.
Choose Schedule 40 when
- Pressure is modest and the line is not cyclic
- Flow area matters — pumping cost follows the bore
- The line is above ground, protected and inspectable
- Cost and weight drive the design; Sch 40 is the stocked default
- Threading is not required above NPS 2
Choose Schedule 80 when
- Design pressure exceeds what Schedule 40 allows
- The service is corrosive or erosive and needs a corrosion allowance
- The pipe will be threaded — threads cut into the wall, and many codes will not permit threading Schedule 40 in small bore
- The line is buried, in a rack subject to impact, or in a vibration-prone service
- Steam, compressed gas or any stored-energy service
Where STD and XS come in
Standard weight and Extra Strong are the older designations. Through NPS 10, STD and Schedule 40 are the same wall; through NPS 8, XS and Schedule 80 are the same wall. Above those sizes the two series split — STD freezes at 0.375 in and XS at 0.500 in while the numbered schedules keep climbing. The STD vs XS comparison works through where each pairing breaks.
Both schedules, size by size
Full tables: Schedule 40 dimensions · Schedule 80 dimensions · all 14 schedules · what a schedule actually is.
Common questions
Is Schedule 80 pipe bigger than Schedule 40?
No — the outside diameter is identical. Schedule 80 has a thicker wall, so it is heavier and has a smaller bore. NPS 4 is 4.500 in on the outside in both.
Can Schedule 40 and Schedule 80 pipe be welded together?
Yes, but the bore mismatch must be handled. ASME B31.3 requires the inside of the thicker component to be tapered no steeper than 30° to the thinner bore when the mismatch exceeds the allowable, so the joint is counterbored and back-bevelled rather than butted square.
How much heavier is Schedule 80 than Schedule 40?
Between 26% and 73% depending on size, averaging about 45% across the 27 sizes that publish both. The penalty is largest in small bore, where the wall is a bigger fraction of the diameter.
Does Schedule 80 hold twice the pressure of Schedule 40?
No. Pressure capacity scales roughly with wall thickness, not with the schedule number. Schedule 80 is typically 40–50% thicker than Schedule 40, so it carries roughly 40–50% more pressure in the same material — not double.
Which schedule is standard for water piping?
Schedule 40 in nearly all cases. Schedule 80 appears in water service mainly where the pipe is threaded, buried under load, or exposed to impact.