3/8 in schedule 40 pipe flow and volume calculator
3/8 in schedule 40 pipe has an outside diameter of 17.1 mm and a bore of 12.5 mm, a wall thickness of 2.30 mm. Nominal pipe size is a label, not a measurement — nothing about a 1-inch pipe is one inch.
Domestic water is usually kept between 1 and 2.4 m/s to limit noise and erosion.
Copper and plastic about 140; new steel 130; old steel 100.
- Flow in US gallons per minute
- 2.918
- Inside diameter
- 12.50
- Wall thickness
- 2.300
- Bore area
- 122.72 mm2
- Water content per metre
- 0.1227
- Water in the run
- 3.07
- Weight of that water
- 3.07
- Friction head loss
- 6.343
- Pressure drop
- 62.205 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 12.5 = 12.5
- outside = 17.1 = 17.1
- Wall thickness = (17.1 - 12.5) / 2 = 2.3
- Bore area = pi * 12.5^2 / 4 = 122.718
- area_m2 = 122.718 / 1e6 = 0.000 122 718
- Water content per metre = 122.718 / 1000 = 0.122 718
- Water in the run = 0.122 718 * 25 = 3.067 96
- flow_m3s = 0.000 122 718 * 1.5 = 0.000 184 078
- Flow at that velocity = 0.000 184 078 * 60000 = 11.0447
- Flow in US gallons per minute = 11.0447 / 3.785411784 = 2.917 69
- loss_per_m = 10.67 * (0.000 184 078)^1.852 / (140^1.852 * (12.5/1000)^4.87) = 0.253 726
- Friction head loss = 0.253 726 * 25 = 6.343 16
- Pressure drop = 6.343 16 * 1000 * g0 / 1000 = 62.2052
- Weight of that water = 3.067 96 * 1000 / 1000 = 3.067 96
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- bore area = pi d^2 / 4
- flow = area x velocity
- Hazen-Williams head loss per metre = 10.67 Q^1.852 / (C^1.852 d^4.87), SI units
Nominal pipe size is a legacy label. Above 14 inch NPS the outside diameter equals the nominal size; below it, neither dimension matches.
Hazen-Williams is an empirical formula fitted to water near room temperature. For other fluids or for gases, use Darcy-Weisbach.
Fittings add loss too. A rule of thumb is to add 30 to 50% to the straight-run figure for a typical domestic circuit.
3/8 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 3.7 | 0.97 |
| 1 | 7.4 | 1.95 |
| 1.5 | 11.0 | 2.92 |
| 2 | 14.7 | 3.89 |
| 2.5 | 18.4 | 4.86 |
| 3 | 22.1 | 5.84 |
Frequently asked
What is the inside diameter of 3/8 in schedule 40 pipe?
12.5 mm, against an outside diameter of 17.1 mm.
How much water does 3/8 in pipe hold?
0.123 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.