1/4 in schedule 40 pipe flow and volume calculator
1/4 in schedule 40 pipe has an outside diameter of 13.7 mm and a bore of 9.2 mm, a wall thickness of 2.25 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
- 1.581
- Inside diameter
- 9.20
- Wall thickness
- 2.250
- Bore area
- 66.48 mm2
- Water content per metre
- 0.0665
- Water in the run
- 1.66
- Weight of that water
- 1.66
- Friction head loss
- 9.068
- Pressure drop
- 88.930 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 9.2 = 9.2
- outside = 13.7 = 13.7
- Wall thickness = (13.7 - 9.2) / 2 = 2.25
- Bore area = pi * 9.2^2 / 4 = 66.4761
- area_m2 = 66.4761 / 1e6 = 0.000 066 476 1
- Water content per metre = 66.4761 / 1000 = 0.066 476 1
- Water in the run = 0.066 476 1 * 25 = 1.6619
- flow_m3s = 0.000 066 476 1 * 1.5 = 0.000 099 714 2
- Flow at that velocity = 0.000 099 714 2 * 60000 = 5.982 85
- Flow in US gallons per minute = 5.982 85 / 3.785411784 = 1.5805
- loss_per_m = 10.67 * (0.000 099 714 2)^1.852 / (140^1.852 * (9.2/1000)^4.87) = 0.362 732
- Friction head loss = 0.362 732 * 25 = 9.068 31
- Pressure drop = 9.068 31 * 1000 * g0 / 1000 = 88.9297
- Weight of that water = 1.6619 * 1000 / 1000 = 1.6619
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.
1/4 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 2.0 | 0.53 |
| 1 | 4.0 | 1.05 |
| 1.5 | 6.0 | 1.58 |
| 2 | 8.0 | 2.11 |
| 2.5 | 10.0 | 2.63 |
| 3 | 12.0 | 3.16 |
Frequently asked
What is the inside diameter of 1/4 in schedule 40 pipe?
9.2 mm, against an outside diameter of 13.7 mm.
How much water does 1/4 in pipe hold?
0.066 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.