1/8 in schedule 40 pipe flow and volume calculator
1/8 in schedule 40 pipe has an outside diameter of 10.3 mm and a bore of 6.8 mm, a wall thickness of 1.75 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
- 0.863
- Inside diameter
- 6.80
- Wall thickness
- 1.750
- Bore area
- 36.32 mm2
- Water content per metre
- 0.0363
- Water in the run
- 0.91
- Weight of that water
- 0.91
- Friction head loss
- 12.900
- Pressure drop
- 126.508 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 6.8 = 6.8
- outside = 10.3 = 10.3
- Wall thickness = (10.3 - 6.8) / 2 = 1.75
- Bore area = pi * 6.8^2 / 4 = 36.3168
- area_m2 = 36.3168 / 1e6 = 0.000 036 316 8
- Water content per metre = 36.3168 / 1000 = 0.036 316 8
- Water in the run = 0.036 316 8 * 25 = 0.907 92
- flow_m3s = 0.000 036 316 8 * 1.5 = 0.000 054 475 2
- Flow at that velocity = 0.000 054 475 2 * 60000 = 3.268 51
- Flow in US gallons per minute = 3.268 51 / 3.785411784 = 0.863 45
- loss_per_m = 10.67 * (0.000 054 475 2)^1.852 / (140^1.852 * (6.8/1000)^4.87) = 0.516 009
- Friction head loss = 0.516 009 * 25 = 12.9002
- Pressure drop = 12.9002 * 1000 * g0 / 1000 = 126.508
- Weight of that water = 0.907 92 * 1000 / 1000 = 0.907 92
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.
1/8 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 1.1 | 0.29 |
| 1 | 2.2 | 0.58 |
| 1.5 | 3.3 | 0.86 |
| 2 | 4.4 | 1.15 |
| 2.5 | 5.4 | 1.44 |
| 3 | 6.5 | 1.73 |
Frequently asked
What is the inside diameter of 1/8 in schedule 40 pipe?
6.8 mm, against an outside diameter of 10.3 mm.
How much water does 1/8 in pipe hold?
0.036 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.