1 1/4 in schedule 40 pipe flow and volume calculator
1 1/4 in schedule 40 pipe has an outside diameter of 42.2 mm and a bore of 35.1 mm, a wall thickness of 3.55 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
- 23.006
- Inside diameter
- 35.10
- Wall thickness
- 3.550
- Bore area
- 967.62 mm2
- Water content per metre
- 0.9676
- Water in the run
- 24.19
- Weight of that water
- 24.19
- Friction head loss
- 1.903
- Pressure drop
- 18.664 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 35.1 = 35.1
- outside = 42.2 = 42.2
- Wall thickness = (42.2 - 35.1) / 2 = 3.55
- Bore area = pi * 35.1^2 / 4 = 967.618
- area_m2 = 967.618 / 1e6 = 0.000 967 618
- Water content per metre = 967.618 / 1000 = 0.967 618
- Water in the run = 0.967 618 * 25 = 24.1905
- flow_m3s = 0.000 967 618 * 1.5 = 0.001 451 43
- Flow at that velocity = 0.001 451 43 * 60000 = 87.0857
- Flow in US gallons per minute = 87.0857 / 3.785411784 = 23.0056
- loss_per_m = 10.67 * (0.001 451 43)^1.852 / (140^1.852 * (35.1/1000)^4.87) = 0.076 126 3
- Friction head loss = 0.076 126 3 * 25 = 1.903 16
- Pressure drop = 1.903 16 * 1000 * g0 / 1000 = 18.6636
- Weight of that water = 24.1905 * 1000 / 1000 = 24.1905
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 1/4 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 29.0 | 7.67 |
| 1 | 58.1 | 15.34 |
| 1.5 | 87.1 | 23.01 |
| 2 | 116.1 | 30.67 |
| 2.5 | 145.1 | 38.34 |
| 3 | 174.2 | 46.01 |
Frequently asked
What is the inside diameter of 1 1/4 in schedule 40 pipe?
35.1 mm, against an outside diameter of 42.2 mm.
How much water does 1 1/4 in pipe hold?
0.968 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.