3/4 in schedule 40 pipe flow and volume calculator
3/4 in schedule 40 pipe has an outside diameter of 26.7 mm and a bore of 20.9 mm, a wall thickness of 2.90 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
- 8.157
- Inside diameter
- 20.90
- Wall thickness
- 2.900
- Bore area
- 343.07 mm2
- Water content per metre
- 0.3431
- Water in the run
- 8.58
- Weight of that water
- 8.58
- Friction head loss
- 3.483
- Pressure drop
- 34.161 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 20.9 = 20.9
- outside = 26.7 = 26.7
- Wall thickness = (26.7 - 20.9) / 2 = 2.9
- Bore area = pi * 20.9^2 / 4 = 343.07
- area_m2 = 343.07 / 1e6 = 0.000 343 07
- Water content per metre = 343.07 / 1000 = 0.343 07
- Water in the run = 0.343 07 * 25 = 8.576 74
- flow_m3s = 0.000 343 07 * 1.5 = 0.000 514 605
- Flow at that velocity = 0.000 514 605 * 60000 = 30.8763
- Flow in US gallons per minute = 30.8763 / 3.785411784 = 8.156 65
- loss_per_m = 10.67 * (0.000 514 605)^1.852 / (140^1.852 * (20.9/1000)^4.87) = 0.139 339
- Friction head loss = 0.139 339 * 25 = 3.483 47
- Pressure drop = 3.483 47 * 1000 * g0 / 1000 = 34.1612
- Weight of that water = 8.576 74 * 1000 / 1000 = 8.576 74
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.
3/4 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 10.3 | 2.72 |
| 1 | 20.6 | 5.44 |
| 1.5 | 30.9 | 8.16 |
| 2 | 41.2 | 10.88 |
| 2.5 | 51.5 | 13.59 |
| 3 | 61.8 | 16.31 |
Frequently asked
What is the inside diameter of 3/4 in schedule 40 pipe?
20.9 mm, against an outside diameter of 26.7 mm.
How much water does 3/4 in pipe hold?
0.343 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.