18 in schedule 40 pipe flow and volume calculator
18 in schedule 40 pipe has an outside diameter of 457.2 mm and a bore of 428.7 mm, a wall thickness of 14.25 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
- 3431.834
- Inside diameter
- 428.70
- Wall thickness
- 14.250
- Bore area
- 144 343.37 mm2
- Water content per metre
- 144.3434
- Water in the run
- 3608.58
- Weight of that water
- 3608.58
- Friction head loss
- 0.103
- Pressure drop
- 1.009 kPa
Show the working
- Flow velocity = 1.5
- Pipe run = 25
- Hazen-Williams C = 140
- Fluid density = 1000
- Inside diameter = 428.7 = 428.7
- outside = 457.2 = 457.2
- Wall thickness = (457.2 - 428.7) / 2 = 14.25
- Bore area = pi * 428.7^2 / 4 = 144 343
- area_m2 = 144 343 / 1e6 = 0.144 343
- Water content per metre = 144 343 / 1000 = 144.343
- Water in the run = 144.343 * 25 = 3608.58
- flow_m3s = 0.144 343 * 1.5 = 0.216 515
- Flow at that velocity = 0.216 515 * 60000 = 12 990.9
- Flow in US gallons per minute = 12 990.9 / 3.785411784 = 3431.83
- loss_per_m = 10.67 * (0.216 515)^1.852 / (140^1.852 * (428.7/1000)^4.87) = 0.004 114 07
- Friction head loss = 0.004 114 07 * 25 = 0.102 852
- Pressure drop = 0.102 852 * 1000 * g0 / 1000 = 1.008 63
- Weight of that water = 3608.58 * 1000 / 1000 = 3608.58
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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.
18 in pipe at common velocities
| Velocity (m/s) | Flow (L/min) | Flow (US gpm) |
|---|---|---|
| 0.5 | 4330.3 | 1143.94 |
| 1 | 8660.6 | 2287.89 |
| 1.5 | 12990.9 | 3431.83 |
| 2 | 17321.2 | 4575.78 |
| 2.5 | 21651.5 | 5719.72 |
| 3 | 25981.8 | 6863.67 |
Frequently asked
What is the inside diameter of 18 in schedule 40 pipe?
428.7 mm, against an outside diameter of 457.2 mm.
How much water does 18 in pipe hold?
144.343 litres per metre of run.
Where the numbers come from
- Dimensions are the ASME B36.10 schedule 40 values for the nominal size.