Pressure Drop Calculator
Pipe pressure drop for water, air or a custom fluid by Darcy-Weisbach, or Hazen-Williams for water, in gpm, inches and psi or metric units. How it works
Use the Pressure Drop Calculator
About the Pressure Drop Calculator
Pressure drop in a pipe comes from the Darcy-Weisbach equation: ΔP = f × (L ÷ D) × ρv² ÷ 2, where f is the friction factor, L the pipe length, D the inside diameter, ρ the fluid density and v the velocity. As a yardstick, water at 50 gpm loses about 2 psi in every 100 ft of 2 in Schedule 40 steel pipe.
Water pressure drop per 100 ft of Schedule 40 steel pipe
| Nominal size | Inside diameter | Flow | Velocity | Pressure drop per 100 ft |
|---|---|---|---|---|
| 1/2 in | 0.622 in | 2 gpm | 2.1 ft/s | 2.02 psi |
| 1/2 in | 0.622 in | 5 gpm | 5.3 ft/s | 10.95 psi |
| 3/4 in | 0.824 in | 5 gpm | 3.0 ft/s | 2.68 psi |
| 3/4 in | 0.824 in | 10 gpm | 6.0 ft/s | 9.78 psi |
| 1 in | 1.049 in | 10 gpm | 3.7 ft/s | 2.91 psi |
| 1 in | 1.049 in | 20 gpm | 7.4 ft/s | 10.73 psi |
| 1-1/4 in | 1.380 in | 20 gpm | 4.3 ft/s | 2.70 psi |
| 1-1/4 in | 1.380 in | 30 gpm | 6.4 ft/s | 5.79 psi |
| 1-1/2 in | 1.610 in | 30 gpm | 4.7 ft/s | 2.67 psi |
| 1-1/2 in | 1.610 in | 50 gpm | 7.9 ft/s | 7.02 psi |
| 2 in | 2.067 in | 50 gpm | 4.8 ft/s | 1.99 psi |
| 2 in | 2.067 in | 100 gpm | 9.6 ft/s | 7.45 psi |
| 3 in | 3.068 in | 100 gpm | 4.3 ft/s | 1.02 psi |
| 3 in | 3.068 in | 200 gpm | 8.7 ft/s | 3.80 psi |
| 4 in | 4.026 in | 200 gpm | 5.0 ft/s | 0.97 psi |
| 4 in | 4.026 in | 400 gpm | 10.1 ft/s | 3.63 psi |
Water at 20 °C (68 °F) in commercial steel pipe with a roughness of 0.045 mm, straight pipe only, calculated with the Darcy-Weisbach and Colebrook-White equations the calculator uses. Enter the inside diameter in the calculator, not the nominal size.
Hazen-Williams C factors against Darcy-Weisbach: 50 gpm through 100 ft of 2 in Schedule 40 (2.067 in)
| Method and pipe | C factor or roughness | Pressure drop |
|---|---|---|
| Darcy-Weisbach, commercial steel | 0.045 mm | 1.99 psi |
| Hazen-Williams, PVC / plastic | C 150 | 1.72 psi |
| Hazen-Williams, copper / stainless steel | C 140 | 1.96 psi |
| Hazen-Williams, new steel or new cast iron | C 130 | 2.24 psi |
| Hazen-Williams, smooth concrete or galvanized iron | C 120 | 2.60 psi |
| Hazen-Williams, old cast iron | C 100 | 3.65 psi |
Darcy-Weisbach uses water at 20 °C. Hazen-Williams is hf = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), head in meters with Q in m³/s and L and D in meters, turned into pressure at 9,806.65 Pa per meter. It has no temperature or viscosity term, so the water temperature field does not change it.
Frequently asked questions
- How do you calculate pressure drop in a pipe?
- Find the velocity from the flow rate and the pipe’s inside area, then the Reynolds number, Re = ρvD ÷ μ. Get the friction factor from Re and the pipe roughness, and put it into ΔP = f × (L ÷ D) × ρv² ÷ 2. The calculator shows each of these numbers for your inputs.
- How much does pipe size change the pressure drop?
- A great deal. For the same flow, friction loss rises with roughly the fifth power of the diameter ratio. At 20 gpm, 100 ft of 1 in pipe loses 10.73 psi and the same length of 1-1/4 in pipe loses 2.70 psi.
- What is the difference between Darcy-Weisbach and Hazen-Williams?
- Darcy-Weisbach works for any fluid, using its density and viscosity and the roughness of the pipe. Hazen-Williams is a simpler formula for water only, with a C factor for the pipe material. The calculator offers both for water and Darcy-Weisbach for everything else.
- How do I include elbows, tees and valves?
- Add up the loss coefficient K of each fitting and enter the total. Each unit of K costs one velocity head, ρv² ÷ 2. K values are listed in fitting manufacturers’ data and in handbooks such as Crane Technical Paper 410.
- Can I calculate pressure drop for air?
- Yes, for a round duct or pipe at near atmospheric pressure and about 20 °C. The Air setting uses the density of air at 20 °C and 1 atm and treats it as incompressible, which holds while the drop is under about 10% of the line pressure. For compressed air, choose Custom and enter the density at your line pressure.
- How much flow will a pipe carry for a given pressure drop?
- Choose Solve for: Flow rate and type the drop you can allow. The page searches for the flow that gives exactly that total drop, fittings included: 50 kPa over 100 m of 50 mm pipe is 11.43 m³/h by Hazen-Williams with C = 150, and 10.53 m³/h of water at 20 °C by Darcy-Weisbach in commercial steel. The working below the result also gives the head loss, ΔP ÷ (ρ × g), about 2.31 ft per psi for water.
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