Calculate the friction loss in a water pipe by Hazen-Williams or Darcy-Weisbach, using the real inside diameter of steel, copper, PVC, PPR, HDPE or ductile iron pipe. Get the pressure drop in m, kPa, bar and psi, the velocity, and a table that compares every size at your flow.
SI: hf = 10.67 × L × Q1.852 ÷ (C1.852 × d4.87) (hf in m, Q in m³/s, d in m)
US: p = 4.52 × Q1.85 ÷ (C1.85 × d4.87) (psi per foot, Q in gpm, d in inches)
C depends on the pipe material and age: about 100 for old steel, 120 for new steel, 130–140 for copper and ductile iron, and 150 for plastic pipe. Note that the US form gives the loss per foot of pipe; multiply by the length in feet, not by length ÷ 100. Hazen-Williams is accurate for water at normal temperatures and is the method used in NFPA 13 sprinkler calculations.
Darcy-Weisbach, hf = f × (L ÷ d) × v² ÷ 2g, with the friction factor f from the Colebrook equation, works for any fluid and temperature. Use it for hot water systems: water at 60 °C is about half as viscous as at 15 °C, which lowers the friction loss in a way Hazen-Williams cannot show. The calculator uses the water density and viscosity at the temperature you enter.
| Service | Typical maximum |
|---|---|
| Domestic cold water mains | 2.4 m/s (8 ft/s) |
| Domestic hot water, copper | 1.5 m/s (5 ft/s) |
| Branches near occupied rooms | 1.2–1.5 m/s (4–5 ft/s) |
| Pump suction | 1.0–1.5 m/s (3–5 ft/s) |
| Fire mains | up to about 6 m/s (20 ft/s) in calculations |
Higher velocity means smaller pipe but more noise, erosion and water hammer. Check pump duties with the Pump Head Calculator, surge with the Water Hammer Calculator, and pipe dimensions with the Pipe Schedule Reference.
2 L/s through 50 m of 1½" Schedule 40 steel pipe (ID 40.9 mm), C = 120, 25% fittings allowance:
Use Hazen-Williams for cold water: in SI units, head loss in metres = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87) with Q in m³/s and d in metres. For hot water or other fluids use Darcy-Weisbach with the Colebrook friction factor. Add an allowance for fittings and valves.
About 120 for new steel, 100 for old corroded steel, 130–140 for copper and cement-lined ductile iron, and 150 for PVC, CPVC, PPR and HDPE. Fire protection codes often require 120 for black steel and 100 for dry systems.
No. The common US form p = 4.52 Q^1.85 ÷ (C^1.85 d^4.87) gives psi per foot of pipe. Multiply by the length in feet; multiply by 100 for psi per 100 ft.
Around 2.4 m/s (8 ft/s) for cold water mains and 1.5 m/s (5 ft/s) for hot water in copper, lower near quiet rooms and at pump suctions.
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