Pipe flow rate calculator
This pipe flow rate calculator works from the inside diameter and the flow velocity, or the other way round, and gives the volume flow, Reynolds number, friction factor and pressure drop for water.
Source: Wikipedia – Rohrreibungszahl (Darcy-Weisbach, Swamee-Jain, Rauheiten). Updated: .
How it is calculated
Flow rate Q = v · A with A = π·d²/4. In a pipe with a 20 mm inside diameter, 20 l/min flows at about 1.06 m/s.
How it is calculated
The velocity follows from flow rate and cross-section, and the Reynolds number from Re = v·d/ν. Below Re ≈ 2,320 water flows laminar, above that turbulent. The friction factor λ is 64/Re for laminar flow and, for turbulent flow, comes from the explicit Swamee–Jain formula using Re and the relative roughness k/d. The pressure drop follows from Darcy–Weisbach: Δp = λ · (L/d) · ρ · v²/2.
Which velocity makes sense?
- Drinking water and heating pipes are usually designed for 0.5 to 2 m/s.
- Water that is too fast makes noise and raises the pressure drop, water that is too slow encourages deposits.
- The pressure drop rises with the square of the velocity and falls sharply with a larger diameter.
Pipe material and water temperature act through roughness and viscosity. Bends, valves and fittings add minor losses that are not included here.
Frequently asked questions
How do you calculate the flow rate in a pipe?
With Q = v · π·d²/4: flow velocity times cross-sectional area. At 1 m/s in a pipe with a 20 mm inside diameter that is about 18.8 l/min.
What flow velocity is normal in water pipes?
Usually 0.5 to 2 m/s. Lower risks deposits, higher causes flow noise and high pressure drop.
How do you calculate pressure drop in a pipe?
With Darcy–Weisbach: Δp = λ · (L/d) · ρ · v²/2. The friction factor λ depends on the Reynolds number and the roughness.
Sources and legal basis
- Wikipedia – Rohrreibungszahl (Darcy-Weisbach, Swamee-Jain, Rauheiten)
- Wikipedia – Darcy–Weisbach equation
- Wikipedia – Swamee–Jain equation (explicit friction factor)
- Wikipedia – Eigenschaften des Wassers (properties of water: density and viscosity by temperature)
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