Pipe flow calculator (Darcy-Weisbach)
Estimate the velocity, Reynolds number and friction head loss of water flowing through a straight pipe.
How it is calculated
v = Q ÷ A, Re = v × D ÷ ν (water at 20 °C, ν = 1.0 × 10⁻⁶ m²/s). Swamee-Jain: f = 0.25 ÷ [log₁₀(ε ÷ 3.7D + 5.74 ÷ Re0.9)]². Head loss h = f × (L ÷ D) × v² ÷ (2g). Laminar flow (Re below 2000): f = 64 ÷ Re.
Worked example
Q = 10 L/s in a 100 mm steel pipe, 100 m long: v = 1.27 m/s, Re = 127,324, f = 0.0196, head loss = 1.62 m.
Steps
- Enter the flow rate in L/s.
- Enter the inside diameter and the pipe length.
- Enter the roughness for the pipe material, then read the head loss.
Frequently asked questions
Which roughness should I use?
Commercial steel about 0.045 mm, PVC about 0.0015 mm, cast iron about 0.26 mm. Use the manufacturer value when available.
Is Swamee-Jain accurate?
It is within about 1 % of the Colebrook equation for Re from 5,000 to 10⁸ and relative roughness from 10⁻⁶ to 10⁻².
Does this include fittings?
No. Add minor losses K × v² ÷ (2g) for valves, elbows and tees.
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References
Darcy-Weisbach equation; Swamee and Jain (1976) explicit friction factor; Moody chart (Moody, 1944).
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.