Calculate head loss and pressure drop in pipes using the Darcy-Weisbach equation. Compare pipe materials, sizes, and flow conditions for plumbing, HVAC, and hydraulic systems.
Friction loss is the head and pressure drop caused by viscous shear as fluid moves through a pipe. The Darcy-Weisbach equation — h_f = f(L/D)(v²/2g) — describes that loss for any Newtonian fluid, with the friction factor set by Reynolds number and pipe roughness.
That makes pipe sizing a real design choice, not just a formatting detail. A smaller diameter increases velocity, which raises friction loss, pump power, and operating cost. The effect is especially important in long runs, chilled-water loops, fire protection systems, and process piping.
This calculator turns pipe length, diameter, roughness, fluid properties, and velocity into head loss, pressure drop, and pump power. It also compares common pipe materials and sizes so you can see how one change in diameter alters the result.
Pipe losses depend on several linked inputs: roughness, Reynolds number, diameter, length, and flow rate. Doing that by hand means bouncing between a Moody chart, unit conversions, and diameter tables. This calculator keeps those pieces together and shows how each pipe size changes head loss, pressure drop, and pump power.
Darcy-Weisbach Equation: h_f = f × (L/D) × v²/(2g) Pressure Drop: ΔP = f × (L/D) × ρv²/2 Pump Power: P = ΔP × Q = ΔP × Av where: f = Darcy friction factor (from Colebrook-White or 64/Re for laminar) L = pipe length (m), D = pipe diameter (m) v = flow velocity (m/s), g = 9.81 m/s² ρ = fluid density (kg/m³), Q = volumetric flow rate (m³/s)
Result: h_f = 4.26 m, ΔP = 41.7 kPa
100 m of 4" commercial steel pipe (ε = 0.045 mm) with water at 2 m/s: Re = 202,390, f = 0.0218. Head loss = 0.0218 × (100/0.1016) × 4/(2 × 9.81) = 4.26 m. Pressure drop = 998 × 9.81 × 4.26 = 41.7 kPa (6.1 psi). Pump power = 41,700 × 0.0162 = 676 W to overcome friction.
The most important practical insight in pipe hydraulics is the fifth-power relationship between diameter and head loss at constant flow rate. If you need a specific flow rate Q, the velocity in a pipe of diameter D is v = Q/(πD²/4). Substituting into Darcy-Weisbach: h_f ∝ v² × L/D ∝ Q² × L/D⁵. This means doubling the diameter reduces friction loss by a factor of 32 — often justifying the higher cost of larger pipe.
Engineers typically size pipes by selecting a maximum acceptable velocity (1.5–2.5 m/s for water supply, 3–5 m/s for fire protection) or a maximum head loss per unit length (often 0.03–0.05 m per meter for distribution mains). This calculator\'s diameter comparison table directly supports this workflow: enter your required flow rate, then read which pipe sizes meet your head-loss budget.
For continuously operating systems (water treatment, industrial processes), friction loss translates directly to electricity cost. Power = ΔP × Q / η_pump. At $0.10/kWh, a pump overcoming 50 kPa at 10 L/s costs about $4,400/year. Reducing friction by 50% through pipe upsizing pays for itself quickly — a calculation this tool supports with its pump power output.
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Head loss is measured as meters of fluid column and describes the energy lost per unit weight. Pressure drop is the same loss expressed as Pascals, so it depends on fluid density. Use head loss for pump curves and pressure drop for pressure ratings.
No. Friction factor depends on Reynolds number and relative roughness, not length. Pipe length changes the total head loss because the Darcy-Weisbach equation scales linearly with L.
A smaller pipe raises velocity for the same flow rate, and the Darcy-Weisbach terms compound quickly. For constant flow, friction loss scales roughly with 1/D⁵, which is why upsizing even one pipe size can make a large difference.
This calculator covers straight-pipe or major losses only. Fittings, valves, and reducers add separate minor-loss terms, which are easy to include after you know the main pipe loss.
Higher temperature lowers water viscosity, which changes Reynolds number and usually reduces friction loss a little. The effect is modest compared with changes in pipe size or flow rate.
The calculator uses SI inputs and shows pressure loss in both Pascals and common engineering units like psi and bar. The comparison table is there to help with nominal pipe sizes and metric diameters.