Pipe Flow & Pressure Drop Calculator
Velocity, Reynolds number and friction pressure loss in a round pipe.
Steady, single-phase, full-bore flow in a round pipe. Two-phase flow, compressible gas across a large pressure ratio, and non-Newtonian fluids all need different treatment.
How it works
Pressure drop along a pipe is Darcy–Weisbach:
Δp = f × (L ÷ D) × (ρ v² ÷ 2)
Velocity is squared, so doubling the flow through a given pipe roughly quadruples the loss. Diameter is worse than it looks: velocity itself depends on D², so the whole expression scales close to D⁻⁵. Going up one pipe size — 50 mm to 63 mm — cuts the pressure drop by about two thirds. Almost every case of "the pump is not strong enough" is a pipe one size too small, and no pump fixes that economically.
The friction factor f depends on Reynolds number and on relative roughness. Below Re 2300 the flow is laminar and f is exactly 64/Re, independent of the pipe wall — in laminar flow the fluid slides over itself, not the surface. Above about 4000 it is turbulent and roughness starts to matter. This uses the Swamee–Jain explicit fit to the Colebrook equation, within about 1% of it across the turbulent range and without needing to iterate.
Velocity is also a design limit in its own right. Water above roughly 3 m/s is noisy and starts eroding fittings; hydraulic pressure lines run 3–6 m/s and suction lines much slower, because a starved pump cavitates. Those limits usually bind before pressure drop does.
Fittings are handled as equivalent straight length, the standard shop approximation: a 90° elbow is worth about 30 diameters, a gate valve 8, a globe valve 340.
Common questions
Why does one pipe size make such a difference?
Because the loss scales roughly with the fifth power of diameter. Going from 50 mm to 63 mm — a 26% increase — cuts pressure drop to about a third. It is nearly always cheaper than a bigger pump, and it saves energy for the life of the installation.
What velocity should I aim for?
Water: 1–2.5 m/s in distribution, under 1.5 on a suction. Hydraulic oil: 3–6 m/s pressure, 1–1.5 suction, 2–3 return. Compressed air: under 9 m/s in a ring main. Exceeding these gives noise, erosion and, on a pump suction, cavitation.
How do I handle bends and valves?
Add their equivalent length in the field above. Rules of thumb in pipe diameters: 90° elbow 30, 45° elbow 16, tee through-run 20, tee branch 60, gate valve 8, ball valve 3, globe valve 340, swing check 100.
What about elevation?
Not included — this is friction only. Static head is separate and simply ρgh: 10 m of water is about 0.98 bar, whichever way the pipe runs. Add it to the friction loss to size a pump.