Estimate the friction pressure loss along a full pipe with the Darcy-Weisbach equation. Enter the flow, a Schedule 40 size or any inside diameter, the length and pipe material, and get the pressure drop in psi and kPa, head loss, velocity, Reynolds number and Colebrook friction factor, with optional fittings and elevation change.
Last updated:
Flow and fluid
kg/m³
cP
cP = mPa·s; cSt × density in g/cm³ = cP
Pipe
Optional
e.g. 90° elbow ≈ 0.3–0.9, open gate valve ≈ 0.2, tee branch ≈ 1.0–1.8
Same unit as length; negative if the outlet is lower
Enter your values to see the result.
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How to use the Pipe Pressure Drop Calculator
Enter the flow rate and choose the fluid.
Choose the pipe size (Schedule 40) or enter a custom inside diameter.
Enter the length and pick the pipe material for its roughness.
Optionally add ΣK for elbows, valves and tees, and the elevation rise.
Read the pressure drop and head loss, then use the table to see how the loss changes with flow.
Pressure drop formulas
Velocity: v = Q ÷ (π D² / 4)
Reynolds number: Re = ρ v D ÷ μ
Darcy-Weisbach: ΔP = f × (L / D) × ρ v² / 2; head loss h_f = ΔP ÷ (ρ g)
Minor losses: ΔP = ΣK × ρ v² / 2; elevation: ΔP = ρ g Δz
This is the Darcy friction factor. Some references use the Fanning factor, which is one quarter of it. In the transitional range (Re 2,300–4,000) the real friction factor is uncertain; the calculator uses Colebrook there, which is usually conservative.
Typical pipe roughness
Pipe material
ε (mm)
ε (in)
PVC, CPVC, copper, drawn tubing
0.0015
0.00006
PE / PEX
0.007
0.0003
Stainless steel
0.015
0.0006
Commercial steel (new)
0.045
0.0018
Asphalted cast iron
0.12
0.0047
Galvanized steel
0.15
0.006
Cast iron (new)
0.26
0.010
Concrete
0.3–3
0.012–0.12
Riveted steel
0.9–9
0.035–0.35
Typical fitting loss coefficients (K)
Fitting
Approximate K
90° elbow, long radius
0.2–0.4
90° elbow, standard threaded
0.7–1.5
45° elbow
0.2–0.4
Tee, flow through run
0.2–0.6
Tee, flow through branch
1.0–1.8
Gate valve, fully open
0.1–0.2
Globe valve, fully open
6–10
Swing check valve
2
Sharp pipe entrance / exit to tank
0.5 / 1.0
K values vary with size and manufacturer. Use catalog data for design, and add a margin for aging pipe.
Frequently asked questions
What is the Darcy-Weisbach equation?
ΔP = f × (L/D) × ρv²/2, where f is the Darcy friction factor, L the pipe length, D the inside diameter, ρ the fluid density and v the mean velocity. Divided by ρg it gives the head loss h_f = f (L/D) v²/(2g). It works for any Newtonian liquid and for gases when the pressure drop is small compared with the pressure.
How is the friction factor found?
For laminar flow (Re < 2,300) f = 64/Re. For turbulent flow this calculator solves the Colebrook-White equation 1/√f = −2 log₁₀(ε/3.7D + 2.51/(Re√f)) by iteration, which is what the Moody chart plots. The explicit Haaland formula is shown as a check; it is usually within about 2%.
What roughness should I use?
Typical absolute roughness values are about 0.0015 mm for PVC, copper and drawn tubing, 0.045 mm for new commercial steel, 0.15 mm for galvanized steel and 0.26 mm for new cast iron. Old, corroded or scaled pipe can be several times rougher, so add margin for aging.
How do fittings add pressure drop?
Each fitting adds ΔP = K × ρv²/2, where K is its loss coefficient. Add up the K values of all elbows, tees and valves and enter ΣK. The calculator also shows the equivalent length of straight pipe that would cause the same loss (K × D / f).
What is the difference between head loss and pressure drop?
They are the same loss in different units. Head loss is expressed as the height of fluid column (feet or meters), pressure drop as pressure. For water, 1 psi ≈ 2.31 ft of head. Pump curves use head, gauges read pressure.
Why does doubling the flow roughly quadruple the pressure drop?
In turbulent flow ΔP is proportional to v², and the friction factor changes only slowly, so twice the flow gives about 3.5 to 4 times the loss. In laminar flow the loss is directly proportional to flow. The table under the results shows this for your pipe.
Is Darcy-Weisbach better than Hazen-Williams?
Darcy-Weisbach is valid for any fluid, temperature and flow regime, while Hazen-Williams is an empirical formula limited to water at ordinary temperatures in turbulent flow. Results are similar for cold water in typical pipes, but Darcy-Weisbach is preferred for oils, hot water and small pipes.
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