Pipe Pressure Drop Calculator (Darcy–Weisbach)
By Santaji GadepatilinUpdated October 27, 2026
Friction between a moving fluid and the pipe wall causes a pressure drop that pumps must overcome. The Darcy–Weisbach equation is the most accurate general method — see how to calculate pressure drop in pipes and its link to Bernoulli's equation. This free pipe pressure drop calculator solves for whichever value you leave blank and shows the formula and steps.
Pressure drop (bar) vs Flow velocity (m/s) — other inputs held at your current values.
How to use the pipe pressure drop calculator
Enter the friction factor, pipe length, inner diameter, velocity and density. You get the friction head loss and pressure drop. Get the velocity from flow using the pipe flow velocity calculator.
Pipe pressure drop formula
hf = f × (L ÷ D) × v² ÷ 2g ΔP = ρ g hf
Worked example
100 m of 100 mm pipe, water at 1.77 m/s, f = 0.02:
- hf = 0.02 × (100 ÷ 0.1) × 1.77² ÷ (2 × 9.81) = 3.19 m
- ΔP = 998 × 9.81 × 3.19 = 31,270 Pa = 0.313 bar
Velocity matters most
Pressure drop rises with the square of velocity. Doubling the flow in the same pipe gives four times the loss — the chart shows the curve. Going up one pipe size usually cuts losses dramatically.
Finding the friction factor
The Darcy friction factor depends on Reynolds number and pipe roughness:
- Laminar flow (Re < 2,300): f = 64 ÷ Re.
- Turbulent flow: use the Colebrook equation or the explicit Swamee–Jain approximation:
f = 0.25 ÷ [log₁₀(ε ÷ 3.7D + 5.74 ÷ Re0.9)]²
Find Re with the Reynolds number calculator.
Pipe roughness values
| Material | Absolute roughness ε (mm) |
|---|---|
| Drawn copper, PVC, PE | 0.0015 |
| Stainless steel | 0.015 |
| Commercial steel (new) | 0.045 |
| Galvanised steel | 0.15 |
| Cast iron | 0.26 |
| Concrete | 0.3–3.0 |
Old pipes become rougher with scale and corrosion, which raises friction losses over time.
Minor losses in fittings
Valves, bends and tees add losses: h = K × v² ÷ 2g.
| Fitting | Typical K |
|---|---|
| 90° long-radius elbow | 0.3–0.5 |
| 90° standard elbow | 0.75–0.9 |
| Tee, flow through branch | 1.0–1.8 |
| Gate valve, fully open | 0.15–0.2 |
| Globe valve, fully open | 6–10 |
| Pipe entrance (sharp) | 0.5 |
| Pipe exit | 1.0 |
Add the K values for all fittings and include them with the straight-pipe loss to get total head loss for pump sizing.
Typical friction factors (turbulent water)
| Pipe | Typical f |
|---|---|
| Smooth plastic / copper | 0.012 – 0.018 |
| New commercial steel | 0.015 – 0.022 |
| Old or corroded steel | 0.025 – 0.04 |
For laminar flow, f = 64 ÷ Re — check the Reynolds number first.
Frequently asked questions
What is the Darcy–Weisbach equation?
It gives the friction loss in a full pipe — head loss = f × (L/D) × v²/2g. It works for any liquid and any flow regime, as long as the right friction factor is used.
How do I find the friction factor?
For laminar flow (Re below 2,300), f = 64/Re. For turbulent flow, use the Moody chart or the Colebrook equation with the pipe's roughness. 0.015–0.03 covers most commercial steel pipes in turbulent water flow.
Does this include losses from valves and fittings?
No — only straight pipe. Add fittings as an equivalent length of pipe, or use their K factors separately.
What is the difference between Darcy and Fanning friction factors?
The Darcy factor is four times the Fanning factor. Make sure your chart or correlation uses the same one as your formula.
How does diameter affect head loss?
For the same flow, head loss varies roughly with 1 ÷ D⁵, so a slightly larger pipe greatly reduces losses.