Pressure Drop Calculator
Calculate friction pressure loss along a pipe using Darcy–Weisbach.
Please note: These are idealised textbook models using representative material data. They are for study, sizing and sanity-checking only — never for final design. Real engineering requires code-compliant analysis, verified material certificates and a qualified engineer’s sign-off.
What the Pressure Drop Calculator does
Friction converts pressure into heat as fluid moves along a pipe. Darcy–Weisbach quantifies it from the friction factor, the length-to-diameter ratio and the dynamic pressure — and because dynamic pressure carries velocity squared, modest flow increases cost a great deal of pressure.
Formula
Δp = f × (L ÷ D) × (ρ v² ÷ 2)Head loss = Δp ÷ (ρ g)Laminar: f = 64 ÷ ReTurbulent (Swamee–Jain): f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]²
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Fluid | one of 7 options | Yes | — |
| Pipe material | one of 6 options | Yes | — |
| Pipe inside diameter | number | Yes | — |
| Pipe inside diameter unit | one of 9 options | Yes | — |
| Pipe length | number | Yes | — |
| Pipe length unit | one of 9 options | Yes | — |
| Flow velocity | m/s | Yes | Accepts more than 0. |
| Total fittings loss coefficient | number | Optional | Sum of K values. An elbow is about 0.9, a gate valve 0.2, a tee 1.8. Accepts 0 or more. |
| Elevation gain | number | Optional | — |
| Elevation gain unit | one of 9 options | Yes | — |
How to use it
- Choose Fluid and Pipe material.
- Enter Pipe inside diameter, Pipe length and Flow velocity.
- Optionally add Total fittings loss coefficient and Elevation gain.
- Select Calculate.
Worked example
Water at 2 m/s through 100 m of 50 mm commercial steel pipe.
- Fluid
- Water at 20 °C
- Diameter
- 50 mm
- Length
- 100 m
- Velocity
- 2 m/s
Re = 99,621 and ε/D = 0.0009, giving f = 0.0220. Δp = 0.0220 × 2,000 × 1,996 = 87.8 kPa, or 8.97 m of head.
Frequently asked questions
Why does pipe diameter matter so much?
Velocity rises with the square of the diameter reduction and pressure drop with velocity squared, on top of the L/D term. The combined effect is roughly a fifth-power relationship.
How accurate is the friction factor?
Swamee–Jain sits within about 1% of the implicit Colebrook equation over the normal turbulent range, which is well inside the uncertainty of real pipe roughness.
Method and sources
Method. Darcy-Weisbach pressure drop, Δp = f × (L ÷ D) × (ρv² ÷ 2), with the friction factor from the flow regime and relative roughness.
Assumptions
- Steady, fully developed, incompressible flow in a pipe of constant diameter and uniform roughness.
Limitations
- Roughness values are representative for a material and degrade in service — scaling and corrosion increase drop substantially over a pipe's life.
- Fittings, bends and valves frequently contribute more than the straight run and must be added as equivalent lengths or loss coefficients.
- Friction-factor correlations carry their own uncertainty, typically a few per cent, and more in the transition region where the regime is not well defined.
Sources
- The Darcy-Weisbach equation with Colebrook-White friction factors — Classical fluid mechanics. The pressure-drop relationship and the friction-factor treatment.