Calculator
Formula Used
- Velocity: v = Q / A, with A = πD²/4
- Reynolds: Re = ρvD / μ
- Major loss: ΔPₘ = f (L/D) (ρv²/2)
- Minor loss: ΔPₖ = (ΣK) (ρv²/2)
- Total: ΔP = ΔPₘ + ΔPₖ
- Laminar: f = 64/Re (Re < 2000)
- Turbulent: f = 0.25 / [log10(ε/3.7D + 5.74/Re^0.9)]²
- Head loss: h = ΔP / (ρg)
How to Use
- Select a method. Use Darcy–Weisbach for most cases.
- Enter pipe length and inner diameter with correct units.
- Provide flow rate. Pick a fluid preset or set custom properties.
- Add roughness and total fitting coefficient if fittings exist.
- Choose the output unit, then press Calculate.
- Use the CSV/PDF buttons to save your results.
Example Data Table
| Scenario | Key Inputs | Typical Output |
|---|---|---|
| Water in 50 mm steel, moderate flow | L=25 m, D=50 mm, Q=2 L/s, ΣK=3 | ΔP ≈ 5–25 kPa (depends on roughness) |
| Air in small tube, high velocity | L=10 m, D=12 mm, Q=20 L/s, ΣK=6 | ΔP can exceed 50 kPa |
| Water using Hazen–Williams | L=200 m, D=100 mm, Q=10 L/s, C=130 | Head loss ≈ few meters |
Pressure drop fundamentals
Pressure drop is the pressure decrease as a fluid travels through a pipe because mechanical energy is dissipated. For a straight run, Darcy–Weisbach estimates it as ΔP = f·(L/D)·(ρ·v²/2), combining friction factor, length, diameter, density, and velocity. Since v depends on flow rate and area, a small diameter change can raise losses sharply in practice. Good estimates size pumps, validate inlet pressures, and cut avoidable power consumption.
Flow regime and friction factor
The friction factor f depends strongly on flow regime and relative roughness. Reynolds number Re = ρvD/μ separates laminar flow (Re<2000), where f = 64/Re, from turbulent flow, where roughness and turbulence dominate. The calculator can use a Swamee–Jain approximation to the Colebrook equation, requiring pipe roughness ε and diameter D to form ε/D. Higher viscosity lowers Re and often reduces losses. Always confirm units before comparing materials directly.
Minor losses and fittings
Real systems include fittings that add minor losses beyond straight‑pipe friction. Each elbow, valve, tee, or entrance can be modeled with a loss coefficient K, producing ΔP_minor = K·(ρ·v²/2). A practical way to combine many items is to sum K values or convert them to an equivalent length Le = K·D/f. When short piping has many fittings, minor losses can exceed major losses, often especially at high velocity.
Using the calculator for design checks
To use the calculator, start by selecting a method and entering consistent fluid properties. For gases, choose whether density is treated as constant or based on operating pressure and temperature. Enter pipe length, diameter, and roughness, then set flow rate or velocity. If you have fittings, add total K or equivalent length so the model includes them. Run several cases to see quickly how ΔP responds to diameter, viscosity, and temperature assumptions.
Interpreting results and engineering limits
Interpret results by comparing calculated ΔP with available pump head, compressor discharge pressure, or allowable drop across equipment. A common design check is to limit velocity to reduce noise, erosion, and water hammer risk; lower velocity also improves energy efficiency. If the calculator reports high Re and large ΔP, consider a larger diameter, smoother material, or shorter routing. Always include safety margins and verify critical lines with vendor data and field measurements.
FAQs
1) What is the difference between pressure drop and head loss?
Head loss expresses the same energy loss as an equivalent fluid column height (meters or feet). Pressure drop is that loss in pressure units. They convert using ΔP = ρ·g·h.
2) Which diameter should I use, inner or outer?
Use the internal diameter because it determines flow area and velocity. For tubing or lined pipe, measure or look up the true bore, not the nominal size.
3) How do I choose pipe roughness?
Roughness depends on material and condition. New smooth plastic is low, new steel is higher, and corroded pipe can be much higher. Use published ε values, then apply a conservative margin.
4) When are minor losses important?
Minor losses matter when fittings are numerous, piping is short, or velocity is high. In compact manifolds, K-sums can dominate the total drop even if the straight length is small.
5) Can I use this for compressible gas flow?
You can get a first estimate if density changes are small. For large pressure changes, high Mach numbers, or choked flow risk, use a compressible-flow model and compare with equipment vendor curves.
6) Why does pressure drop rise so fast with flow rate?
Velocity increases with flow, and friction and K-loss terms scale with v². That square relationship means doubling flow can roughly quadruple the pressure drop, depending on regime and roughness.