Pipe Flow Calculator
Calculate flow rate, velocity, pressure drop, and Reynolds number for pipes using Darcy-Weisbach.
📖 What is a Pipe Flow Calculator?
A pipe flow calculator determines how fluids move through pipes under pressure. It is used daily by plumbing engineers, civil engineers, HVAC designers, and process engineers to size pipes correctly, estimate pressure losses, and ensure systems deliver the required flow rate to where it is needed.
This calculator applies the Darcy-Weisbach equation — the most accurate and universal model for pipe friction losses. Unlike the older Hazen-Williams formula (which only applies to water), Darcy-Weisbach works for any Newtonian fluid at any temperature and any pipe material, in both laminar and turbulent regimes.
Two modes are available: Find Pressure Drop calculates how much pressure is lost for a given flow rate through a pipe — used when you know the required flow and need to know the pump head required. Find Flow Rate calculates how much flow a pipe delivers given the available pressure head — used when designing gravity-fed systems or checking existing pipework.
The calculator also computes the Reynolds number to classify the flow regime (laminar or turbulent), and the Darcy friction factor using the Swamee-Jain approximation (accurate within 3% for turbulent flows). For laminar flow (Re < 2300), it uses the exact formula f = 64/Re.
📝 Pipe Flow Formulas
v = Q / A where A = π(D/2)²
Reynolds number:
Re = v × D / ν
Friction factor (Swamee-Jain, turbulent Re > 4000):
f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]²
Laminar (Re ≤ 2300): f = 64/Re
Darcy-Weisbach pressure drop:
ΔP = f × (L/D) × (ρv²/2) [Pa]
Head loss: h_f = ΔP / (ρg) [m]
Where: Q = flow rate (m³/s) | D = diameter (m) | ν = kinematic viscosity (m²/s) | ε = roughness (m) | L = length (m) | ρ = density (kg/m³) | g = 9.81 m/s²
✍️ How to Use This Calculator
- Select Find Pressure Drop (you know flow) or Find Flow Rate (you know available head).
- Enter the pipe inner diameter in mm and length in metres.
- Enter the flow rate in L/s (or available pressure head in metres).
- Enter the pipe roughness ε in mm. Common values: PVC = 0.0015, copper = 0.0015, galvanised steel = 0.15, cast iron = 0.26.
- Select the fluid — water at 20°C is the default.
- Click Calculate. Review velocity (aim for 0.5–3 m/s), Reynolds number (flow regime), and pressure drop.
📄 Example Calculations
Pipe: 50 mm dia PVC, 100 m long, ε = 0.0015 mm, water at 20°C, flow = 2 L/s
Area = π × (0.025)² = 0.001963 m²
v = 0.002 / 0.001963 = 1.02 m/s ✓ (within 0.5–3 m/s)
Re = 1.02 × 0.05 / 1.004×10⁻⁶ = 50,797 (turbulent)
f (Swamee-Jain) ≈ 0.0198
ΔP = 0.0198 × (100/0.05) × (998 × 1.02²/2) = 20.6 kPa = 2.1 m head
Example 2 — Larger diameter comparison:
Same conditions but 80 mm pipe: v = 0.398 m/s, ΔP = 1.5 kPa = 0.15 m head
Doubling diameter reduces pressure drop by ~14× — pipe diameter has an enormous impact on flow resistance.