Pipe Flow Calculator
Calculate pipe flow rate in GPM and CFM, flow velocity, Reynolds number, and pressure drop using Darcy-Weisbach for water, oil, air, and gas.
in
ft/s
Flow Rate (GPM)
—
Flow Rate (CFM) —
Reynolds Number —
Extended More scenarios, charts & detailed breakdown ▾
in
ft/s
Flow Rate (GPM)
—
Flow Rate (CFM) —
Pipe Area —
Professional Full parameters & maximum detail ▾
GPM
in
ft
%
Flow Conditions
Flow Velocity —
Reynolds Number —
Flow Regime —
Pressure & Head Loss
Moody Friction Factor —
Head Loss —
Pressure Drop —
Pump Sizing
Pump Power Required —
How to Use This Calculator
- Enter pipe diameter in inches and flow velocity in ft/s.
- Results show flow rate in GPM and CFM, plus Reynolds number.
- Use the Professional tier for pressure drop and friction factor calculations.
Formula
Area = π × (D/2)²
Flow (ft³/s) = Velocity × Area
GPM = Flow × 448.83 | CFM = Flow × 60
Re = V × D / ν (kinematic viscosity)
Example
Example: 2-inch pipe at 3 ft/s → Area = 0.0218 ft² → Flow = 0.0654 ft³/s → 29.4 GPM.
Frequently Asked Questions
- Flow rate = velocity × cross-sectional area. Q (ft³/s) = V (ft/s) × π × (D/2)². Convert to GPM by multiplying ft³/s × 448.83.
- Reynolds number = (velocity × diameter) / kinematic viscosity. Re < 2300 is laminar flow, Re > 4000 is turbulent flow.
- Use the Darcy-Weisbach equation: ΔP = f × (L/D) × (ρV²/2). This calculator computes it automatically with the Moody friction factor.
- Water in residential pipes: 2–5 ft/s for supply lines, up to 8 ft/s for short runs. Above 8 ft/s causes erosion and noise.
- Pipe roughness (ε) affects friction factor. PVC/smooth: 0.000005 ft; copper: 0.000005 ft; steel: 0.00015 ft; cast iron: 0.00085 ft.