Pipe Flow Rate Calculator
Calculate flow rate from pipe diameter and velocity (or velocity from flow), in L/min, GPM and m³/h, plus the Reynolds number for water.
How to use
- 1Enter the pipe's inner diameter.
- 2Enter the flow velocity to get the flow rate, or the flow rate to get the velocity.
How it's calculated
Q = A × v, with A = π d² ÷ 4. Reynolds number Re = v d ÷ ν (ν ≈ 1.0 × 10⁻⁶ m²/s for water at 20 °C); Re > 4,000 is turbulent.
Frequently asked questions
What velocity is typical in water pipes?
Around 1–2.5 m/s (3–8 ft/s) in household plumbing. Higher speeds cause noise, erosion and water hammer.
Should I use inner or outer diameter?
The inner diameter — the actual flow area.
What does the Reynolds number tell me?
Whether flow is laminar (Re < 2,300), transitional, or turbulent (Re > 4,000), which affects friction losses.