How to use this calculator
Enter the diameter and velocity at section 1 and the diameter at section 2. The calculator finds the flow rate and the velocity at section 2.
Formulas
Continuity (incompressible)Q = A1V1 = A2V2
Round pipesV2 = V1(D1D2)² ṁ = ρQ
Worked example
100 mm pipe at 1.5 m/s narrowing to 50 mm (the default)
- A1 = π(0.1)²/4 = 7.854 × 10⁻³ m².
- Q = 7.854 × 10⁻³ × 1.5 = 11.8 L/s.
- V2 = 1.5 × (100/50)² = 6 m/s.
Frequently asked questions
Does this work for gases?
At low speeds (Mach < 0.3) yes. At higher speeds density changes, so use mass flow ρ₁A₁V₁ = ρ₂A₂V₂.
Why does a garden hose nozzle spray faster?
The opening is smaller, so the same flow rate must move faster: halving the diameter quadruples the speed.