Flow Rate Calculator
Find flow rate from pipe dimensions and velocity.
Calculate
Formulas
Q = A × v = π(d/2)² × vVolumetric flow rate from cross-section and velocity
Flow Rate
A 50mm pipe at 2 m/s: Q = 3.93 L/s. Household tap: ~0.1-0.2 L/s. Fire hose: ~15-20 L/s. Used in plumbing, HVAC, chemical processing.
How Volumetric Flow Rate Works
Flow rate measures the volume of fluid passing a point per unit time. For flow through a pipe or channel, it equals the cross-sectional area multiplied by the average velocity:
Q = A × v = π(d/2)² × vQ is the volumetric flow rate (m³/s), A is the cross-sectional area, v is the average flow velocity, and d is the pipe diameter.
This is the continuity principle: for an incompressible fluid, if the pipe narrows, the velocity must rise to keep Q constant.
Worked Example
A 50 mm (0.05 m) diameter pipe with flow velocity 2 m/s:
A = π(0.025)² = 0.00196 m²
Q = 0.00196 × 2 = 0.00393 m³/s ≈ 3.9 L/s
The Continuity Equation
For steady, incompressible flow, the flow rate is conserved along a pipe: A₁v₁ = A₂v₂. When a pipe's diameter halves, its area drops to one-quarter, so the velocity must increase fourfold to maintain the same Q. This is why water speeds up through a nozzle.
Frequently Asked Questions
What is the difference between flow rate and velocity?
Velocity is how fast the fluid moves (m/s); flow rate is how much volume passes per second (m³/s). Flow rate equals velocity times area.
How do I convert to litres per minute?
Multiply m³/s by 60000. For example, 0.004 m³/s × 60000 = 240 L/min.
Does this work for gases?
The formula gives volumetric flow for any fluid, but gases are compressible, so their density and volume change with pressure. Mass flow rate is often more useful for gases.