MotionLab

Flow Rate Calculator

How much fluid volume passes a point per second, from a pipe's size and the fluid's speed.

Cross-sectional area (m²)

Flow velocity (m/s)

Flow rate (m³/s)

0.02

Q = A · v

What you entered

Cross-sectional area (m²): 0.01Flow velocity (m/s): 2

1
Q = A × v
0.01 × 2= 0.02 m³/s

Result

Flow rate (m³/s): 0.02

Fluid moving at 2 m/s through a 0.01 m² pipe flows at 0.02 m³/s.

Volumetric flow rate from pipe area and fluid speed

Flow rate is the cross-sectional area times the fluid velocity, in cubic metres per second.

Area (m2)	Velocity (m/s)	Flow rate (m3/s)	Litres per second
0.002	10	0.0200	20.0
0.01	2	0.0200	20.0
0.05	1.5	0.0750	75.0
0.1	3	0.3000	300.0
1	0.5	0.5000	500.0

The first two rows deliver identical flow from very different pipes - a narrow one running fast and a wider one running slow. That is the continuity principle: for an incompressible fluid the flow rate stays constant along a pipe, so where the pipe narrows the fluid must speed up. It is why a thumb over a hose end makes the water shoot further, and it feeds directly into Bernoulli's equation, where that increased speed comes at the cost of reduced pressure. Multiply cubic metres per second by 1000 for litres per second.

Continuity: narrower pipes mean faster flow

For an incompressible fluid, flow rate stays constant through a pipe of varying width — so fluid speeds up where the pipe narrows (like a thumb over a garden hose) and slows down where it widens, keeping A×v constant.

Real pipes have friction losses

This formula gives the ideal flow rate from geometry and speed alone — real-world flow also depends on pipe roughness, length, and viscosity, which this simple calculation doesn't account for.

Frequently asked questions

Water flows through a 5 cm diameter pipe at 2 m/s. What's the flow rate?

Area = π × (0.025)² = 0.00196 m². Q = A × v = 0.00196 × 2 = 0.00393 m³/s = 3.93 liters/second. That's about 236 liters per minute — a reasonable rate for a garden hose.

If I put my thumb over a garden hose, why does water spray farther?

The continuity equation: A₁v₁ = A₂v₂. Reducing the opening area forces the same volume of water through faster. Halving the area doubles the velocity. The total flow rate stays the same — the water just exits faster through the smaller opening.

How is flow rate related to Bernoulli's equation?

Flow rate (Q = Av) gives you the volume per second. Bernoulli's equation explains the pressure-velocity trade-off: where flow speeds up (narrow section), pressure drops. Together, they describe both how much fluid flows and what happens to pressure along the way. See the Bernoulli's equation calculator.