Engineering
Flow Rate Calculator
Flow rate (Q) is the volume of fluid passing a point per unit time. This calculator supports two methods: cross-section × velocity (for pipe sizing) or volume ÷ time (for tank fill/drain problems). Outputs in liters per second, cubic meters per hour, and US gallons per minute.
Calculator
Formula
Formula
Q = A × v or Q = V / t
For area × velocity: Q = π × (d/2)² × v, where d is internal diameter in meters and v is fluid velocity in m/s. Result in m³/s, then converted. For volume ÷ time: Q = V (liters) / t (seconds), giving L/s. 1 L/s = 3.6 m³/h = 15.850 US GPM.
Worked example
Water flows at 2 m/s through a 50 mm internal diameter pipe. Find the flow rate.
- 1.Radius r = 0.025 m
- 2.Area A = π × (0.025)² = 0.001963 m²
- 3.Q = A × v = 0.001963 × 2 = 0.003927 m³/s
- 4.In L/s: 0.003927 × 1,000 = 3.927 L/s
- 5.In m³/h: 3.927 × 3.6 = 14.14 m³/h
- 6.In GPM: 3.927 × 15.850 = 62.24 GPM
Flow rate is 3.93 L/s = 14.1 m³/h = 62.2 US GPM.
Assumptions and limitations
- Assumes fully developed, uniform flow (no turbulence model).
- Diameter is internal bore in mm; velocity in m/s.
- Volume/time method assumes constant flow rate over the measurement period.
- For compressible fluids (gases), volumetric flow rate changes with pressure — use mass flow rate instead.