Quick answer: Volumetric flow rate is the volume moving past a point per unit of time: Q = A × v (area × velocity) or Q = V ÷ t (volume ÷ time). Water at 2 m/s through a 0.01 m² cross-section flows at 0.02 m³/s, about 317 US gallons per minute.
The volumetric flow rate formulas
Q = A × v or Q = V ÷ tArea × velocity, or volume ÷ timeThe first form is standard for pipe flow, HVAC ducts and open channels where you know the geometry and speed. The second is simpler when you can time a fill, such as how long a 5-gallon bucket takes to fill from a hose.
Flow rate units
| Unit | Equal to | Common use |
|---|---|---|
| GPM (US gal/min) | 0.06309 L/s | Plumbing, pumps, irrigation |
| CFM (ft³/min) | 0.4719 L/s | HVAC, ventilation, ducts |
| L/min | 0.01667 L/s | Metric plumbing, lab equipment |
| m³/h | 0.2778 L/s | Industrial and process flow |
| m³/s | 1,000 L/s | SI base unit |
Worked examples
Area × velocity: water at 2 m/s through 0.01 m²
- Q = 0.01 × 2 = 0.02 m³/s
- = 20 L/s
- × 15,850.32 = 317 GPM
Result: about 317 GPM
Volume ÷ time: a 50 gallon tank fills in 60 seconds
- Q = 50 gal ÷ 60 s = 0.833 gal/s
- × 60 = 50 GPM
Result: 50 GPM
A 0.01 m² cross-section is about a 4.4 in bore. A nominal 4 in pipe is 0.0081 m².
Tips for accurate results
- Use the inside diameter to work out area; wall thickness matters.
- Measure velocity where you measure the cross-section.
- Time at least 10 to 15 seconds of a container fill.
- Match units to the trade: GPM for plumbing, CFM for HVAC, L/s or m³/h for metric engineering.
Who uses a volumetric flow rate calculator
- Plumbers and irrigation designers sizing pipes and pumps.
- HVAC technicians balancing duct airflow in CFM.
- Engineering students checking homework.
- Process engineers converting metric and imperial units.
Frequently asked questions
What is volumetric flow rate?
The volume of fluid passing a point per unit of time, written Q. It is Q = A × v (area × velocity) or Q = V ÷ t (volume ÷ time).
How do I convert flow rate to GPM?
Multiply m³/s by 15,850.32 for US gallons per minute. The calculator shows GPM alongside five other units.
What is the difference between GPM and CFM?
GPM measures liquid flow, common in plumbing and irrigation. CFM measures air or gas flow, common in HVAC. The formula is the same.
How is flow rate different from velocity?
Velocity is how fast the fluid moves (ft/s). Flow rate is how much volume passes per time (GPM). Flow rate = area × velocity.
How do I find a pipe’s cross-sectional area?
Area = π × r² using the inside radius. A 4 in bore has r = 2 in, so area ≈ 12.57 in² ≈ 0.0081 m².
Why does measured flow not match the pipe rating?
Friction, fittings, elevation changes and blockages reduce real flow. Q = A × v assumes ideal, unobstructed flow.
Related tools
Accuracy note: These are ideal-flow formulas for planning and learning. Real systems lose flow to friction and fittings, so confirm critical sizing with a qualified engineer or technician.
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