What Is Flow Rate? Volumetric and Mass Flow Rate
Flow rate describes how much fluid passes through a given cross-section per unit of time. Our Flow Rate Calculator handles both types of flow rate in one tool — making it useful for plumbing, engineering, medicine, and everyday life.
Volumetric Flow Rate (Q)
Volumetric flow rate measures the volume of fluid passing a point per unit of time. It is usually represented by the symbol Q (sometimes written as V̇):
Q = V / t (volume ÷ time)
Alternatively, if you know the cross-sectional area of the pipe and the average velocity of the fluid:
Q = A × v (area × velocity)
Common units for volumetric flow rate include:
- Metric: m³/s, m³/h, L/s, L/min, L/h, mL/s
- US / Imperial: GPM (gallons per minute), gal/h, CFM (ft³/min), CFS (ft³/s)
Mass Flow Rate (ṁ)
Mass flow rate measures the mass of a substance passing through a cross-section per unit of time. It is denoted by ṁ (m with a dot):
ṁ = m / t (mass ÷ time)
Mass flow rate is critical in industrial processes, wastewater treatment, and chemical engineering. Common units:
- Metric: kg/s, kg/min, kg/h, g/s, g/min
- US / Imperial: lb/s, lb/min, lb/h, oz/s
How to Use This Flow Rate Calculator
- Select a calculation mode:
- Volume ÷ Time — enter a volume and the time it took to flow
- Area × Velocity — enter the pipe's cross-sectional area and flow velocity
- Mass ÷ Time — enter a mass and the time it took to flow (for mass flow rate)
- Enter your values and select the appropriate units (metric or US).
- Click Calculate — the calculator instantly shows results in all common units for both systems.
Practical Examples
- Garden hose: A typical hose delivers about 10–15 L/min (2.6–4 GPM).
- Shower head: Standard flow rate is ~8 L/min (2.1 GPM); low-flow models ~6 L/min.
- Human heart (cardiac output): Pumps approximately 5 L/min at rest — a value you can verify in our cardiac output calculator.
- River flow: A small river may carry 10–50 m³/s; large rivers can exceed 10,000 m³/s.
- Pipe velocity calculation: If a 10 cm diameter pipe (area ≈ 0.00785 m²) has water moving at 2 m/s, the flow rate is Q = 0.00785 × 2 ≈ 0.0157 m³/s = 15.7 L/s.
Frequently Asked Questions
- What is the difference between flow rate and velocity?
- Velocity (v) is the speed of the fluid in m/s or ft/s. Flow rate (Q) is the volume passing a cross-section per second. They are related: Q = A × v, where A is the cross-sectional area.
- When should I use mass flow rate instead of volumetric flow rate?
- Use mass flow rate (ṁ) when the density of the fluid matters — for gases, high-temperature liquids, or chemical processes. Volumetric flow rate is more intuitive for incompressible liquids like water.
- What is GPM?
- GPM stands for gallons per minute (US gallons). It is the standard unit for water flow in the United States. 1 GPM ≈ 3.785 L/min ≈ 0.0631 L/s.
- What is CFM?
- CFM stands for cubic feet per minute. It is commonly used for air flow in HVAC systems and ventilation. 1 CFM ≈ 28.32 L/min.
- Can I use this calculator for air flow?
- Yes — the volumetric mode (Q = V/t or Q = A × v) works for any fluid including air. For air mass flow, use the mass flow mode and enter air density × volume as mass.
Flow Rate Formulas Summary
| Formula | Description | Use Case |
|---|---|---|
| Q = V / t | Volume divided by time | Tank filling, pipe discharge |
| Q = A × v | Area times velocity | Pipe flow, open channels |
| ṁ = m / t | Mass divided by time | Industrial processes, gas flow |
| ṁ = ρ × Q | Density times volumetric flow rate | Converting between Q and ṁ |