Flow Rate Converter
Convert flow rate across industrial units, in both of its forms. Volumetric flow measures volume per time — litres per second, cubic metres per hour, US gallons per minute, cubic feet per minute. Mass flow measures mass per time — kilograms per second, tonnes per hour, pounds per hour. Converting within either form uses fixed ratios; crossing between them needs the fluid’s density, which links them by the relation mass flow = density × volumetric flow. Enter a density (water is about 998 kg/m³) when converting across.
How to use this tool
- Enter a flow value.
- Pick the unit you have and the unit you want — volumetric and mass units are grouped in the menus.
- If you convert between a volumetric and a mass unit, enter the fluid density.
- Converting within volumetric-only or mass-only units ignores the density.
- Read the result, with the density used shown when it applies.
The formula
Flow rate comes in two kinds: volumetric (volume per time) and mass (mass per time). Within each kind the units are fixed ratios; crossing between them needs the fluid’s density, since mass flow equals density times volumetric flow.
ṁ = ρ · Q
Q = ṁ ÷ ρ- Q
- Volumetric flow rate, m³/s
- ṁ
- Mass flow rate, kg/s
- ρ
- Fluid density, kg/m³
Volumetric flow alone does not fix mass flow: a cubic metre of air and a cubic metre of water carry very different masses.
Worked examples
Pump rating
- Given
- 1 L/s
- Result
- 3.6 m³/h
A litre per second is 3.6 cubic metres per hour — a common way to state pump duty.
Water mass flow
- Given
- 10 L/s of water (ρ = 998)
- Result
- ≈ 9.98 kg/s
Multiplying volumetric flow by density gives mass flow: 10 L/s of water is about 9.98 kg/s.
US units
- Given
- 100 GPM
- Result
- ≈ 6.31 L/s
A US gallon per minute is about 0.063 L/s, so 100 GPM is roughly 6.3 L/s.
Frequently asked questions
Volumetric flow is how much space of fluid passes per unit time; mass flow is how much matter. They differ by density: a cubic metre of air weighs far less than a cubic metre of water, so the same volumetric flow gives very different mass flows depending on the fluid.
Only when converting between a volumetric unit and a mass unit. Volumetric-to-volumetric and mass-to-mass conversions use fixed ratios and ignore density. The tool asks for density and applies it only when the conversion actually crosses between the two kinds.
The density of the flowing fluid at its actual conditions. Water near room temperature is about 998 kg/m³; air at sea level is about 1.2 kg/m³; oils are typically 850–950 kg/m³. Using the wrong density is the main source of error in mass-flow conversions.
Both are volumetric, but GPM (US gallons per minute) is used for liquids and CFM (cubic feet per minute) for gases and air handling. They convert directly as volumes; a US gallon is about 0.1337 cubic feet.
The unit ratios are exact, since gallons, feet and pounds have defined metric values. The density-based crossing is exact given the density you supply — its accuracy depends on how well that density matches the real fluid. Internal precision is preserved and only the display is rounded.