Mass Flow Rate Converter

Mass flow rate measures how much mass passes through a point per unit of time. It is the working unit of chemical engineering, HVAC and fluid mechanics.

All conversions use kg/s as the base unit.

Metric conversions:

• 1 kg/s = 1,000 g/s = 3,600 kg/h = 3.6 tonne/h

Imperial conversions:

• 1 kg/s = 2.20462 lb/s = 7,936.64 lb/h
• 1 lb/s = 0.453592 kg/s

Cross-system:

• 1 tonne/h = 2,204.62 lb/h
• 1 lb/h = 0.000126 kg/s

Common applications:

The reason engineers track mass flow rather than volume comes down to one fact: mass is conserved, volume isn’t. Heat a gas or drop its pressure and it expands, so a fixed volumetric flow can carry very different amounts of actual material. Mass flow sidesteps that entirely, which is why it’s the unit of choice for combustion, chemical reactions, and anything where the quantity of substance, not the space it fills, is what counts.

That distinction is the usual source of error. A “1,000 m³/h” gas reading represents different masses at different temperatures and pressures, while “1 kg/s” is unambiguous. When a process depends on how much material is actually moving, such as fuel into an engine or reactant into a reactor, mass flow is the figure to trust.

Crossing between the two takes one number: density.

mass flow = volumetric flow × density

At 4 kg/s of water, density 1,000 kg/m³, that is 0.004 m³/s or 4 litres a second. The same 4 kg/s of air at room conditions, density about 1.2 kg/m³, is 3.33 m³/s, more than eight hundred times the volume for exactly the same mass. Pipe sizing follows the volume; the process follows the mass.

Reference densities, all at roughly 20 °C and one atmosphere unless noted:

The gas figures are the ones to treat with suspicion, because they move with temperature and pressure while the liquids barely do. Double the absolute pressure on a gas and you roughly double its density, so the same volumetric reading now carries twice the mass. That is exactly why gas is metered in normal or standard cubic metres, Nm³ or scm, which are volumes already corrected back to a stated reference condition. An Nm³ of any gas is a fixed mass of that gas; a plain m³ is whatever happened to be in the pipe at the time.

One more trap worth naming. A tonne here is the metric tonne of 1,000 kg. The US short ton is 907.2 kg and the UK long ton is 1,016 kg, so a flow quoted in “tons per hour” is ambiguous by up to ten percent until somebody says which. On a boiler feed line that difference is not a rounding error, it is a different pump.