Mass Flow Rate

Mass flow rate describes how much mass crosses a chosen section per unit time. It is written ṁ (“m dot”) and uses mass-per-time units. Volumetric flow rate Q measures volume per time; the two are related through fluid density, but they are not interchangeable without it.

Common mass-flow units

Mass-flow units combine a mass unit with a time unit. Examples include kilograms per second (kg/s), grams per second (g/s), kilograms per minute (kg/min), and tonnes per hour (t/h).

Formula and example

For uniform density across the section, mass flow rate is ṁ = ρQ, where ρ is fluid density and Q is volumetric flow rate. Keep units consistent: kg/m³ × m³/s gives kg/s.

For example, 2 L/s is 0.002 m³/s. At a density of 800 kg/m³, the mass flow rate is 800 × 0.002 = 1.6 kg/s. A different fluid density gives a different mass rate at the same volume rate.