The one-line relationship
Mass flow equals volumetric flow times density. Trivial arithmetic — the entire engineering content of this conversion is which density you use. Density belongs to a specific temperature and pressure, and using the wrong reference is one of the quietest, most common flow measurement errors in industry.
Worked example
A hot oil system pumps 45 m³/h. At the pumping temperature of 180 °C the oil density is 780 kg/m³ (versus 850 at ambient):
- Correct: 45 × 780 = 35,100 kg/h = 35.1 t/h
- Using the ambient density instead: 45 × 850 = 38,250 kg/h — a 9% error from temperature alone
Field notes
- What does your meter actually measure? Magnetic, vortex, ultrasonic and DP meters sense volume (or velocity); Coriolis and thermal meters sense mass. Any conversion between the two happens somewhere — in the transmitter, DCS or a spreadsheet — with an assumed density. Find that assumption before trusting a converted number.
- Liquids: density falls roughly 0.05–0.1% per °C for hydrocarbons — a 50 °C swing is a 3–5% mass error if uncompensated.
- Gases: density is proportional to pressure and inversely to absolute temperature. Never convert gas flows with a fixed density unless conditions truly are fixed.
- Steam: use steam-table density at the operating pressure — saturated steam at 10 barg is ~5.6 kg/m³, at 1 barg only ~1.1 kg/m³.
Frequently asked questions
How do I convert kg/h to m³/h?
Divide mass flow by density: m³/h = kg/h ÷ kg/m³. For water (≈1000 kg/m³), 5000 kg/h is 5 m³/h.
What density should I use?
The density at the actual operating temperature and pressure at the measurement point — not the density at standard conditions. For liquids the temperature effect dominates; for gases both matter enormously.
Why do steam and gas flows usually use mass units?
Because gas volume changes with pressure and temperature while mass does not. Energy and material balances need mass (or standard volume), so boiler steam is metered in kg/h or t/h.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.