Why this conversion needs a third number
Mass and volume are different physical quantities. Converting between them always requires density, and that is why typing "kg/h to m³/h" into a search box cannot give you a number — the answer depends entirely on what is flowing:
1,000 kg/h in m³/h — by fluid
| Fluid | Density (kg/m³) | 1,000 kg/h = |
|---|---|---|
| Water, 20 °C | 998.2 | 1.0018 m³/h |
| Water, 80 °C | 971.8 | 1.0290 m³/h |
| Diesel / gas oil | 840.0 | 1.1905 m³/h |
| Light crude (35 °API) | 850.0 | 1.1765 m³/h |
| Condensate | 750.0 | 1.3333 m³/h |
| LPG liquid (propane) | 500.0 | 2.0000 m³/h |
| Saturated steam, 2 bara | 1.1294 | 885.4 m³/h |
| Saturated steam, 10 bara | 5.1467 | 194.3 m³/h |
| Saturated steam, 20 bara | 10.0 | 99.5700 m³/h |
| Air, 1 atm, 20 °C | 1.2040 | 830.6 m³/h |
| Air, 7 bara, 20 °C | 8.3178 | 120.2 m³/h |
| Natural gas, 1 atm, 15 °C | 0.6784 | 1474.1 m³/h |
| Nitrogen, 1 atm, 20 °C | 1.1650 | 858.4 m³/h |
| CO₂, 1 atm, 20 °C | 1.8420 | 542.9 m³/h |
Water and air differ by a factor of about 830 for the same mass flow. Anyone quoting a kg/h figure without naming the fluid — and, for a gas, the pressure — has not given you enough to work with.
Gases: density comes from pressure and temperature
For a gas, density is not a property you look up once. It follows the ideal gas law and moves with operating conditions:
Air at atmospheric pressure is 1.204 kg/m³; the same air at 7 bar absolute is 8.32 kg/m³, so the volumetric flow for a given mass flow drops by a factor of seven. This calculator derives gas density from the pressure and temperature you enter, and also reports the flow at normal and standard reference conditions — see the actual ↔ standard converter for more on that distinction and the ideal gas law calculator for density on its own.
Steam: use the specific volume at pressure
Saturated steam is the case people most often get wrong, because its volume collapses as pressure rises:
| Pressure (bar abs) | vg (m³/kg) | 1,000 kg/h = |
|---|---|---|
| 1 | 1.694 | 1,694 m³/h |
| 2 | 0.8854 | 885 m³/h |
| 5 | 0.3747 | 375 m³/h |
| 10 | 0.1943 | 194 m³/h |
| 20 | 0.09957 | 99.6 m³/h |
| 40 | 0.04977 | 49.8 m³/h |
Pick "Steam — enter pressure" above and the specific volume is interpolated from the saturation line. Full properties are in the saturated steam table.
Worked example
A boiler consuming 3,100 kg/h of feedwater at 80 °C:
- Water density at 80 °C = 971.8 kg/m³
- 3,100 ÷ 971.8 = 3.19 m³/h = 53.2 L/min
- If that same 3,100 kg/h left as steam at 10 bar absolute, it would occupy 602 m³/h — which is why steam lines dwarf feedwater lines
Field notes
- Use operating density, not reference density. A gas quoted at standard conditions and metered at line pressure will not reconcile unless both are on the same basis.
- Absolute pressure, always. Feeding gauge pressure into the gas law understates density by one atmosphere — most damaging at low pressures. The gauge converter handles it.
- Liquid density moves with temperature too, by around 0.7% per 10 °C for hydrocarbons. For custody work use the corrected figure from the density and °API converter.
- Two-phase flow has no single density. Wet steam or a gassy liquid cannot be converted with one number; separate the phases or meter them separately.
- Sizing a line from this result? Take the volumetric figure into the pipe velocity calculator with the true bore from the pipe schedule lookup.
Frequently asked questions
How do I convert kg/h to m³/h?
Divide the mass flow by the fluid density: m³/h = kg/h ÷ ρ (kg/m³). For water at about 998 kg/m³, 1,000 kg/h is 1.002 m³/h. The conversion is impossible without a density, which is why no generic converter can answer it.
What is 1000 kg/h in m³/h?
It depends entirely on the fluid. 1,000 kg/h is 1.00 m³/h of water, 1.19 m³/h of diesel, 194 m³/h of saturated steam at 10 bar absolute, and 831 m³/h of air at atmospheric pressure and 20 °C — a range of nearly a thousand to one.
How do I convert kg/h of steam to m³/h?
Use the specific volume of steam at the operating pressure. Saturated steam at 10 bar absolute has a specific volume of 0.1943 m³/kg, so 1,000 kg/h becomes 194.3 m³/h. Steam volume changes dramatically with pressure, so the pressure must be stated.
What is the difference between m³/h, Nm³/h and Sm³/h?
m³/h is the actual volume at operating conditions. Nm³/h is normalised to 0 °C and 1 atm; Sm³/h is normalised to a standard temperature, commonly 15 °C. For gases these differ substantially — the same mass flow gives different numbers in each.
How do I convert m³/h back to kg/h?
Multiply by density: kg/h = m³/h × ρ. The same density caveat applies — for a gas it must be the density at the actual operating pressure and temperature, not at standard conditions.
Provided for reference and education. Gas densities use the ideal gas law and steam properties are interpolated from the saturation line — confirm against validated property data for design work. See our disclaimer.