Guide
Nm³ to kg and back: a conversion table for oxygen nitrogen, argon, CO₂, hydrogen, helium, N₂O, acetylene, air and propane
One normal cubic metre (Nm³, gas at 0 °C and 1.01325 bar) of oxygen weighs 1.429 kg, so 1 kg of oxygen is 0.700 Nm³. Nitrogen is 1.250 kg/Nm³, argon 1.784, carbon dioxide 1.977, hydrogen 0.0899 and acetylene 1.172. Multiply Nm³ by the density to get kg; divide kg by it to get Nm³.
- Oxygen
- 1.4290 kg/Nm³; 0.6998 Nm³/kg; 1 litre of liquid oxygen at its boiling point gives 0.799 Nm³
- Nitrogen
- 1.2504 kg/Nm³; 0.7997 Nm³/kg; 1 litre of liquid nitrogen gives 0.645 Nm³
- Carbon dioxide
- 1.977 kg/Nm³; 0.506 Nm³/kg; 0.534 m³/kg at 15 °C
- Hydrogen
- 0.08988 kg/Nm³; 11.13 Nm³ per kg
- Acetylene
- 1.17167 kg/Nm³; 0.8535 Nm³/kg; 0.918 m³/kg at 20 °C
To convert normal cubic metres of a gas to kilograms, multiply by the gas's density at 0 °C and 1.01325 bar; to go from kilograms to Nm³, divide by it. Oxygen weighs 1.429 kg/Nm³, nitrogen 1.250, argon 1.784 and carbon dioxide 1.977. The difficulty is not the arithmetic but the reference conditions, which differ between Nm³, Sm³ and scf, and between suppliers. This page fixes them, gives NIST-based densities for ten gases, and adds the litres of liquid that one Nm³ represents for gases supplied as liquid.
Nm³, Sm³ and scf: say which one you mean
| Unit | Temperature | Pressure | Common user | Factor from 1 Nm³ |
|---|---|---|---|---|
| Normal cubic metre, Nm³ | 0 °C (273.15 K) | 101.325 kPa (1 atm) | European and Indian plant ratings, PSA and ASU capacity | 1 |
| Standard cubic metre, Sm³ | 15 °C (288.15 K) | 101.325 kPa | Natural gas (ISO 13443); Gas Cylinders Rules 2016 use 15 °C | 1.055 |
| Cylinder contents to IS 309 / IS 1747 | 27 °C (300.15 K) | 760 mm Hg | Oxygen and nitrogen cylinder contents in India | 1.099 |
| Standard cubic foot, scf (60 °F) | 15.6 °C | 14.696 psia | Natural gas practice | 37.33 scf |
| Standard cubic foot, scf (70 °F) | 21.1 °C | 14.696 psia | US industrial-gas practice | 38.04 scf |
The factors use the ideal-gas temperature ratio at constant pressure, which holds within a fraction of a per cent at atmospheric pressure. One cubic foot is 0.0283168 m³.
A PSA oxygen plant rated at 50 Nm³/h and a cylinder quoted at 7.6 m³ (at 27 °C) are therefore on different scales: 7.6 m³ at 27 °C is only 6.9 Nm³.
The Nm³ to kg conversion table
| Gas | kg per Nm³ | Nm³ per kg | kg per m³ at 15 °C, 1 atm | Litres of liquid per Nm³ | Nm³ per litre of liquid | Liquid basis |
|---|---|---|---|---|---|---|
| Oxygen, O₂ | 1.4290 | 0.6998 | 1.354 | 1.252 | 0.799 | 1,141.2 kg/m³ at −182.96 °C |
| Nitrogen, N₂ | 1.2504 | 0.7997 | 1.185 | 1.551 | 0.645 | 806.1 kg/m³ at −195.80 °C |
| Argon, Ar | 1.7840 | 0.5605 | 1.690 | 1.278 | 0.782 | 1,395.4 kg/m³ at −185.85 °C |
| Carbon dioxide, CO₂ | 1.977 | 0.5058 | 1.872 | 1.919 | 0.521 | about 1,030 kg/m³, saturated at −20 °C |
| Hydrogen, H₂ | 0.08988 | 11.126 | 0.0852 | 1.269 | 0.788 | 70.85 kg/m³ at −252.78 °C |
| Helium, He | 0.17848 | 5.603 | 0.1692 | 1.432 | 0.698 | 124.7 kg/m³ at −268.93 °C |
| Nitrous oxide, N₂O | 1.9779 | 0.5056 | 1.873 | 1.608 | 0.622 | 1,230.4 kg/m³ at −88.47 °C |
| Acetylene, C₂H₂ | 1.17167 | 0.8535 | 1.109 | — | — | Never handled as liquid |
| Air (dry) | 1.2929 | 0.7735 | 1.225 | — | — | Not supplied as liquid |
| Propane, C₃H₈ | 2.0105 | 0.4974 | 1.899 | 3.962 | 0.252 | 507.5 kg/m³, saturated at 15 °C |
Sources: NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems, for all gas and liquid densities except acetylene, air and the CO₂ liquid figure. Acetylene: Bottomley, Reeves and Seitlow, as used on the acetylene technical data sheet. Air: standard dry air at 0 °C, and the ICAO standard atmosphere at 15 °C. CO₂ liquid: NIST saturation data as tabulated on carbon dioxide technical data. Litres of liquid per Nm³ = gas density ÷ liquid density × 1,000.
Liquid figures are for liquid at its boiling point at 1 atm unless stated. Liquid in a pressurised storage tank is a few degrees warmer and slightly less dense, so it yields a little less gas per litre than the table shows.
Worked examples
Oxygen for a hospital or a cutting shop
A hospital's medical oxygen use is 500 Nm³ per day.
- Mass: 500 × 1.429 = 714.5 kg per day.
- As liquid oxygen: 500 × 1.252 = 626 litres of liquid per day.
- As cylinder contents to IS 309 (27 °C): 500 × 1.099 = 549 m³.
A PSA plant rating
A PSA oxygen plant producing 50 Nm³/h delivers 50 × 1.429 = 71.5 kg/h, or about 1,715 kg in 24 hours. KIGL's PSA oxygen plants and PSA nitrogen plants are rated in Nm³/h, which is the comparison basis to use against liquid or cylinder supply; see cylinder supply or on-site generation.
Liquid nitrogen to gas
A 1,000 litre delivery of liquid nitrogen weighs 1,000 × 0.8061 = 806 kg and gives 806 ÷ 1.2504 = 645 Nm³, or about 680 m³ at 15 °C. The commonly quoted expansion ratio of about 694 is the same liquid measured as gas at 21 °C. More on liquid nitrogen properties and safety.
Carbon dioxide by weight
A cylinder holding 30 kg of carbon dioxide gives 30 × 0.5058 = 15.2 Nm³, or 30 ÷ 1.872 = 16.0 m³ at 15 °C. Because CO₂ is liquid in the cylinder, its contents can only be checked by weighing; see permanent vs liquefied gases.
Acetylene by weight
Dissolved acetylene is sold by net weight. Each kilogram is 0.8535 Nm³, 0.902 m³ at 15 °C and 0.918 m³ at 20 °C, from the measured densities of Bottomley, Reeves and Seitlow. The weighing procedure is on how to check acetylene cylinder contents by weight.
Hydrogen
One kilogram of hydrogen is 11.13 Nm³. Hydrogen's real-gas behaviour at cylinder pressure (Z above 1) means a cylinder holds less than Boyle's law suggests; see gas laws for cylinder users.
Why not just use 22.414 litres per mole
The ideal-gas molar volume at 0 °C and 1 atm is 22.414 litres, so the ideal density is molar mass ÷ 22.414. For the permanent gases it is close: oxygen 1.4276 against the real 1.4290 kg/m³. For gases near condensation it is not:
| Gas | Ideal density, kg/Nm³ | Real density, kg/Nm³ | Real heavier by |
|---|---|---|---|
| Oxygen | 1.4276 | 1.4290 | 0.1 % |
| Carbon dioxide | 1.9635 | 1.977 | 0.7 % |
| Acetylene | 1.1617 | 1.17167 | 0.85 % |
| Propane | 1.9674 | 2.0105 | 2.2 % |
For billing CO₂, acetylene or LPG by volume, the difference is commercially significant; use the measured density.
Three conversion mistakes that recur in quotations
- Mixing Nm³ and m³ at 27 °C. A cylinder quoted at "7 m³" under IS 309 conventions and a plant quoted at "7 Nm³" differ by almost 10 %. Convert both to Nm³ before comparing cost per unit of gas.
- Using the room-temperature expansion ratio for liquid. The 694:1 ratio for liquid nitrogen (gas at 21 °C) and roughly 860:1 for liquid oxygen (gas at 20 °C) are not normal-condition figures; in Nm³ they are 645 and 799. A bulk contract stated in Nm³ needs the Nm³ figures.
- Treating a pressure gauge as a contents meter for liquefied gases. For CO₂, N₂O and propane, convert from weighed kilograms, never from pressure.
Rules that matter
Rules to apply
- Write the reference conditions next to every volume: Nm³ (0 °C), Sm³ (15 °C) or m³ at 27 °C.
- Convert kg to Nm³ by dividing by the density in the table; convert Nm³ to kg by multiplying.
- To move between reference temperatures, multiply by the ratio of absolute temperatures: Nm³ × 1.055 = m³ at 15 °C; Nm³ × 1.099 = m³ at 27 °C.
- For liquids, use the density of the liquid as delivered, and accept that warm tank liquid yields slightly less than the boiling-point figure.
- For gas properties beyond density, see the industrial gas properties table.
Nm³ — frequently asked questions
How many kg is 1 Nm³ of oxygen?
1.429 kg. The NIST Chemistry WebBook gives oxygen a density of 1.4290 kg/m³ at 0 °C and 1.01325 bar, the conditions that define the normal cubic metre. Going the other way, 1 kg of oxygen is 0.700 Nm³. At 15 °C the same kilogram occupies 0.738 m³, and at 27 °C, the temperature IS 309:2023 uses for cylinder contents, about 0.77 m³.
What is the difference between Nm³ and Sm³?
The reference temperature. A normal cubic metre is gas at 0 °C and 101.325 kPa (DIN 1343). A standard cubic metre is usually gas at 15 °C and 101.325 kPa (ISO 13443), the temperature the Gas Cylinders Rules 2016 also use. The same quantity of gas is about 5.5 % larger in Sm³: 1 Nm³ ≈ 1.055 Sm³. Always ask which convention a quotation uses.
How do I convert a gas flow in Nm³/h to kg/h?
Multiply by the gas density at normal conditions. A 50 Nm³/h oxygen supply is 50 × 1.429 = 71.5 kg/h; 50 Nm³/h of nitrogen is 62.5 kg/h; 50 Nm³/h of acetylene is 58.6 kg/h. To convert kg/h back to Nm³/h, divide by the same density. The figures are in the table on this page, from NIST data and, for acetylene, measured densities.
How many Nm³ of carbon dioxide are in 1 kg?
0.506 Nm³, since carbon dioxide weighs 1.977 kg/Nm³ (NIST). At 15 °C and 1 atm the kilogram occupies 0.534 m³, and at 20 °C about 0.544 m³. A cylinder holding 30 kg of CO₂ therefore delivers about 15.2 Nm³ or 16.0 m³ at 15 °C, which is why CO₂ is bought and checked by weight rather than volume.
What is 1 Nm³ in standard cubic feet?
About 37.3 scf if the standard is 60 °F (15.6 °C) and 14.696 psia, or about 38.0 scf at 70 °F (21.1 °C), the convention common in the US industrial-gas trade. One cubic foot is 0.0283168 m³, and the gas expands with temperature from 0 °C. The figures use the ideal-gas temperature ratio, which is within a fraction of a per cent at 1 atm.
Why not use 22.4 litres per mole for every gas?
Because 22.414 litres per mole is the ideal-gas molar volume at 0 °C and 1 atm, and real gases differ. For oxygen, nitrogen and argon the error is under 0.1 %, but carbon dioxide is 0.7 % denser than ideal, acetylene 0.85 % and propane 2.2 %. Use measured densities, as in the table here, for any figure that affects billing or filling.
How many kg is 1 Nm³ of argon?
1.784 kg, from the NIST density of argon at 0 °C and 1 atm. One kilogram of argon is 0.5605 Nm³, or 0.592 m³ at 15 °C. Argon is the densest of the three air gases, so a cylinder of argon weighs noticeably more than an oxygen or nitrogen cylinder of the same size and pressure: about 12.5 kg of gas in a 46.7 litre cylinder at 150 bar, against about 10.1 kg of oxygen.
References
- Bureau of Indian Standards. (2023). Oxygen and IS 1747:2023 Nitrogen — Specifications (cl 6.2 contents at 27 °C) (IS 309:2023, Fifth Revision).
- CGA — Compressed Gas Association
- Government of India. (2016). Gas Cylinders Rules, 2016 (G.S.R. 1081(E)). Petroleum and Explosives Safety Organisation.
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [, SRD 69 — Thermophysical Properties of Fluid Systems] https://webbook.nist.gov/chemistry/fluid/
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