Density of each gas — the number the conversion turns on
Every volume-to-weight conversion is just the gas density. One Nm³ of hydrogen weighs 90 grams; one Nm³ of carbon dioxide weighs almost two kilograms. Using the wrong gas density is the most common cause of a disputed delivery note.
| Gas | Formula | Molecular weight | Density at 0 °C, 1 atm (g/l) | 1 Nm³ weighs | 1 kg = |
|---|---|---|---|---|---|
| Acetylene | C₂H₂ | 26.038 | 1.1716 | 1.172 kg | 0.854 Nm³ |
| Oxygen | O₂ | 31.999 | 1.4290 | 1.429 kg | 0.700 Nm³ |
| Nitrogen | N₂ | 28.013 | 1.2506 | 1.251 kg | 0.800 Nm³ |
| Argon | Ar | 39.948 | 1.7840 | 1.784 kg | 0.561 Nm³ |
| Carbon dioxide | CO₂ | 44.010 | 1.9770 | 1.977 kg | 0.506 Nm³ |
| Hydrogen | H₂ | 2.016 | 0.0899 | 0.090 kg | 11.123 Nm³ |
| Helium | He | 4.003 | 0.1786 | 0.179 kg | 5.599 Nm³ |
| Nitrous oxide | N₂O | 44.013 | 1.9780 | 1.978 kg | 0.506 Nm³ |
| Compressed air | Air | 28.960 | 1.2930 | 1.293 kg | 0.773 Nm³ |
Nm³ or SCF? They are not the same
A normal cubic metre (Nm³) is measured at 0 °C and 1 atm — the convention used across India and Europe. A standard cubic foot (SCF) in US practice is measured at 60 °F (15.56 °C). Because gas expands as it warms, the same mass occupies more volume at 60 °F than at 0 °C.
The practical consequence: 1 Nm³ = 37.35 SCF, not the 35.31 you get from a pure cubic-metre-to-cubic-foot conversion. Quoting one and invoicing the other is a 5.8% error in the buyer's favour or yours, depending which way round it goes.
| Quantity | Equals | Note |
|---|---|---|
| 1 Nm³ | 37.35 SCF | Includes the 0 °C → 60 °F temperature correction |
| 1 Nm³ | 1,000 litres of gas | At the same 0 °C, 1 atm basis |
| 1 Nm³ | 1.055 m³ at 15 °C | The same gas, measured warmer |
| 1 SCF | 0.02677 Nm³ | The reverse conversion |
| 1 kg | Depends entirely on the gas | See the density table above |
Common conversions, pre-calculated
The values engineers look up most often, worked out for each gas so you do not have to.
| Gas | 1 Nm³ → kg | 10 Nm³ → kg | 1 kg → Nm³ | 1 Nm³ → SCF | 1 Nm³ → lb |
|---|---|---|---|---|---|
| Acetylene | 1.172 | 11.72 | 0.854 | 37.35 | 2.583 |
| Oxygen | 1.429 | 14.29 | 0.700 | 37.35 | 3.150 |
| Nitrogen | 1.251 | 12.51 | 0.800 | 37.35 | 2.757 |
| Argon | 1.784 | 17.84 | 0.561 | 37.35 | 3.933 |
| Carbon dioxide | 1.977 | 19.77 | 0.506 | 37.35 | 4.359 |
| Hydrogen | 0.090 | 0.90 | 11.123 | 37.35 | 0.198 |
| Helium | 0.179 | 1.79 | 5.599 | 37.35 | 0.394 |
| Nitrous oxide | 1.978 | 19.78 | 0.506 | 37.35 | 4.361 |
| Compressed air | 1.293 | 12.93 | 0.773 | 37.35 | 2.851 |
What a standard 47-litre cylinder holds
A 47-litre water-capacity cylinder charged to 150 kg/cm² holds about 6.87 Nm³ of gas. What that weighs — and therefore what it costs to freight — again depends on the gas.
| Gas | Nm³ in a 47 L cylinder at 150 kg/cm² | Weight of gas | Notes |
|---|---|---|---|
| Oxygen | 6.87 Nm³ | 9.82 kg | Permanent gas — content scales with pressure |
| Nitrogen | 6.87 Nm³ | 8.59 kg | Permanent gas — content scales with pressure |
| Argon | 6.87 Nm³ | 12.26 kg | Permanent gas — content scales with pressure |
| Hydrogen | 6.87 Nm³ | 0.62 kg | Permanent gas — content scales with pressure |
| Helium | 6.87 Nm³ | 1.23 kg | Permanent gas — content scales with pressure |
| Compressed air | 6.87 Nm³ | 8.88 kg | Permanent gas — content scales with pressure |
| Acetylene | Not applicable | Sold by weight | Dissolved in acetone over a porous mass — cylinder pressure does not indicate content |
| Carbon dioxide | Not applicable | ~30 kg typical | Liquid under its own vapour pressure — gauge reads full until nearly empty |
| Nitrous oxide | Not applicable | Sold by weight | Liquefied under pressure |
Cylinder figures assume ideal-gas behaviour, which is how filling stations reckon it and is accurate to well within a percent at these pressures.