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The essentials in 30 seconds
Vactual = Vn × (Pn / P) × (T / Tn)
- A normal cubic metre (Nm³) is a quantity of gas brought back to 0 °C and 1,013.25 hPa (DIN 1343).
- Use the absolute pressure (gauge + atmospheric) and temperatures in kelvins.
- The actual m³/h, not the Nm³/h, sets the velocity and the pressure drop in the pipe.
Key words
- Nm³
- Normal cubic metre: the volume a gas would occupy at 0 °C and 1,013.25 hPa (DIN 1343) or 15 °C (ISO 2533).
- absolute pressure
- Gauge pressure plus atmospheric pressure, about 101,325 Pa at sea level.
- kelvins
- Absolute temperature: °C + 273.15.
- mass flow
- Mass of gas per unit time, in kg/h; it does not change when the gas is compressed or heated.
How do you convert Nm3/h to m3/h?
A normal cubic metre (Nm³) is a volume of gas brought back to normal conditions: 0 °C and 1,013.25 hPa under the DIN 1343 standard, or 15 °C and 1,013.25 hPa under ISO 2533. To convert a normal flow rate in Nm³/h into the actual flow rate in m³/h at operating conditions, the ideal gas law applies: the actual volume is proportional to the absolute temperature and inversely proportional to the absolute pressure. The actual flow rate, not the normal one, is the value to use for the velocity and the pressure drop in a pipe.
Nm³/h to m³/h conversion formula
- Vn: normal volume or flow rate, in Nm³ or Nm³/h
- Vactual: actual volume or flow rate at operating conditions, in m³ or m³/h
- Pn: normal pressure, 101,325 Pa; P: absolute pressure of the gas, in Pa (gauge pressure + atmospheric pressure)
- Tn: normal temperature, 273.15 K (DIN 1343) or 288.15 K (ISO 2533); T: gas temperature in kelvins (°C + 273.15)
Example: 100 Nm³/h of compressed air at 6 bar gauge and 20 °C (DIN 1343): Vactual = 100 × 101,325 / 701,325 × 293.15 / 273.15 = 15.5 m³/h. The reverse conversion, from m³/h to Nm³/h, divides instead of multiplying.
Conversion table: actual m³ per Nm³ against pressure
| Gauge pressure (bar) | Gas at 0 °C | Gas at 20 °C |
|---|---|---|
| 0 | 1.000 | 1.073 |
| 1 | 0.503 | 0.540 |
| 2 | 0.336 | 0.361 |
| 3 | 0.252 | 0.271 |
| 4 | 0.202 | 0.217 |
| 5 | 0.167 | 0.179 |
| 6 | 0.144 | 0.155 |
| 7 | 0.128 | 0.137 |
| 8 | 0.112 | 0.121 |
| 10 | 0.092 | 0.099 |
Reading: 1 Nm³ of air occupies 0.155 m³ at 6 bar gauge and 20 °C; conversely 1 actual m³ contains 1 / 0.155 = 6.45 Nm³.
Detailed explanations and calculation in Mecaflux
The gas being compressible, it is possible to change a volume of the same amount of gasby compressing or changing its temperature. It becomes very difficult to speak of a quantity of gas volume without giving the pressure and temperature of the gas when the volume was measured. It would be difficult to give a temperature and pressure whenever we speak of a volume of gas. Then the temperature and pressure at the time of measurement of the volume, are normalized at called normal conditions. This is called normal m3.
Attention, there are two standards, and thus potential errors in conversions:
same pressure of 1013 millibars (average atmospheric pressure) for 2 standards:
DIN 1343 : temperature 273.15K (0°C)
ISO 2533 : temperature 288.15K (15°C)
For calculations of head loss, the size of volumes of gas (compressed or not) moved into the ducts must be given in cubic meters (m3).
For conversions or Nm3 Nm3 m3 / h m3 / h:
P1V1/T1 = P2V2/T2
p1 and T1 being normal temperatures and pressures v1 is the normal volume (relaxed)
p2 and T2 being the temperature and pressure gas V2 is the volume of the compressed gas
The converter integrated in Mecaflux Standard
The conversion interface normal volume (nm3) and real (m3) converts the normal flow (Nm3 / h) to real (m3 / h)
Frequently asked questions
What is the difference between a Nm³ and a m³?
The m³ is an actual volume measured at the pressure and temperature of the moment. The Nm³ is the same gas brought back to normal conditions (0 °C and 1,013.25 hPa): it represents a fixed quantity of matter, whatever the compression. One Nm³ of air always weighs 1.293 kg.
What is the difference between DIN 1343 and ISO 2533 for the Nm³?
Both standards use the same pressure, 1,013.25 hPa, but DIN 1343 sets the normal temperature at 0 °C (273.15 K) and ISO 2533 at 15 °C (288.15 K). An ISO Nm³ therefore contains 5.5 % less gas than a DIN Nm³. Always state which standard is used.
What is a Sm³ or standard cubic metre?
The Sm³ (standard cubic metre) is the equivalent of the Nm³ in many industries, most often defined at 15 °C and 1,013.25 hPa, sometimes at 60 °F (15.56 °C) or 20 °C. Check the reference conditions before converting.
How do I convert the gas flow rates of my own installation to Nm³/h?
The unit converter of Mecaflux Standard goes from actual flow to normal flow and mass flow from the pressure and temperature of your gas, in either standard, and the fluid editor supplies the density of air and common gases for the duct calculations that follow. For a gas, compressed-air or flue gas network to size, Mecaflux carries out the study of your project, or offers training on the software applied to your own installation.