Quick answer: Absolute humidity is the mass of water vapour in a volume of air, usually in g/m³. Most instruments do not measure it directly: they measure temperature and relative humidity (or dew point) and calculate it. The formula is AH (g/m³) ≈ 216.7 × e ÷ (T + 273.15), where e is the vapour pressure in hPa and T the temperature in °C. For engineering work, the related humidity ratio (g of water per kg of dry air) is more useful because it does not change when air is heated or cooled. For direct, high-accuracy measurement, use a chilled-mirror dew point hygrometer.
Quick answer
- Absolute humidity: g of water per m³ of air.
- Humidity ratio (mixing ratio): g of water per kg of dry air.
- Both stay the same when air warms or cools (without condensing); RH does not.
Three ways to express humidity
| Quantity | Unit | Changes with temperature? | Use |
|---|---|---|---|
| Relative humidity | % | Yes | Comfort, mold, material equilibrium |
| Absolute humidity | g/m³ | Slightly (air volume changes) | Comparing amounts of water in air; meteorology |
| Humidity ratio | g/kg dry air | No | HVAC, dehumidifier sizing, ventilation calculations |
| Dew point | °C | No | Condensation risk |
The US National Weather Service explains RH, dew point and their relation (NWS).
How to calculate absolute humidity
- Saturation vapour pressure at T (°C), in hPa: e_s = 6.108 × exp(17.27 × T ÷ (T + 237.3))
- Vapour pressure: e = RH ÷ 100 × e_s
- AH (g/m³) = 216.7 × e ÷ (T + 273.15)
Example: 20 °C and 50 % RH: e_s = 23.4 hPa, e = 11.7 hPa, AH = 216.7 × 11.7 ÷ 293.15 ≈ 8.6 g/m³.
Humidity ratio: x (g/kg) = 622 × e ÷ (p − e), with p the air pressure in hPa (1013 at sea level). At 20 °C / 50 % RH, x ≈ 7.3 g/kg.
Absolute humidity table (g/m³)
| Temperature | 30 % RH | 50 % RH | 70 % RH | 90 % RH | 100 % RH |
|---|---|---|---|---|---|
| 0 °C | 1.5 | 2.4 | 3.4 | 4.4 | 4.8 |
| 10 °C | 2.8 | 4.7 | 6.6 | 8.5 | 9.4 |
| 20 °C | 5.2 | 8.6 | 12.1 | 15.6 | 17.3 |
| 30 °C | 9.1 | 15.2 | 21.2 | 27.3 | 30.3 |
| 40 °C | 15.3 | 25.5 | 35.7 | 45.9 | 51.0 |
Notice that cold air at 90 % RH (10 °C: 8.5 g/m³) holds about the same water as room air at 50 % RH (20 °C: 8.6 g/m³). That is why ventilating with cold, "humid" winter air can dry a building.
Instruments
| Instrument | How it gets absolute humidity | Typical use |
|---|---|---|
| Thermo-hygrometer (capacitive RH + temperature) | Calculates from T and RH | Buildings, warehouses, greenhouses, HVAC |
| Dew point transmitter | Measures dew point; calculates AH or humidity ratio | Process air, dry rooms, compressed air |
| Chilled-mirror hygrometer | Measures dew point directly by cooling a mirror | Reference, calibration, laboratories |
| Psychrometer (wet and dry bulb) | Calculates from two temperatures | Field checks |
| Optical / spectroscopic analysers | Measure water vapour concentration directly | Gas processing, research |
NIST maintains humidity standards and hygrometer calibration services (NIST).
Accuracy
The calculated value inherits errors from both sensors. At 20 °C and 50 % RH, an RH error of ±2 % gives about ±4 % error in absolute humidity; a temperature error of ±0.3 °C adds about ±2 %, depending on sensor design. For tight process control, use dew point instruments, which measure moisture content more directly.
Using absolute humidity in practice
- Ventilation decisions: ventilate only when outdoor air holds less water (g/kg or g/m³) than indoor air.
- Dehumidifier sizing: water load = air mass flow × difference in humidity ratio; see dehumidifier sizing.
- Drying processes: track moisture removal by the humidity ratio difference between air entering and leaving.
- Condensation: convert to dew point and compare with surface temperatures; see dew point sensor placement.
FAQ
Frequently asked questions
What is absolute humidity?
The mass of water vapour in a cubic metre of air, in g/m³.
How do you measure absolute humidity?
Usually by measuring temperature and RH (or dew point) and calculating it; chilled-mirror hygrometers measure dew point directly for high accuracy.
What is a normal indoor absolute humidity?
About 7–11 g/m³ at typical room conditions (20–22 °C and 40–55 % RH).
What is the difference between absolute humidity and humidity ratio?
Absolute humidity is per volume of air (g/m³); humidity ratio is per mass of dry air (g/kg) and does not change with temperature.
Sources
- US National Weather Service, Humidity.
- NIST, Humidity programmes and hygrometers.
- ASHRAE Handbook—Fundamentals, Psychrometrics; Measurement and instruments.
