Hong Life CalculatorsUnderstanding the absolute humidity calculator — concept and principle
Absolute humidity is the mass of water vapour in one cubic metre of air. This calculator finds it from air temperature and relative humidity, and also gives the vapour pressure, humidity ratio, specific humidity and dew point. It is useful in HVAC, drying, storage, meteorology and indoor-climate work.
How it is calculated
- Saturation vapour pressure pws at the air temperature from the Hyland–Wexler equations used by ASHRAE (over ice below 0.01 °C).
- Actual vapour pressure pw = RH × pws.
- Absolute humidity AH = pw ÷ (Rw × T), with Rw = 461.5 J/(kg·K) and T in kelvin.
- Humidity ratio W = 0.621945 × pw ÷ (p − pw); specific humidity = W ÷ (1 + W).
- Dew point = temperature at which pws equals pw.
The principle
Warm air can hold more water vapour than cold air, so relative humidity alone does not tell you how much water is present. Absolute humidity does. Treating water vapour as an ideal gas, its density follows from its partial pressure and temperature. The humidity ratio, used in psychrometrics, compares the mass of vapour with the mass of dry air, which is why it depends on the total air pressure.
Worked example
Default inputs: 25 °C, relative humidity 60%, pressure 101.325 kPa.
- Saturation vapour pressure at 25 °C ≈ 31.69 hPa.
- Vapour pressure = 0.60 × 31.69 ≈ 19.02 hPa.
- AH = 1,902 Pa ÷ (461.5 × 298.15 K) ≈ 0.01382 kg/m³ = 13.82 g/m³; at saturation 23.03 g/m³.
- Humidity ratio = 0.621945 × 1.902 ÷ (101.325 − 1.902) ≈ 11.9 g/kg dry air; specific humidity ≈ 11.76 g/kg.
- Dew point ≈ 16.7 °C.
| Air temperature (°C) | Absolute humidity (g/m³) |
|---|---|
| 6.25 | 4.43 |
| 12.5 | 6.6 |
| 18.8 | 9.64 |
| 25 | 13.82 |
| 31.2 | 19.48 |
| 37.5 | 27.01 |
| 50 | 49.68 |
How to read the result
The dew point is often the most practical number: surfaces colder than 16.7 °C in this air will gather condensation. Absolute humidity lets you compare air at different temperatures; for example, cold outdoor air at high RH may contain far less water than warm indoor air at moderate RH.
Common mistakes
- Comparing relative humidities at different temperatures as if they described the same water content.
- Entering pressure in hPa or mmHg instead of kPa.
- Forgetting that pressure at altitude changes the humidity ratio.
- Using absolute humidity when an HVAC calculation needs the humidity ratio.
Frequently asked questions
What is the difference between absolute and relative humidity?
Absolute humidity is the actual mass of water vapour per volume; relative humidity is the vapour pressure as a percentage of the saturation value at that temperature.
What is a comfortable indoor humidity?
Comfort and health guidance often cites relative humidity of roughly 30–60% indoors; check guidance relevant to your setting.
How is dew point related to absolute humidity?
Both depend only on the amount of water vapour; the dew point is the temperature at which that vapour would saturate the air.
Sources: ASHRAE Handbook — Fundamentals, Chapter 1 Psychrometrics · WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), humidity