Absolute Humidity Calculator
Enter air temperature and relative humidity to find the mass of water vapor in each cubic metre of air. You also get actual vapor pressure, saturation vapor pressure, dew point, and the specific humidity. Switch between Celsius and Fahrenheit. All outputs update instantly as you type.
Formula
Worked example
At 25 degC and 60% RH: Psat = 611.657 * exp(17.625*25 / (243.04+25)) = 3167 Pa. Pact = 3167 * 0.60 = 1900 Pa. AH = 1900 / (461.5 * 298.15) * 1000 = 13.83 g/m3. Dew point = (243.04 * gamma) / (17.625 - gamma) where gamma = ln(0.6) + 17.625*25/268.04 = 15.3 degC.
What is absolute humidity?
Absolute humidity is the total mass of water vapor present in a given volume of air, expressed in grams per cubic metre (g/m3). Unlike relative humidity, which is a percentage of the maximum moisture air can hold at a given temperature, absolute humidity is an actual physical quantity. Warm air can hold far more water vapor than cold air: at 35 degC saturated air holds roughly 40 g/m3, while at 0 degC it holds only about 5 g/m3. This is why a cold winter day can feel extremely dry even at 90% relative humidity - the absolute amount of moisture is tiny.
How absolute humidity is calculated
The calculation uses two steps. First, the saturation vapor pressure at the given temperature is found using the Magnus formula: Psat = 611.657 * exp(17.625 * T_C / (243.04 + T_C)), where T_C is temperature in Celsius. Second, the actual vapor pressure is Pact = Psat * (RH / 100). Absolute humidity then follows from the ideal gas law for water vapor: AH (kg/m3) = Pact / (Rv * T_K), where Rv is the specific gas constant for water vapor (461.5 J per kg per Kelvin) and T_K is the absolute temperature in Kelvin. Multiply by 1000 to express the result in g/m3.
Dew point and saturation deficit
The dew point is the temperature at which air becomes saturated if cooled at constant pressure. It is calculated by inverting the Magnus formula. When the air temperature equals the dew point, relative humidity is 100% and condensation begins forming on surfaces. The saturation deficit is the difference between saturation vapor pressure and actual vapor pressure. A large deficit means the air can absorb a lot more moisture before it becomes saturated. HVAC engineers and agronomists use the saturation deficit to assess plant stress and ventilation requirements.
Why absolute humidity matters
Relative humidity is the most commonly reported humidity measure, but absolute humidity is often more practical. In HVAC design, engineers need to know the actual mass of water to add or remove to condition air, not just a percentage. In meteorology and aviation, specific humidity (g/kg of moist air) and mixing ratio are related measures used in weather models and aircraft performance calculations. In building science, the dew point tells you when condensation will form on wall cavities or window glass. Biologically, absolute humidity is directly linked to virus survival and respiratory comfort: studies have found that viral pathogens persist longer at both very low and very high absolute humidity.
Typical absolute humidity by condition
| Condition | Temp (degC) | RH (%) | Absolute Humidity (g/m3) | Comfort |
|---|---|---|---|---|
| Arctic winter | -30 | 80 | 0.2 | Extremely dry |
| Cold winter indoors (heated) | 20 | 30 | 5.2 | Dry |
| Comfortable indoor air | 22 | 50 | 9.7 | Comfortable |
| Recommended office range | 21 | 40 | 7.3 | Comfortable |
| Warm humid summer | 30 | 70 | 21.3 | Muggy |
| Tropical rainforest | 35 | 95 | 38.8 | Very humid |
| ASHRAE comfort zone lower | 20 | 30 | 5.2 | Dry edge |
| ASHRAE comfort zone upper | 26 | 60 | 15.6 | Humid edge |
Approximate absolute humidity values across common climates and indoor environments at standard pressure.
Frequently asked questions
What is the difference between absolute humidity and relative humidity?
Relative humidity (RH) is a percentage: how much water vapor the air currently holds compared to the maximum it could hold at that temperature. Absolute humidity is a mass concentration, typically grams of water vapor per cubic metre of air. The same absolute humidity at different temperatures gives very different relative humidities, because warmer air can hold more vapor. For example, 9.4 g/m3 is about 50% RH at 20 degC but only about 23% RH at 35 degC.
What is a comfortable absolute humidity level?
Most people find 6 to 13 g/m3 comfortable for indoor environments. This broadly corresponds to relative humidities of 30 to 60% at typical indoor temperatures (18 to 25 degC). Below about 4 g/m3 the air feels very dry and can irritate the eyes and respiratory tract. Above 15 to 17 g/m3 conditions feel muggy and sweat evaporation slows, which increases perceived heat.
What formula is used to calculate absolute humidity?
The standard approach is: (1) calculate saturation vapor pressure using the Magnus formula Psat = 611.657 * exp(17.625 * T / (243.04 + T)); (2) multiply by relative humidity to get actual vapor pressure Pact = Psat * RH / 100; (3) apply the ideal gas law for water vapor AH = Pact / (Rv * T_K) where Rv = 461.5 J/(kg*K) and T_K is temperature in Kelvin. This gives kg/m3, which is multiplied by 1000 to get g/m3.
How does altitude affect absolute humidity?
This calculator includes a barometric pressure input for this reason. At altitude, lower air pressure means less total air per cubic metre, so the same relative humidity corresponds to a lower absolute humidity. This calculator uses atmospheric pressure in the specific humidity output. However, the absolute humidity formula itself (via the ideal gas law) depends on vapor partial pressure rather than total pressure, so the barometric pressure mainly affects the specific humidity and mixing ratio outputs.
What is specific humidity and how is it different from absolute humidity?
Specific humidity is the ratio of the mass of water vapor to the total mass of moist air, expressed in grams of vapor per kilogram of moist air (g/kg). Absolute humidity is mass per unit volume (g/m3). The two are closely related but not identical because a kilogram of moist air occupies a volume that depends on temperature and pressure. Specific humidity is commonly used in meteorology and aviation because it is conserved when air parcels move vertically and change pressure.