Absolute Humidity Calculator

Absolute humidity: mass of water vapour per cubic metre

Absolute humidity (AH) is the mass of water vapour per unit volume of air, expressed in g/m³ (metric) or lb/ft³ (imperial). It comes from the ideal gas law applied to the water vapour component: AH = P_w × M_w / (R × T_K), where P_w is the partial vapour pressure in Pa, M_w = 18.015 g/mol is the molar mass of water (the reference value published in NIST's Chemistry WebBook for water vapour thermophysical data), and R = 8.314 J/(mol·K) is the universal gas constant. Simplified: AH (g/m³) = P_w (Pa) × 2.167 / T_K. This gives a direct measurement of moisture mass in a fixed volume of air, independent of elevation as long as P_w is computed from the local RH and saturation pressure at the measured temperature.

Unlike humidity ratio (g/kg of dry air), absolute humidity changes when air is heated or cooled because the volume of the air changes with temperature even when no moisture is added. A cubic metre of air at 10°C holds less mass than a cubic metre at 30°C (the warmer air is less dense), so the same moisture content in g/kg corresponds to a lower AH in g/m³ at higher temperatures. This means AH is not conservative through a sensible heating or cooling process, making it less useful than humidity ratio for HVAC mass-balance calculations. However, AH is the direct output of many sensor technologies and appears in some cleanroom, pharmaceutical, and greenhouse standards. See the moisture content calculator for humidity ratio in g/kg and gr/lb.

Saturation deficit and absolute humidity in Canadian dehumidification

Saturation deficit (AH_sat − AH) is the gap between the actual absolute humidity and the maximum the air could hold at the same temperature. It has the same units as AH (g/m³) and directly measures the drying capacity of the air. High saturation deficit drives rapid moisture evaporation from surfaces, building materials, and occupant skin. In a Canadian winter scenario, outdoor air near saturation at −20°C carries only about 0.9 g/m³ of moisture, so little that the air is functionally bone-dry. Bringing that same air indoors and heating it to 21°C without adding moisture actually pushes its absolute humidity down further, to roughly 0.76 g/m³, because the same mass of water vapour now occupies a larger volume of warmer, less dense air. Since saturation AH at 21°C is about 18.3 g/m³, the resulting deficit is close to 17.6 g/m³. That extreme deficit explains the severe dryness of heated winter air in Canadian buildings and the need for a properly sized whole-home humidifier to close the gap.

Absolute humidity in IAQ monitoring and pharmaceutical HVAC

Capacitive and resistive humidity sensors directly measure relative humidity, but most IAQ data loggers and building automation systems report both RH and absolute humidity after computing AH from the measured T and RH. For pharmaceutical cleanrooms under Health Canada and USP standards, temperature and RH limits are specified as absolute conditions at set points, and compliance calculations often check AH against a maximum g/m³ limit to prevent moisture-driven particulate behaviour; the clean room calculator covers the related particle count and air change requirements for these spaces. Similarly, in cannabis production and food processing facilities across Canada, absolute humidity targets (rather than RH targets) are used to maintain consistent product quality across varying process temperatures. Use this tool to convert any measured T and RH reading into an absolute humidity value suitable for these compliance checks. Cross-reference with the vapor pressure calculator to verify the underlying P_w values.