Absolute Humidity Calculator - Air Water Vapor Density

An absolute humidity calculator turns air temperature, relative humidity, and atmospheric pressure into the mass of water vapor suspended in one cubic meter of moist air. The calculator uses the ideal gas law applied to water vapor, with the Tetens equation feeding in the saturation vapor pressure, so the result matches the tables printed in HVAC, psychrometrics, and meteorology references.

• • HVAC and indoor air-quality work: Translate a building sensor reading into the absolute water content the air actually carries, in g/m^3.
• • Meteorology and weather data: Convert a surface weather observation into a humidity value comparable across temperature ranges.
• • Classroom psychrometrics and physics: Work a standard absolute humidity problem set against the ideal gas derivation and the Tetens reference without rebuilding the formula by hand.
• • Drying, storage, and condensation checks: Read the water content of air entering a desiccant, a cold-storage room, or a compressed-air line to judge condensation risk.

Absolute humidity is the mass of water vapor per unit volume of moist air, reported in g/m^3 in most references and kg/m^3 in some meteorological data sets. Relative humidity compares the current vapor pressure to the saturation vapor pressure at the same temperature, so it changes whenever the temperature changes even if the actual water content stays fixed.

If the reason you are converting relative humidity into absolute humidity is a plant or HVAC control loop, the Vapor Pressure Deficit Calculator covers the matching saturation minus actual pressure using the same Tetens input set.

The calculator takes air temperature, relative humidity, and atmospheric pressure. It runs the Tetens equation to find saturation vapor pressure in pascals, multiplies by relative humidity to get the actual vapor pressure, and uses the ideal gas law to convert that pressure into grams of water vapor per cubic meter of moist air. Mixing ratio and specific humidity are then derived from the same vapor pressure.

• T_C (air temperature): Air temperature in degrees Celsius. Drives the Tetens exponent and the kelvin conversion T_K = T_C + 273.15.
• RH (relative humidity): Relative humidity as a percent from 0 to 100. The ratio RH/100 scales the saturation pressure to the actual pressure.
• P_atm (atmospheric pressure): Atmospheric pressure in kilopascals, default 101.325 kPa. Used in the mixing ratio.
• P_sat (saturation vapor pressure): Maximum water vapor pressure the air could hold at T_C, in pascals.
• P_v (actual vapor pressure): Water vapor pressure present, equal to P_sat times RH/100.
• AH (absolute humidity): Mass of water vapor per cubic meter of moist air, in g/m^3. Equals (P_v x M_w) / (R x T_K).

The result is split into visible pieces so the same intermediate numbers stay exposed: saturation pressure, actual pressure, absolute humidity in g/m^3, and the mixing ratio. That makes it easy to see whether the result is moving because the temperature changed or because the humidity shifted at fixed temperature.

According to Engineering Toolbox: Water Vapor and Saturation Pressure, absolute humidity of moist air can be expressed in g/m^3 as P_v x M_w / (R x T_K), and the saturation absolute humidity at 25 degrees C and 100 percent RH is 23.02 g/m^3, the reference value most building-science tables print.

The same PV = nRT derivation that powers the Ideal Gas Calculator turns the actual vapor pressure into a grams-per-cubic-meter result here.

Four ideas sit behind the formula: an ideal-gas conversion of vapor pressure into mass per volume, a temperature-driven saturation pressure, a humidity-scaled actual pressure, and the mixing ratio that pairs the vapor with dry air.

These four quantities explain why absolute humidity, mixing ratio, and specific humidity are almost equal at room temperature but diverge at the high vapor pressures reached in tropical air.

For a closer look at the partial-pressure picture behind P_v, the Gas Laws Calculator covers the ideal gas law and Dalton's law for the same problem.

Three numbers go in, and four water-content outputs plus the actual vapor pressure come out. Defaults are 25 degrees C, 50 percent RH, 101.325 kPa

1. 1 Enter the air temperature: Type the dry-bulb air temperature in degrees Celsius, with 0.1 degree precision if the sensor is calibrated that fine.
2. 2 Enter the relative humidity: Type the relative humidity as a percent. Values above 100 are clamped to 100, which reproduces a fully saturated air sample.
3. 3 Enter the atmospheric pressure: Type the local pressure in kilopascals. Leave the default 101.325 kPa for sea-level work, or lower it to about 80 kPa for an 1800 m site.
4. 4 Read the absolute humidity: The primary row reports the mass of water vapor in grams inside one cubic meter of moist air, with a secondary kg/m^3 readout underneath.
5. 5 Read the saturation absolute humidity: The saturation row reports the maximum absolute humidity the air could hold at the chosen temperature.
6. 6 Read the mixing ratio and specific humidity: The mixing ratio and specific humidity rows report the same water content in g/kg of dry or moist air.

For a 22 degrees C, 45 percent RH server room, the absolute humidity calculator reads AH = 8.73 g/m^3. ASHRAE recommends server-room dew points between 5.5 and 15 degrees C, which lines up with AH in the 6 to 12 g/m^3 band.

If the same reading is being used to judge human comfort, the Heat Index Calculator converts it into the perceived temperature you can quote to a building occupant.

The calculator is most useful when the goal is to translate a sensor reading into the actual water content of the air, expressed in the unit the downstream calculation expects.

• • Six outputs from three inputs: Computes absolute humidity in g/m^3 and kg/m^3, saturation absolute humidity, mixing ratio, specific humidity, and actual vapor pressure.
• • Ideal-gas anchored: Uses the AH = P_v x M_w / (R x T_K) form, matching the ASHRAE Handbook of Fundamentals.
• • Saturation ceiling shown: Reports the saturation absolute humidity alongside the actual value.
• • Altitude-aware mixing ratio: Lets the user override atmospheric pressure so the mixing ratio stays correct at high-altitude sites.
• • Input clamping: Clamps out-of-range humidity entries to 0 or 100 and surfaces a small validation message.

These benefits matter most when the same calculation is repeated many times a day, as in a building automation system that logs a sensor every minute.

According to ASHRAE Handbook, absolute humidity is the mass of water vapor per unit volume of moist air, while humidity ratio and specific humidity are the same water content per unit mass of dry or moist air.

Absolute humidity and surface temperature together drive condensation risk, and the Heat Transfer Conduction Calculator covers the conductive heat flux through a wall or window.

Three numbers drive the result, and three contextual factors decide whether the absolute humidity is comfortable, productive, or out of range.

• • The Tetens equation is accurate within about 0.4 percent for 0 to 50 degrees C. Outside that range a Goff-Gratch or Buck equation is needed.
• • The ideal gas law treats water vapor as an ideal gas, which is a good approximation below about 200 kPa.
• • Below 0 degrees C the Tetens equation gives the saturation pressure over supercooled water, but ice has a slightly different saturation curve.

These caveats matter because the ideal gas derivation is a textbook fit, not a real-world measurement.

According to Wikipedia: Humidity, absolute humidity is the mass of water vapor per unit volume of moist air, given in g/m^3, and is computed from the actual vapor pressure using the ideal gas law AH = M_w x P_v / (R x T_K).

For an indoor-cultivation site, absolute humidity sets the transpiration load on a leaf, and the CO2 Grow Room Calculator covers the carbon dioxide side of the same indoor-cultivation problem.