When to use: Use to determine moist air properties from dry-bulb temperature and relative humidity. Essential for AHU coil selection, dehumidification design, economizer analysis, and cooling tower performance. Enthalpy (BTU/lb) is used to calculate the energy content difference across coils. Dew point indicates when moisture will condense on surfaces — critical for preventing mold and condensation issues.
This psychrometric calculator computes the properties of moist air — relative humidity, humidity ratio, dew point, wet-bulb temperature, enthalpy, and specific volume — from two known properties such as dry-bulb temperature and relative humidity. It does the chart-reading and saturation-pressure math for you, returning a full air state in seconds. It's built for HVAC engineers, mechanical designers, energy modelers, commissioning agents, and students who need to size coils, analyze dehumidification, evaluate economizers, or check for condensation risk.
Moist air is described by a handful of interrelated properties, and fixing any two of them fixes all the rest at a given barometric pressure:
• Dry-bulb temperature (DB): the air temperature read by an ordinary thermometer. • Wet-bulb temperature (WB): the temperature read by a thermometer with a wetted wick in moving air, reflecting evaporative cooling. WB equals DB only when the air is fully saturated. • Relative humidity (RH): the ratio of the actual water-vapor pressure to the saturation pressure at the same temperature, expressed as a percent. • Humidity ratio (W): the mass of water vapor per unit mass of dry air, in pounds of water per pound of dry air (lb/lb) or grains per pound (7,000 grains = 1 lb). • Dew point (DP): the temperature to which air must be cooled at constant pressure to reach saturation, where condensation begins. • Enthalpy (h): the total heat content of the air, sensible plus latent, in Btu per pound of dry air. • Specific volume: the volume occupied per pound of dry air (ft³/lb), used to convert between mass and volumetric airflow (cfm).
The psychrometric chart plots all of these at once: DB on the horizontal axis, humidity ratio on the vertical axis, with curves of constant RH, WB, enthalpy, and specific volume overlaid. Reading any point gives the complete air state.
Worked example: air at 75°F dry-bulb and 50% relative humidity has a dew point of roughly 55°F and a humidity ratio of about 0.0093 lb water per lb dry air (≈65 grains/lb), with an enthalpy near 28 Btu/lb — a typical indoor comfort condition.
1. Enter two known properties — most commonly dry-bulb temperature and relative humidity. 2. The calculator solves the saturation-pressure relationship to fix the air state. 3. Read the remaining properties: wet-bulb, dew point, humidity ratio, enthalpy, and specific volume. 4. Use the outputs to size equipment — for example, compare enthalpy at the entering and leaving coil conditions to find total cooling load, or check the dew point against surface temperatures to assess condensation risk.
Psychrometrics is the foundation of load calculations and air-side design. Total cooling load splits into sensible heat (changing dry-bulb temperature) and latent heat (removing moisture, i.e. lowering humidity ratio); the enthalpy difference across a coil captures both at once, which is essential for correct coil sizing and dehumidification design.
Comfort standards such as ASHRAE Standard 55 define an acceptable envelope of temperature and humidity, so designers use the chart to confirm supply conditions land inside the comfort zone. Dew point control prevents condensation on cold ducts, windows, and equipment, avoiding mold and water damage — a surface below the air's dew point will sweat. Getting the psychrometrics right is what keeps a system comfortable, efficient, and dry.
Psychrometrics is the study of the thermodynamic properties of moist air — mixtures of dry air and water vapor. It governs how HVAC systems heat, cool, humidify, and dehumidify air, and the psychrometric chart is its central design tool.
Dry-bulb temperature is the air temperature measured by a standard thermometer. Wet-bulb temperature is measured with a wetted wick in moving air and reflects evaporative cooling, so it accounts for moisture. The two are equal only when the air is fully saturated at 100% relative humidity; otherwise wet-bulb is lower.
Humidity ratio (also called mixing ratio or specific humidity, symbol W) is the mass of water vapor per unit mass of dry air, expressed in pounds of water per pound of dry air or in grains per pound (7,000 grains = 1 lb). Unlike relative humidity, it does not change as the air is simply heated or cooled, which makes it convenient for tracking moisture through a system.
The dew point is the temperature to which air must be cooled at constant pressure for water vapor to begin condensing; at the dew point the air is saturated at 100% relative humidity. It matters because any surface colder than the air dew point will collect condensation — controlling dew point prevents sweating ducts, mold growth, and moisture damage.
Enthalpy is the total heat content of moist air (sensible plus latent) in Btu per pound of dry air. Because it bundles both temperature and moisture, comparing the enthalpy of the entering and leaving air across a coil directly gives the total cooling or dehumidification energy: load (Btu/hr) ≈ 4.5 × cfm × Δh. It is also the basis of enthalpy economizer control.
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