Psychrometric Calculator

• •Use the Psychrometric Calculator to check your hand calculations on homework or exam problems — comparing its result with your own working helps identify arithmetic errors before they propagate.
• •Use it during the early design phase to rapidly iterate on parameters and narrow down feasible configurations before committing time to detailed finite element simulations or full design packages.
• •Use it when reviewing a colleague's calculation or checking a vendor's data sheet for plausibility — a quick sanity check can prevent costly downstream errors.
• •Use it to generate reference data for a technical report or presentation without manual computation, ensuring consistent, reproducible numbers throughout the document.
• •Use it in the field when a quick estimate is needed and a full engineering software package is not available.

The Psychrometric Calculator is a precision engineering calculation tool designed for students, engineers, and technical professionals. Humidity ratio, dew point, wet-bulb temperature, enthalpy and specific volume of moist air from dry-bulb temperature, relative humidity and pressure, using the ASHRAE 2017 equations All calculations are performed using established engineering formulas from the relevant scientific literature and standards. Follow the units printed beside each field. Where a unit selector is provided, the selected unit is converted internally. Results are computed in your browser without sending data to a server, which keeps your inputs private. This calculator is intended as a supplementary tool for learning and design exploration; always verify results against authoritative references for safety-critical applications.

Psychrometrics is the study of moist air — mixtures of dry air and water vapor. The key properties are: dry-bulb temperature (T_db, the air temperature from an ordinary thermometer), wet-bulb temperature (T_wb, the temperature of a water-wetted thermometer bulb that represents evaporative cooling), relative humidity (φ, the ratio of current vapor pressure to saturation vapor pressure at the same temperature), humidity ratio (W, mass of water vapor per unit mass of dry air), dew point (the temperature at which condensation begins when the air is cooled at constant pressure), enthalpy (h, total energy per unit mass of dry air), and specific volume (v). These properties are related through saturation curves and the psychrometric chart — a graphical representation that shows all properties simultaneously and allows any two to specify the complete state. For air-conditioning calculations, the enthalpy is particularly important because it captures both sensible heat (temperature change) and latent heat (moisture change) in a single value. Cooling and dehumidification moves air across the psychrometric chart from high h, high W to low h, low W — and the required refrigeration capacity equals the mass flow times the enthalpy difference. Common calculations include: sensible heating or cooling (constant W, varying T); evaporative cooling (constant enthalpy, crossing the saturation curve); humidification (adding moisture at constant T or following an adiabatic saturation line); and dehumidification (usually by cooling below dew point, then reheating). Building HVAC load calculations rely heavily on psychrometric analysis to determine cooling capacity, reheat, and humidity control requirements. The calculator implements the perfect-gas psychrometric equations of the ASHRAE Handbook—Fundamentals (2017), chapter 1, for dry-bulb temperatures from −100 °C to 200 °C and any absolute pressure above the water-vapour partial pressure (down to about 0.0014 Pa); they were checked against PsychroLib at 10, 80, 101.325 and 200 kPa, and the page shows a caution above 200 kPa. Below 0.01 °C it takes saturation (and therefore relative humidity) over ice, as ASHRAE does.

• •HVAC cooling load analysis: compute required refrigeration capacity for space conditioning given outdoor air conditions and desired indoor conditions. The total load is mass flow × enthalpy difference from outdoor to indoor state.
• •Dehumidification sizing: determine how much moisture must be removed to reach a target humidity ratio. Compare the humidity ratio of the entering air with that of the target state; a coil that dehumidifies must cool below the target dew point, which is why reheat is often needed afterwards.
• •Evaporative cooling effectiveness: from the inlet dry-bulb temperature and relative humidity, the calculator gives the wet-bulb temperature, the limit an evaporative cooler approaches. Useful for swamp cooler sizing in dry climates.
• •Outdoor air calculations: for 100% outdoor air systems, compute the enthalpy and moisture content of fresh air at various design conditions to size cooling and heating coils properly.
• •Drying process design: compute the moisture-carrying capacity of heated air used to dry grain, lumber, laundry, or industrial products. The air's enthalpy and humidity determine how much water it can absorb before saturation.