Psychrometric Calculator
Enter the dry-bulb temperature, relative humidity and atmospheric pressure to instantly compute every key psychrometric property of moist air: wet-bulb temperature, dew point, humidity ratio, enthalpy, specific volume, vapor pressure and degree of saturation. Switch between metric and imperial units. Used by HVAC engineers, meteorologists, and process designers.
Formula
Worked example
At 25 C dry-bulb and 50% RH at sea level (101.325 kPa): saturation vapor pressure is 3.167 kPa. Actual vapor pressure is 0.5 x 3.167 = 1.584 kPa. Humidity ratio W = 0.62198 x 1.584 / (101.325 - 1.584) = 0.00988 kg/kg. Enthalpy = 1.006 x 25 + 0.00988 x (2501 + 1.86 x 25) = 25.15 + 25.22 = 50.37 kJ/kg. Dew point is approximately 13.9 C and wet-bulb is approximately 17.7 C.
What is psychrometrics?
Psychrometrics is the science of moist air and its thermodynamic properties. Air is a mixture of dry air and water vapor, and the relative proportion of those two components determines properties such as temperature, humidity, density, and the energy needed to heat or cool the air. Engineers use psychrometrics daily to design heating, ventilation, and air conditioning (HVAC) systems, dryers, cooling towers, greenhouses, and food storage facilities. The psychrometric chart is the standard graphical tool for visualizing all these properties simultaneously, and this calculator computes the same quantities from your inputs instantly.
Key psychrometric properties explained
Dry-bulb temperature is the ordinary air temperature measured by a shielded thermometer. Wet-bulb temperature is the lowest temperature achievable by evaporating water into the air stream under adiabatic conditions; the difference between dry-bulb and wet-bulb (called the wet-bulb depression) tells you how much evaporative cooling is possible. Dew point is the temperature at which moisture begins to condense on surfaces; when surfaces fall below the dew point, condensation forms. Humidity ratio (or mixing ratio) is the mass of water vapor per kilogram of dry air, an absolute measure that does not change when you heat or cool the air. Relative humidity is the ratio of actual vapor pressure to the saturation vapor pressure at the same temperature, expressed as a percentage. Enthalpy combines the sensible heat of the dry air with the latent heat carried by the water vapor, giving the total energy content. Specific volume is the volume occupied by one kilogram of dry air including the vapor it carries.
How the calculator works: equations and accuracy
This calculator uses the Buck (1981) equation for saturation vapor pressure, which is accurate to within 0.02% over the range -40 to 60 C and is preferred in HVAC engineering over simpler approximations. The humidity ratio and relative humidity follow directly from the ratio of partial to total pressures. Enthalpy uses the ASHRAE fundamental formula: h = 1.006 Tdb + W(2501 + 1.86 Tdb) in kJ/kg. Wet-bulb temperature is found by iteratively solving the Sprung psychrometer equation, which is the standard method when a psychrometric chart or steam tables are not available. Pressure is accounted for throughout: at higher altitudes the lower pressure shifts all the humidity and enthalpy values, which is why the pressure input matters for accurate results.
HVAC and engineering applications
In air conditioning design, the enthalpy difference between outdoor and indoor air determines the cooling or heating load on the coil. Knowing the dew point allows engineers to specify coil surface temperatures that avoid condensation in supply ducts. Wet-bulb temperature drives evaporative cooler sizing and cooling tower performance: the approach temperature (supply water temperature minus inlet wet-bulb) quantifies how close the tower comes to the thermodynamic limit. In drying processes (food, pharmaceutical, paper), the humidity ratio of the inlet and outlet air streams, combined with the air mass flow rate, gives the actual moisture removal rate. The specific volume is used to convert between volumetric (m3/s or CFM) and mass (kg/s) air flow rates, which change with temperature and humidity.
ASHRAE Comfort Zone and Humidity Guidelines
| Condition | Relative Humidity | Dew Point | Notes |
|---|---|---|---|
| Very dry | Below 20% | Below -10 C | Static electricity, dry skin, respiratory irritation |
| Dry | 20-30% | -10 to -2 C | Marginal comfort, low mold risk |
| Comfortable | 30-60% | -2 to 16 C | ASHRAE recommended comfort zone |
| Humid | 60-70% | 16-21 C | Increasing mold and dust mite risk |
| Very humid | 70-85% | 21-27 C | High condensation and mold risk |
| Near saturation | Above 85% | Above 27 C | Fog, condensation, structural risk |
Recommended indoor humidity and dew point ranges for human comfort and building health (ASHRAE Standard 55).
Frequently asked questions
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb temperature is simply what an ordinary thermometer reads. Wet-bulb temperature is measured by a thermometer wrapped in a wet wick; as water evaporates from the wick it cools the bulb. The drier the air, the more evaporation occurs and the lower the wet-bulb reading. At 100% relative humidity the two temperatures are equal because no evaporation is possible. The gap between them, called the wet-bulb depression, directly measures how much evaporative cooling potential exists in the air.
What is the dew point and why does it matter?
The dew point is the temperature to which you must cool the air (at constant pressure) before water vapor starts to condense into liquid. When any surface in a building, duct, or piece of equipment falls below the dew point, condensation forms. This matters for preventing mold, corrosion, fogged windows and damaged insulation. A dew point above about 21 C (70 F) generally feels muggy to people; below about 2 C (35 F) the air feels very dry.
How does altitude (air pressure) affect the calculation?
At higher altitudes the total barometric pressure is lower, which reduces the partial pressure needed to produce any given humidity ratio. This means the air at altitude can hold more water vapor by mass at the same relative humidity, and it also changes the wet-bulb temperature slightly. For example, Denver at 1609 m has a typical pressure of about 84 kPa compared to 101.3 kPa at sea level. Entering your actual pressure gives accurate results for any elevation.
What humidity ratio is considered comfortable indoors?
ASHRAE Standard 55 defines the acceptable comfort zone for sedentary occupants as roughly 30-60% relative humidity, which corresponds to dew points between about -2 C and 16 C (28 F to 61 F). Below 30% RH the air feels dry and can irritate airways and skin. Above 60% RH dust mites and mold thrive, and above 70% condensation risk increases substantially.
How is enthalpy used in HVAC system design?
Enthalpy quantifies the total heat content of moist air, including both the sensible heat (which changes dry-bulb temperature) and the latent heat (carried by water vapor). The cooling or heating load on an air handling unit equals the air mass flow rate multiplied by the enthalpy difference between the entering and leaving air states. Equipment manufacturers rate cooling coils in kW or BTU/h, which you can match against this enthalpy-based load calculation to select the right coil size.
What is the degree of saturation?
The degree of saturation (sometimes called relative humidity ratio) is the actual humidity ratio divided by the saturation humidity ratio at the same temperature and pressure. It is dimensionless and ranges from 0 (completely dry air) to 1 (fully saturated air). It differs slightly from relative humidity: relative humidity is a pressure ratio while degree of saturation is a mass ratio. For most practical purposes at low pressures they are nearly equal, but for precise engineering calculations the distinction matters.