Psychrometric Calculator
Enter the dry-bulb temperature and one more property — relative humidity, dew point or wet-bulb — and get the full set of moist-air properties: dew point, RH, wet-bulb, humidity ratio, enthalpy and specific volume.
Reading the State of Moist Air
Air is a mix of dry air and water vapour, and a handful of linked properties pin down its exact state. Fix the dry-bulb temperature and any one moisture measure — relative humidity, dew point or wet-bulb — and every other property is determined. That is what a psychrometric chart shows, and what this calculator computes directly.
Dry-Bulb, Wet-Bulb and Dew Point
Dry-bulb is the temperature a normal thermometer reads. Dew point is the temperature at which the air would start to condense — the higher it is, the more moisture the air carries. Wet-bulb sits between the two and reflects evaporative cooling; the gap between dry-bulb and wet-bulb widens as the air gets drier. When all three converge, the air is saturated at 100% relative humidity.
Humidity Ratio, Enthalpy and Volume
The humidity ratio is the actual mass of water per unit mass of dry air — the quantity HVAC equipment must add or remove. Enthalpy is the total heat content, the basis for sizing cooling coils, since cooling humid air means wringing out latent heat as well as sensible heat. Specific volume, the space a unit of dry air occupies, ties airflow in CFM to mass flow.
Comfort and Condensation
Most people are comfortable around 40 to 60% relative humidity. Track the dew point to predict condensation: any surface colder than the dew point — a window, a chilled-water pipe, a duct in a humid attic — will sweat and risk mould. Designers use these properties to size dehumidification, choose insulation and set economizer controls.
Frequently Asked Questions
What is the difference between dew point and wet-bulb?
Dew point is the temperature where condensation begins; wet-bulb is the lowest temperature reachable by evaporative cooling. Wet-bulb is always between the dew point and the dry-bulb temperature.
How do you find relative humidity from dry-bulb and wet-bulb?
Compute the actual vapour pressure from the psychrometric equation using the wet-bulb depression, then divide by the saturation pressure at the dry-bulb temperature.
What is humidity ratio?
The mass of water vapour per unit mass of dry air, often in grams per kilogram or grains per pound. It is what a dehumidifier or cooling coil actually changes.
Why does dew point predict condensation?
Any surface cooler than the dew point chills nearby air below saturation, so moisture condenses on it. Keeping surfaces above the dew point prevents sweating and mould.
Related calculators
- Ventilation Rate Calculator — outdoor air for humidity and air quality.
- CFM & Air Changes Calculator — airflow a space needs.
- Duct Static Pressure Calculator — move that air through ducts.
- BTU Calculator — heating and cooling load.