Psychrometric Calculator

What the psychrometric calculator does

Moist air has a whole set of linked properties, and this calculator works them out together. From the dry bulb temperature, the wet bulb temperature, the altitude, the relative humidity, and the dew point, it reports the enthalpy, humidity ratio, density, specific volume, and vapour pressures of the air, the full thermodynamic picture used in air-conditioning and ventilation design.

Below is what psychrometrics is, what each property means, and a worked example.

How to use it

1. Enter the dry bulb and wet bulb temperatures, the wet bulb being the lower of the two.
2. Enter the altitude above sea level, which sets the atmospheric pressure, along with the relative humidity and dew point.
3. Press Calculate for the full set of moist-air properties, or Reset to clear them.

What psychrometrics is

Psychrometrics is the study of the physical properties of moist air, the mixture of dry air and water vapour that makes up the atmosphere. While a single temperature tells you how warm the air is, it says nothing about how much moisture it carries or how much energy it holds, and those are exactly the things that matter for comfort, condensation, and the sizing of heating and cooling equipment. Psychrometrics ties all of these together.

It is the working language of air-conditioning engineering. Designing a system to cool a building, dehumidify a warehouse, or ventilate a room means tracking not just temperature but moisture content and energy, and predicting how they change as air is heated, cooled, humidified, or mixed. This calculator computes the key psychrometric properties from a handful of measurements, giving the complete state of the air in one go.

The properties it reports

The calculator returns several quantities, each describing a different aspect of the moist air. The humidity ratio is the mass of water vapour carried per unit mass of dry air, the most fundamental measure of how much moisture is present. The enthalpy is the total heat energy of the air per unit mass, combining the warmth of the air with the energy locked in its water vapour, and it is what air-conditioning loads are ultimately about.

Alongside these, the specific volume is the volume that a unit mass of the air occupies, and the density is its inverse, the mass per unit volume. The partial vapour pressure is the share of the total air pressure due to water vapour alone, while the saturation vapour pressure is the most the air could hold at that temperature. Together these properties pin down the state of the moist air completely, which is why an engineer reads them as a set rather than one at a time.

Why wet bulb and altitude matter

The wet bulb temperature is central to psychrometrics because it captures the moisture content through evaporative cooling. A wet, ventilated thermometer reads lower than a dry one by an amount that depends on how dry the air is, so the gap between the dry bulb and wet bulb readings is a direct measure of humidity. This is why the calculator leans on the wet bulb temperature to work out the humidity ratio and the energy content.

Altitude enters because air pressure falls as you climb, and the properties of moist air depend on that pressure. The calculator adjusts the atmospheric pressure for the altitude you enter using the standard barometric relationship, so the results hold whether you are at sea level or high in the mountains. Without that correction, a system designed for a highland city using sea-level assumptions would be sized wrongly, since the thinner air there behaves differently.

The psychrometric chart

Engineers traditionally read these properties off a psychrometric chart, a dense graph that plots all the moist-air quantities against one another so that fixing any two locates a point and reveals the rest. The chart is one of the iconic tools of the trade, and learning to read it is a rite of passage in heating and cooling work.

This calculator does the same job numerically. Where the chart gives a quick visual reading, the calculator returns precise figures for each property, which is handy for calculations and record-keeping. The two complement each other: the chart builds intuition for how the properties move as air is processed, while the calculator delivers the exact numbers behind a point on it.

Method and precision

The calculator uses the standard psychrometric relationships from heating and cooling engineering, including the accepted formulas for humidity ratio, enthalpy, and specific volume, with the atmospheric pressure adjusted for altitude. It takes temperatures in degrees Celsius and altitude in metres, and reports enthalpy in kilojoules per kilogram, the humidity ratio in kilograms per kilogram, density in kilograms per cubic metre, and the pressures in pascals. The results are accurate estimates suitable for design and analysis.

A worked example

Suppose the air has a dry bulb temperature of 30 degrees Celsius and a wet bulb temperature of 20 degrees Celsius, at sea level.

The calculator gives a humidity ratio of about 0.0105 kilograms of water per kilogram of dry air, an enthalpy of about 57 kilojoules per kilogram, and a specific volume of about 0.87 cubic metres per kilogram. The 10-degree gap between the dry and wet bulb readings reflects moderately humid air, and the enthalpy figure is what a cooling system would need to work against to bring this air to a cooler, drier condition.

Questions people ask

What is psychrometrics?

The study of the properties of moist air, the mix of dry air and water vapour. It tracks temperature, moisture content, and energy together, and is the foundation of air-conditioning and ventilation design.

What is the humidity ratio?

The mass of water vapour carried per unit mass of dry air. It is the most fundamental measure of how much moisture the air holds, independent of temperature.

What does the enthalpy tell you?

The total heat energy of the air per unit mass, combining its warmth with the energy held in its water vapour. Air-conditioning loads are ultimately about changing this enthalpy.

Why does altitude matter?

Because air pressure drops with altitude, and moist-air properties depend on pressure. The calculator adjusts the atmospheric pressure for your altitude so the results hold at any elevation.

References

A quick note on where the science comes from. The psychrometric relationships for moist air are standard heating and cooling engineering, set out in the ASHRAE Handbook of Fundamentals and described in the Wikipedia article on psychrometrics. The underlying ideal-gas and humidity concepts follow standard physics.

1. ASHRAE, Handbook of Fundamentals, Psychrometrics chapter. https://www.ashrae.org/technical-resources/ashrae-handbook
2. Wikipedia, Psychrometrics. https://en.wikipedia.org/wiki/Psychrometrics
3. Engineering ToolBox, Psychrometric terms and moist air properties. https://www.engineeringtoolbox.com/psychrometric-terms-d_239.html