Enter pressure (or altitude), dry-bulb temperature, and one more property to compute the full state of moist air, plotted on an interactive psychrometric chart.Learn more ▾Show less ▴
Moist air is fully described once you fix the pressure and any two of its properties. From there the dew point, wet-bulb temperature, relative humidity, humidity ratio, enthalpy, and specific volume all follow from the ASHRAE relationships.
This tool does that calculation in both directions you need: type whichever pair you have measured, and read the rest. The psychrometric chart shows where your air sits relative to saturation so you can reason about heating, cooling, humidification, and condensation at a glance.
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Moist-air properties
Psychrometric chart
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About This Tool
The Psychrometric Calculator computes the complete thermodynamic state of moist air from any two properties at a given pressure, and plots it on an interactive psychrometric chart.
It is built for HVAC and refrigeration engineers, building-science and energy professionals, drying and process engineers, and students — anyone who needs dew point, wet-bulb, humidity ratio, enthalpy, and related properties without reaching for a paper chart or a slide rule.
Set the pressure by altitude or directly, and choose SI or Imperial units.
Enter the dry-bulb temperature and one more property (relative humidity, wet-bulb, dew point, or humidity ratio).
Read the full set of moist-air properties and see the state point on the psychrometric chart.
How to Use
Set the pressure by altitude or directly, and choose SI or Imperial units.
Enter the dry-bulb temperature and one more property (relative humidity, wet-bulb, dew point, or humidity ratio).
Read the full set of moist-air properties and see the state point on the psychrometric chart.
Methodology
All properties follow the ASHRAE Handbook of Fundamentals, Chapter 6. Water-vapor saturation pressure uses the Hyland-Wexler formulation, with separate equations over ice and over liquid water.
Humidity ratio comes from the vapor pressure and total pressure; relative humidity is the ratio of vapor pressure to saturation pressure; enthalpy and specific volume use the standard perfect-gas relationships for moist air. Wet-bulb temperature is found by solving the thermodynamic wet-bulb relation, and dew point by inverting the saturation-pressure equation. Barometric pressure for a given altitude uses the US Standard Atmosphere.
Every property describes the same parcel of air. Relative humidity tells you how close the air is to saturation right now; dew point tells you the surface temperature at which condensation will start; wet-bulb tells you the lowest temperature evaporative cooling can reach.
Humidity ratio and enthalpy stay fixed during pure heating or cooling, which is why engineers track them through a system. On the chart, the distance from your point up to the saturation curve is your margin against condensation, and the horizontal spread of the constant-humidity line shows how far you can heat or cool before adding or removing moisture.
Example 1: Room air at 25 °C and 50% relative humidity at sea level has a dew point of about 13.9 °C, a wet-bulb of about 17.9 °C, a humidity ratio near 9.9 g/kg, and an enthalpy of about 50.3 kJ/kg.
Example 2: The same 25 °C / 50% air cooled to 13 °C is below its dew point, so moisture condenses — the air leaves the coil saturated near 13 °C, having given up both sensible and latent heat. Reading the two enthalpies off the calculator gives the total coil load per kilogram of dry air.
Tips for using psychrometric results
• Set the pressure to your real altitude — at 1500 m it is about 16% below sea level and changes every property.
• To size a cooling coil, work in enthalpy: load equals airflow times the enthalpy drop across the coil.
• To check condensation risk, compare a surface temperature to the dew point; if the surface is colder, moisture will condense.
• Wet-bulb is the target for evaporative cooling — the drier the air, the lower the wet-bulb and the more cooling you get.
• Humidity ratio, not relative humidity, is the honest measure of how much water the air actually carries.
All calculations are performed locally in your browser. No data is sent to any server.
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Frequently Asked Questions
What is a psychrometric calculator?
Psychrometrics is the study of the properties of moist air — the mixture of dry air and water vapor we live and work in. Given the pressure and any two independent properties of that air, every other property is fixed. This calculator takes the dry-bulb temperature plus one of relative humidity, wet-bulb temperature, dew point, or humidity ratio, and returns the full set: the other humidity measures plus enthalpy, specific volume, vapor pressure, degree of saturation, and density.
What is the difference between dry-bulb, wet-bulb, and dew-point temperature?
Dry-bulb temperature is the air temperature a normal thermometer reads. Wet-bulb temperature is what a thermometer reads with a wet wick around the bulb — evaporation cools it, so it falls between dew point and dry-bulb and reflects how much cooling evaporation can provide. Dew-point temperature is the temperature at which the air would become saturated and condensation begins; it depends only on the actual moisture content. At 100% relative humidity all three are equal.
What is humidity ratio, and how is it different from relative humidity?
Humidity ratio (also called mixing ratio or absolute humidity on a mass basis) is the mass of water vapor per unit mass of dry air, usually in grams per kilogram or grains per pound. It does not change when you simply heat or cool the air. Relative humidity is the ratio of the air's vapor pressure to its saturation vapor pressure at the same temperature, as a percentage — it rises as air cools and falls as air warms, even when the actual moisture stays the same.
Why does altitude (barometric pressure) matter?
Almost every moist-air property depends on the total barometric pressure, which drops with altitude. At the same temperature and relative humidity, air at higher elevation holds a different humidity ratio and has a different enthalpy and specific volume than air at sea level. Enter your altitude and the calculator uses the standard-atmosphere pressure for that elevation, or you can type the barometric pressure directly.
What is enthalpy and why is it useful?
Enthalpy is the total heat content of the moist air per unit mass of dry air, combining the sensible heat of the air and the latent heat of its water vapor. It is the quantity HVAC engineers use to size cooling and heating coils, because the energy a coil must add or remove equals the airflow times the change in enthalpy. On the chart, constant-enthalpy lines run as gentle diagonals.
How do I read the psychrometric chart?
Dry-bulb temperature runs along the bottom and humidity ratio up the right side. The curved boundary on the upper-left is the saturation (100% relative humidity) line; the curves below it are constant relative-humidity lines. Your air's state is the marked point. Heating moves the point right along a constant humidity-ratio line; sensible cooling moves it left; adding moisture moves it up; cooling below the dew point removes moisture and the point drops along the saturation curve.
What are common uses for these calculations?
Sizing cooling and dehumidification coils, checking for condensation risk on surfaces and in wall assemblies, designing evaporative cooling, setting drying and curing conditions, validating cooling-tower and air-handler performance, and assessing comfort. Pair it with the Heat Load and HVAC Sizing calculators for a full design workflow.
How accurate is it, and what standard does it use?
The calculator implements the equations from the ASHRAE Handbook of Fundamentals, Chapter 6 — the Hyland-Wexler formulation for water-vapor saturation pressure and the standard relationships for humidity ratio, enthalpy, specific volume, wet-bulb, and dew point. Saturation pressures reproduce the ASHRAE tables to within a fraction of a percent across the −60 to 90 °C range. Results are suitable for engineering design; always confirm against project requirements.
Does it support both SI and Imperial units?
Yes. Switch between SI (°C, kPa, g/kg, kJ/kg, m³/kg) and US/Imperial (°F, psia, grains/lb, Btu/lb, ft³/lb) at any time and every input and result converts instantly. Altitude can be entered in metres or feet.
Is my data private?
Yes. Every calculation and the chart run entirely in your browser. Nothing you enter is stored or sent to any server, so you can use the tool offline once the page has loaded.
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