# Psychrometric Calculator

URL: https://speedycalculator.com/calculators/physics/psychrometric/
Last updated: 2026-10-30

> The psychrometric calculator finds the full set of moist-air properties – vapour pressure, relative humidity, dew point, humidity ratio, degree of saturation, enthalpy and specific volume – from the dry-bulb temperature and either the wet-bulb temperature or relative humidity, at any altitude.

## Inputs

- Pressure from
- Altitude (m, ft)
- Barometric pressure (kPa, hPa, inHg, psi)
- Dry-bulb temperature (°C, °F, K)
- Humidity from
- Wet-bulb temperature (°C, °F, K)
- Relative humidity (%)

## Results

- Barometric pressure
- Saturation vapour pressure
- Vapour pressure
- Relative humidity
- Dew point
- Humidity ratio
- Degree of saturation
- Specific enthalpy
- Specific volume

## Formula

```
p = 101.325·(1 − 2.25577×10⁻⁵·Z)^{5.2559} kPa,  e = e_s(T_wb) − 6.6×10⁻⁴·(1 + 1.15×10⁻³T_wb)·p·(T − T_wb)
W = 0.622·e/(p − e),  h = 1.006T + W·(2501 + 1.86T) kJ/kg,  v = 0.287042·(T + 273.15)·(1 + 1.6078W)/p
```

## Explanation

Psychrometrics – the study of moist air – underpins air conditioning, drying, cooling towers and greenhouse design. A sling psychrometer's wet- and dry-bulb readings give everything else.

### How to use the Psychrometric Calculator

- **Pressure from**
- **Altitude**
- **Barometric pressure**
- **Dry-bulb temperature**
- **Humidity from**
- **Wet-bulb temperature**
- **Relative humidity (%)**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the altitude (or pressure), the dry-bulb temperature and either the wet-bulb temperature or relative humidity.

### Inputs explained

FieldWhat to enterPressure fromChoose one: Altitude, Barometric pressureAltitudeUnits: m, ftBarometric pressureUnits: kPa, hPa, inHg, psiDry-bulb temperatureUnits: °C, °F, KHumidity fromChoose one: Wet-bulb temperature, Relative humidityWet-bulb temperatureUnits: °C, °F, KRelative humidity (%)A number

### What each result means

- **Saturation vapour pressure** = 6.112×exp(17.62×([Dry-bulb temperature])÷(243.12 + ([Dry-bulb temperature])))÷10
- **Relative humidity** = [Vapour pressure]÷[Saturation vapour pressure]×100
- **Humidity ratio** = 622×[Vapour pressure]÷([Barometric pressure] - [Vapour pressure])
- **Degree of saturation** = ([Vapour pressure]÷([Barometric pressure] - [Vapour pressure]))÷([Saturation vapour pressure]÷([Barometric pressure] - [Saturation vapour pressure]))×100
- **Specific enthalpy** = 1.006×[Dry-bulb temperature] + [Humidity ratio]÷1000×(2501 + 1.86×[Dry-bulb temperature])
- **Specific volume** = 0.287042×([Dry-bulb temperature] + 273.15)×(1 + 1.607858×[Humidity ratio]÷1000)÷[Barometric pressure]

## Worked example

Inputs — Pressure from: Altitude; Altitude: 0 m; Dry-bulb temperature: 25 °C; Humidity from: Wet-bulb temperature; Wet-bulb temperature: 18 °C.

Results — Barometric pressure = 101.325 kPa; Saturation vapour pressure = 3.16006 kPa; Vapour pressure = 1.58132 kPa; **Relative humidity = 50.0408 %**; Dew point = 13.8642 °C; Humidity ratio = 9.86107 g/kg; Degree of saturation = 49.2487 %; Specific enthalpy = 50.27107 kJ/kg; Specific volume = 0.85802 m³/kg.

## Key terms

- **Psychrometric chart**: Graph of moist-air properties.

## FAQ

**What is the wet-bulb temperature?**
The temperature of a thermometer cooled by evaporation from a wet wick.

**Why does altitude matter?**
Lower pressure changes the humidity ratio and specific volume.

**What is enthalpy used for?**
Calculating heating and cooling loads of air.

**What is the humidity ratio?**
Grams of water vapour per kilogram of dry air.


## Sources

- [NOAA / National Weather Service – Weather calculators](https://www.weather.gov/epz/wxcalc)
- [Vaisala – Humidity conversion formulas](https://www.vaisala.com/en/lp/humidity-conversion-formulas)
- [ASHRAE Handbook – Fundamentals (psychrometrics)](https://www.ashrae.org/technical-resources/ashrae-handbook)