Humidity and Dew Point Calculator

Dew Point Calculator for Relative Humidity, Dew Point/Frost Point, Moisture Content, Pressure (barg), Water Activity, Molecular Weight (Air, 02, N2, H2, C02, CO) and more parameters – Recommended for Michell Products.

Known Inputs

Molecular Weight

©2023 Michell Instruments Ltd.

While every effort has been made to ensure the formulae used in this calculator are correct, this free software service is provided 'as is' and without warranty or guarantee to the extent of the applicable law.

PST Michell Industrial Dew Point Calculator

PST Michell industrial dew-point calculator facilitates swift and effortless humidity calculations and parameter conversions for engineers. It is designed to be used as a standalone tool or combined with a hygrometer.

The dew-point calculator can calculate humidity values at different pressures, calculate parameters unavailable on a particular instrument, or convert previously measured data to alternative parameters.

Users can calculate multiple parameters, including dew-point temperature, frost-point temperature, relative humidity, absolute humidity, parts per million (volume), parts per million (weight), mixing ratio, wet bulb temperature, water vapor pressure & saturation vapor pressure, and enthalpy.

The dew-point calculator uses the Sonntag formula for water vapor pressure as seen in the British Standard 1339:1, 3.2.2. Users can customize the settings to change which parameters are shown and alter the order to ensure complete ease of use.

Alternatively, you can use our pressure dew point conversion chart over ice and water. The tables include conversions using the Sontag and Wexler methods.

How to use the humidity calculator

Watch the quick video below for a demonstration of how to use the PST Michell Industrial Humidity and Dew-Point Calculator.


  • The PST Michell humidity calculator uses the Sonntag formula for water vapor pressure. It can be found in British Standard 1339:1, 3.2.2

  • The humidity calculator is intended for use within the range of -100°C to +100°C dew point (at atmospheric pressure). Calculations outside this range may yield unexpected results. It's also worth noting that calculations under atmospheric pressure may produce unexpected outcomes due to the distinct behavior of water vapor in vacuum conditions.

    The uncertainties (coverage factor approximately k=2) of the polynomials used in the Sonntag equation are as follows:

    Less than 0.01% of the vapor pressure value for water between 0°C and 100°C.

    Less than 0.6% of the vapor pressure value for supercooled water between 0°C and -50°C.

    Less than 1.0% of the vapor pressure value for ice between 0°C and -100°C.

    This translates to dew-point uncertainties as follows:

    ±0.014°C to ±0.0029°C for water between 0°C and 100°C.

    ±0.07°C to ±0.053°C for supercooled water between 0°C and -50°C.

    ±0.12°C to ±0.049°C for ice between 0°C and -100°C.

  • In Michell chilled mirror products, both water vapor pressure (WVP) and saturation water vapor pressure (SWVP) are calculated using the Sonntag vapor pressure formula (Magnus formula). This formula comes in two variations: the "over water" formula and the "over ice" formula. Generally, the "over water" formula is chosen when the temperature (or dew-point temperature) is above 0 °C, while the "over ice" formula is used for temperatures below 0 °C.

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