What inputs are required?

Enter a dry-bulb temperature and a humidity value. The humidity value can be relative humidity or a supported wet-bulb temperature.

Which humidity modes are supported?

You can use relative humidity, August natural-ventilated wet-bulb temperature, or aspirated Assmann wet-bulb temperature.

Which temperature units can I use?

The calculator accepts °C, K, and °F. Reported temperatures use the selected temperature unit.

Can I change atmospheric pressure?

Yes. Enter atmospheric pressure in kPa or hPa. The default is 101.325 kPa.

What results does the calculator return?

It returns dew-point temperature, saturation vapour pressure, actual vapour pressure, relative humidity, and absolute humidity.

Dew Point Calculator HVAC (Magnus Equation) | Online Free Tool

Execution

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Prepared example runs

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2 examples

25 °C, RH 50%, P=101.325 kPa -> T_dp ≈ 13.86 °C

P_ws(25)=3.1698 kPa, P_w=0.5·3.1698=1.5849 kPa. α=ln(1.5849/0.6108)=0.9537. T_dp=237.3·0.9537/(17.27−0.9537)=13.86 °C. Absolute humidity ρ_v=P_w/(0.4615·T_K)=1.5849/(0.4615·298.15)=0.01152 kg/m³.

{"result":"Dew point at T=25 °C, RH=50%, P=101.325 kPa. T_dp=13.86 °C, P_ws=3.1698 kPa, P_w=1.5849 kPa, ρ_v=0.01152 kg/m³.","metadata":{"input":{"dryBulb":25,"tempUnit":"°C","humidityMode":"rh","humidityValue":50,"pressure":101.325,"pressureUnit":"kPa"},"result":{"dewPoint_C":13.86,"dewPointInputUnit":13.86,"relativeHumidity_pct":50,"saturationPressure_kPa":3.1698,"vapourPressure_kPa":1.5849,"absoluteHumidity_kg_m3":0.01152,"pressure_kPa":101.325}}}

20 °C, Assmann wet-bulb 13 °C, P=101.325 kPa -> T_dp

Aspirated wet-bulb. A=0.000666. P_ws(13)=1.4979 kPa. P_w=1.4979 − 0.000666·101.325·(20−13)=1.4979 − 0.4724=1.0255 kPa. φ=1.0255/2.3388=43.85%. T_dp from inverse Magnus on P_w≈7.39 °C (per code).

{"result":"Dew point at T=20 °C, T_wa=13 °C (Assmann), P=101.325 kPa. T_dp=7.39 °C, φ=43.85%, P_ws=2.3388 kPa, P_w=1.0255 kPa, ρ_v=0.00779 kg/m³.","metadata":{"input":{"dryBulb":20,"tempUnit":"°C","humidityMode":"twas","humidityValue":13,"pressure":101.325,"pressureUnit":"kPa"},"result":{"dewPoint_C":7.39,"dewPointInputUnit":7.39,"relativeHumidity_pct":43.85,"saturationPressure_kPa":2.3388,"vapourPressure_kPa":1.0255,"absoluteHumidity_kg_m3":0.00779,"pressure_kPa":101.325}}}

Inputs

Set the required fields, then run the tool.

7 options

SettingsAdjust formats, ranges, numbers, and modes.7

Dry-bulb TemperaturenumberRequiredDry-bulb (ambient air) temperature, in the selected Temperature Unit.Temperature UnitselectOptionalHumidity Input ModeselectOptionalWhat the Humidity Value represents: relative humidity (φ), August natural-ventilated wet-bulb (T_w), or aspirated/Assmann wet-bulb (T_wa).Humidity ValuenumberRequiredValue of the selected Humidity Input Mode. RH in % (0–100); wet-bulb temperatures in the selected Temperature Unit.Atmospheric PressurenumberOptionalTotal atmospheric pressure P, in the selected Pressure Unit. Default 101.325 kPa (sea level).Pressure UnitselectOptionalDecimal PlacesnumberOptional

Result

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Prepared example runs

Click an example to fill the form automatically. File inputs still need an upload.

2 examples

25 °C, RH 50%, P=101.325 kPa -> T_dp ≈ 13.86 °C

20 °C, Assmann wet-bulb 13 °C, P=101.325 kPa -> T_dp

Inputs

Set the required fields, then run the tool.

7 options

SettingsAdjust formats, ranges, numbers, and modes.7

Result

Ready for a run

Run the tool to preview files, text, structured data, or streamed output here.

Examples

1. Indoor air dew-point check

HVAC technician

Background

An HVAC technician measures indoor air at 25 °C with 50% relative humidity at standard atmospheric pressure.

Problem

The technician needs the dew point and vapour-pressure values to assess the moisture condition of the air.

How to use

Enter 25 as the dry-bulb temperature, select °C and Relative Humidity, then enter 50 as the humidity value. Use the default pressure of 101.325 kPa.

dryBulb: 25; tempUnit: °C; humidityMode: Relative Humidity; humidityValue: 50; pressure: 101.325 kPa

Outcome

The calculator returns a dew point of approximately 13.86 °C, saturation vapour pressure of 3.1698 kPa, actual vapour pressure of 1.5849 kPa, and absolute humidity of about 0.01152 kg/m³.

2. Assmann wet-bulb measurement

Building surveyor

Background

A building surveyor records a 20 °C dry-bulb temperature and a 13 °C aspirated wet-bulb temperature at 101.325 kPa.

Problem

The surveyor needs to derive relative humidity and dew point from the Assmann wet-bulb reading.

How to use

dryBulb: 20; tempUnit: °C; humidityMode: Aspirated Wet-bulb T_wa; humidityValue: 13; pressure: 101.325 kPa

Dew Point Calculator HVAC (Magnus Equation) | Online Free Tool | Elysia Tools

Dew Point Calculator HVAC (Magnus Equation)

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Key facts

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How it works

Use cases

Examples

1. Indoor air dew-point check

2. Assmann wet-bulb measurement

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