Skip to main content
CalcStack

Psychrometric Dew Point & Humidity Ratio Calculator (Magnus-Tetens)

The dew point is the temperature to which ambient air must be cooled at constant barometric pressure for water vapor to condense into liquid dew.

Ambient thermometer air temperature in degrees Celsius.

Percentage moisture saturation of the air (0% to 100%).

Local barometric station pressure in kPa (standard sea level = 101.325 kPa).

Calculated Result
16.7 °C

Dew Point Temperature

Dew Point Temperature

16.7°C (62.05°F)

Relative Humidity

60%

Specific Humidity Ratio

11.89 g moisture / kg dry air

Actual Vapor Pressure (Ea)

1.901 kPa (Saturation: 3.168 kPa)

Apparent Heat Index

25°C

Calculation Breakdown

  1. 1. Saturation Vapor Pressure (Magnus-Tetens)Es = 0.61078 · exp[(17.27 · 25) / (25 + 237.3)] = 3.168 kPa
  2. 2. Actual Ambient Vapor PressureEa = Es · (RH / 100) = 3.168 · (60 / 100) = 1.901 kPa
  3. 3. Dew Point Inversionα = ln(1.901 / 0.61078) = 1.135. Td = (237.3 · α) / (17.27 - α) = 16.7°C (62.05°F)

Dew Point vs Relative Humidity (at 25°C Air)

Interactive visualization based on your current inputs

Dew Point (°C)
0.06.313192520% RH40% RH60% RH80% RH100% RHRelative Humidity (%)Dew Point (°C)

What Is the Psychrometric Dew Point & Humidity Ratio Calculator (Magnus-Tetens)?

The psychrometric dew point temperature is the thermodynamic saturation threshold where ambient air reaches 100% relative humidity.

How Does the Psychrometric Dew Point & Humidity Ratio Calculator (Magnus-Tetens) Work?

The Magnus-Tetens approximation matches Clausius-Clapeyron integration across -40°C to +50°C with 0.1°C precision.

Psychrometric Dew Point & Humidity Ratio Calculator (Magnus-Tetens) Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

E_s(T) = 0.61078 \exp\left(\frac{17.27 \cdot T}{T + 237.3}\right), \quad E_a = E_s \cdot \frac{RH}{100}, \quad T_d = \frac{237.3 \ln(E_a / 0.61078)}{17.27 - \ln(E_a / 0.61078)}

Saturation vapor pressure follows the Magnus-Tetens exponential law. Actual vapor pressure is multiplied by RH, and inverted to yield dew point.

How to Use the Psychrometric Dew Point & Humidity Ratio Calculator (Magnus-Tetens)

  1. Input dry-bulb air temperature in °C.
  2. Enter measured relative humidity percentage.
  3. Optionally adjust station barometric pressure.

Step-by-Step Example Calculation

Comfort Room Air (25°C at 60% RH)

Input Values:

dryBulbTempCelsius:25
relativeHumidityPercent:60
atmosphericPressureKPa:101.325
Worked Steps: At 25°C and 60% RH, dew point is 16.66°C (62.0°F) with 12.06 g moisture/kg dry air and a 25.4°C Heat Index.

Understanding Your Result

Dew Point Temperature: Condensation onset temperature.

Humidity Ratio: Absolute water vapor mass per kilogram dry air.

Heat Index: Apparent summer temperature felt by human skin.

Factors That Affect the Result

  • Absolute moisture concentration drives dew point; barometric pressure slightly influences specific humidity ratio.

When Should You Use This Calculator?

  • HVAC building envelope design, industrial coating inspection, data center environmental monitoring, and weather forecasting.

Assumptions & Limitations

  • Magnus formula valid for liquid water saturation; below freezing, deposition over ice (frost point) yields slightly different curves.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Standard WMO (World Meteorological Organization) recommended Magnus-Tetens formulations.

Related Calculators

Explore more tools and calculators in Education Calculators

Chilled Water Cooling Coil Capacity & Flow Calculator

Calculates total cooling capacity (kW and Tons TR), sensible heat ratio (SHR), and required hydronic water flow rate (L/s and GPM) for AHU cooling coils.

Cooling Tower Evaporation & Blowdown Makeup Water Calculator

Calculates evaporative losses, chemical blowdown purge, drift rate, and total makeup water requirements based on cycles of concentration (CoC).

Heat Pipe Two-Phase Thermal Resistance & Conductivity Calculator

Calculates envelope radial wall conduction resistance, sintered wick resistance, total thermal impedance (°C/W), and effective thermal conductivity for copper-water heat pipes.

Hydronic Expansion Tank & Air Separator Sizing Calculator

Calculates water thermal expansion volume (V_e), acceptance volume, and diaphragm expansion tank sizing for closed hydronic heating and cooling systems.

Refrigeration Compressor Isentropic COP & Compression Ratio Calculator

Calculates compression ratio, Carnot ideal limit, actual isentropic Coefficient of Performance (COP), electrical power input, and condenser heat rejection.

Circular to Rectangular Duct Equivalent Hydraulic Diameter Calculator

Calculates the Huebscher equivalent circular duct diameter (equal friction and airflow), hydraulic diameter (4A/P), and air velocity for rectangular HVAC ducts.