Dew Point Calculator

What this calculator does

The Dew Point Calculator finds the dew point temperature from the air temperature and relative humidity. It also reports the actual water vapor pressure, the saturation water vapor pressure, and the absolute humidity.

The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for water vapor to condense into liquid water. When the dew point is close to the air temperature, the air is near saturation and fog or condensation becomes likely. When the dew point is much lower, the air is dry.

How to use this calculator

1. Enter the air temperature.
2. Choose Fahrenheit, Celsius, or Kelvin.
3. Enter the relative humidity from 1 to 100.
4. Click Calculate.

The default example uses 70 °F and 65% relative humidity. The Magnus formula gives a dew point of about 57.7 °F (14.3 °C). The result card also shows the vapor pressure in pascals, the saturation vapor pressure, and the absolute humidity in grams per cubic meter.

Dew point formulas

γ = ln(RH / 100) + (a × T) / (b + T)
Td = (b × γ) / (a − γ)

a = 17.625
b = 243.04 °C
T = air temperature in °C
RH = relative humidity in percent

The saturation vapor pressure is estimated with the same Magnus constants:

es = 6.1094 × exp((a × T) / (b + T))   [hPa]
e  = (RH / 100) × es                   [hPa]
Absolute humidity = 216.7 × e / (T + 273.15)   [g/m³]

Temperatures entered in Fahrenheit or Kelvin are converted to Celsius before the formula is applied. The dew point is then converted back to Fahrenheit and Kelvin for display.

What the results mean

A dew point below about 50 °F is generally comfortable. Dew points in the 50s feel mild, the 60s feel humid, and the low 70s feel oppressive to many people. A dew point near or above the air temperature means the air is saturated and condensation is imminent.

The vapor pressure is the partial pressure of water vapor in the air. The saturation vapor pressure is the maximum vapor pressure the air could hold at that temperature. Absolute humidity expresses the mass of water vapor per cubic meter of air. These values are useful for HVAC, weather analysis, and drying calculations.

Assumptions and limitations

• The Magnus formula is an approximation. Different constants and formulations can shift the result by a few tenths of a degree.
• The calculation assumes a standard atmospheric pressure. Pressure changes with altitude and weather.
• Relative humidity must be greater than 0% and at most 100%. At 100%, the dew point equals the air temperature.
• Dew point cannot exceed the air temperature in this model. If a weather report shows otherwise, the measurements are inconsistent or taken at different times.
• The calculator does not model condensation onto surfaces, cloud formation, or indoor air quality.
• This is an estimate for planning and not medical advice.

Frequently asked questions

What is the difference between dew point and relative humidity?

Relative humidity is the percentage of moisture the air holds compared with the maximum it could hold at that temperature. Dew point is an absolute temperature. Because warm air can hold more moisture, relative humidity can change during the day even when the dew point stays nearly constant.

Why does dew point matter for comfort?

Dew point reflects the actual amount of moisture in the air, so it is a better comfort indicator than relative humidity alone. A high dew point makes sweat evaporate more slowly, which can make a warm day feel muggy and uncomfortable.

Can the dew point be higher than the air temperature?

No. If air cools below its dew point, water vapor condenses and the dew point falls with the temperature. A reported dew point above the air temperature usually means the measurements came from different times or locations.

Reference table

Temperature unit conversions

Dew point uses the Magnus formula and assumes a standard atmospheric pressure. Values are estimates.

Source

Reference: Calculator.net — Dew Point Calculator. Formula background: Magnus approximation for water vapor pressure.

Disclaimer

This calculator provides an estimate for planning and is not medical advice. Dew point and humidity can affect comfort and health, but they do not diagnose any condition. Use local weather data and professional guidance for critical decisions.