Dew Point Temperature Calculator

What Is the Dew Point?

The dew point is the temperature at which air becomes saturated with water vapor. That is the point where condensation starts. When air cools to its dew point, water droplets form on any surface at or below that temperature, which is what dew and fog are.

The closer the dew point sits to the actual air temperature, the muggier it feels, and the more likely you are to find condensation on cold surfaces.

Below freezing the same calculation gives a frost point instead, since vapour turns straight to ice without passing through liquid. The numbers here use the coefficients for liquid water, so below about 0°C read the answer as approximate, roughly half a degree low against the proper ice values.

The Magnus Formula (Simplified)

This calculator uses the August-Roche-Magnus approximation:

γ = (a × T) ÷ (b + T) + ln(RH/100) Dew Point = (b × γ) ÷ (a − γ)

Where:

• T = Air temperature (°C)
• RH = Relative humidity (%)
• a = 17.625
• b = 243.04°C

This is accurate to within 0.4°C for temperatures between −40°C and 60°C.

Dew Point Comfort Scale

Practical Applications

Building science: Condensation forms on any surface at or below the dew point. If your wall surface is at 15°C and the dew point is 16°C, moisture will condense inside your walls.

HVAC: Air conditioning reduces dew point by removing moisture from the air.

Aviation: Pilots use dew point to predict fog and icing conditions.

Agriculture: Dew point determines overnight frost and dew formation, affecting plant health.

Example

Air temperature: 25°C, Relative humidity: 70% Dew point ≈ 19.1°C

Any surface cooler than 19.1°C in that air will collect condensation, which is why a glass of iced water sweats and a single-glazed window in winter runs with water. It also feels sticky outdoors, since sweat evaporates poorly once the dew point clears 18°C.