Dew Point Temperature Calculator
Calculate the dew point temperature from air temperature and relative humidity.
Understand when condensation, fog, and moisture problems will occur.
What Is the Dew Point?
The dew point is the temperature at which air becomes saturated with water vapor — the point where condensation begins. When the air cools to the dew point temperature, water droplets form on surfaces, creating dew, fog, or frost.
The closer the dew point is to the actual air temperature, the higher the humidity feels — and the more likely condensation is.
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
| Dew Point | How It Feels |
|---|---|
| Below 10°C (50°F) | Dry and comfortable |
| 10–15°C (50–59°F) | Comfortable for most people |
| 15–18°C (59–65°F) | Starting to feel humid |
| 18–21°C (65–70°F) | Noticeably humid and uncomfortable |
| 21–24°C (70–75°F) | Very humid, oppressive |
| Above 24°C (75°F) | Extremely oppressive, dangerous for outdoor work |
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 ≈ 19.0°C
This means: any surface cooler than 19°C will collect condensation. It also feels quite sticky and humid outdoors.