Dew Point Calculator
Calculate the dew point from temperature and relative humidity.
See comfort level and condensation risk.
Supports Fahrenheit and Celsius.
Dew point temperature is the temperature at which air must be cooled — at constant pressure and moisture content — for water vapor to condense into liquid water (dew). It is a direct measure of how much moisture the air contains, independent of temperature.
Magnus formula approximation for dew point:
γ(T, RH) = ln(RH/100) + (b × T) ÷ (c + T)
Dew Point (°C) = c × γ ÷ (b − γ)
Where constants: b = 17.67, c = 243.5°C
Simplified formula (accurate to ±1°C for RH > 50%):
Dew Point ≈ T − [(100 − RH) ÷ 5]
- T = air temperature in °C
- RH = relative humidity as a percentage (0–100)
Worked example: Temperature = 28°C, Relative Humidity = 70%
- γ = ln(0.70) + (17.67 × 28) ÷ (243.5 + 28)
- γ = −0.3567 + 495.76 ÷ 271.5 = −0.3567 + 1.8259 = 1.4692
- Dew Point = 243.5 × 1.4692 ÷ (17.67 − 1.4692) = 357.75 ÷ 16.20 = 22.1°C
Human comfort scale:
| Dew Point | Comfort Level |
|---|---|
| Below 10°C (50°F) | Dry, very comfortable |
| 10–15°C (50–59°F) | Comfortable |
| 15–18°C (59–64°F) | Slightly humid |
| 18–21°C (64–70°F) | Humid, somewhat uncomfortable |
| 21–24°C (70–75°F) | Very humid, oppressive |
| Above 24°C (75°F) | Extremely oppressive |
Dew point vs. relative humidity: Relative humidity is temperature-dependent and can change without any change in actual moisture content. Dew point is an absolute measure — it does not change as temperature fluctuates. Meteorologists prefer dew point for this reason.