Dew Point Calculator

The dew point is the temperature at which air becomes saturated with water vapor — causing moisture to condense out as dew, fog, or frost. It is a more reliable indicator of human discomfort than relative humidity alone.

Magnus formula approximation: Dew Point (°C) = (243.04 × [ln(RH/100) + (17.625 × T)/(243.04 + T)]) ÷ (17.625 − [ln(RH/100) + (17.625 × T)/(243.04 + T)])

Where:

• T = air temperature in °C
• RH = relative humidity in percent
• ln = natural logarithm

Simplified approximation (accurate within 1°C for RH > 50%): Dew Point ≈ T − ((100 − RH) / 5)

Worked example: Temperature: 28°C | Relative Humidity: 75% Dew Point ≈ 28 − ((100 − 75) / 5) = 28 − 5 = 23°C

Human comfort scale by dew point:

Dew point vs. relative humidity: RH of 80% on a cold 5°C day feels dry and crisp. RH of 60% on a hot 35°C day feels muggy and uncomfortable. Dew point above 21°C (70°F) is uncomfortable regardless of temperature, which is why meteorologists prefer it as a humidity metric.

The dew point spread

The gap between air temperature and dew point is called the spread. A small spread (within 3°C) means the air is near saturation, so fog, dew, condensation on cold windows, or frost on lawns is likely overnight. A large spread (10°C or more) means dry air with no condensation risk. Pilots use the spread to predict fog formation at airports: when temperature drops to within 2°C of dew point, fog is imminent.