How to use the Dew Point Calculator
- Enter the air temperature in Celsius.
- Enter the relative humidity as a percentage.
- Read the dew point in Celsius and Fahrenheit.
- Check the comfort band, which reflects how humid the air actually feels.
- Note the spread between air temperature and dew point — a small gap means condensation is close.
How the calculation works
The dew point is the temperature to which air must be cooled, at constant pressure, for it to become saturated and water to condense. Unlike relative humidity it is an absolute measure of moisture content: 60% humidity at 30°C holds far more water than 60% at 10°C, which is why relative humidity alone is a poor guide to how the air feels. Dew point compares directly across conditions.
The calculation uses the Magnus-Tetens approximation to the saturation vapour pressure curve, with the coefficients 17.625 and 243.04 that Alduchov and Eskridge refined in 1996. It is accurate to about 0.1°C over the range from −40°C to 50°C, which comfortably covers all terrestrial weather conditions.
Comfort bands follow meteorological convention: below 10°C dew point the air feels dry, 16°C is where most people start noticing stickiness, 21°C is uncomfortable, and above 24°C is oppressive, because sweat evaporates too slowly to cool effectively. The spread between air temperature and dew point matters separately for buildings — when a surface falls below the dew point, condensation forms, which is the mechanism behind window misting, cold-pipe sweating and interstitial damp in walls.
α = ln(RH/100) + (17.625T)/(243.04+T); Td = (243.04α)/(17.625−α), with T and Td in °CSource: Magnus-Tetens approximation with coefficients from Alduchov & Eskridge (1996), Journal of Applied Meteorology 35(4).
Worked example
A room is at 24°C with 60% relative humidity, and single-glazed windows sit at about 14°C in winter.
- α = ln(0.60) + (17.625 × 24)/(243.04 + 24) = −0.5108 + 1.5836 = 1.0728.
- Td = (243.04 × 1.0728)/(17.625 − 1.0728) = 260.74 / 16.552.
- Dew point = 15.75°C.
- Glass at 14°C sits below that dew point.
A 15.8°C dew point against 14°C glass means condensation will form on the windows — ventilate or raise the surface temperature to stop it.
Frequently asked questions
Why is dew point better than relative humidity?+
It is absolute. A dew point of 20°C feels muggy whatever the air temperature, whereas 70% humidity feels entirely different at 5°C and at 30°C.
Can dew point exceed air temperature?+
No. They are equal at 100% relative humidity, which is the saturation point — fog or dew forms at that boundary.
What dew point stops condensation on windows?+
Keep the indoor dew point below the coldest surface temperature. In winter that usually means holding indoor humidity to 40% or lower.
How does dew point relate to the heat index?+
Both describe humid discomfort, but the heat index converts it into an apparent temperature while dew point states the moisture content directly.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.