How the dew point is found
Air can only hold so much water vapour at a given temperature. The dew point is the temperature you would have to cool that air down to before it becomes fully saturated and moisture starts to condense into dew, fog, or droplets on a cold surface.
This calculator uses the Magnus approximation. It first combines the current temperature and relative humidity into an intermediate term, then rearranges that term to recover the saturation temperature. The result is reported in both Celsius and Fahrenheit.
Reading the result
A dew point close to the air temperature means the air is nearly saturated, so it feels muggy and surfaces fog up easily. A dew point far below the air temperature means the air is dry.
- Below about 10°C (50°F): the air feels comfortably dry.
- Around 16–18°C (60–65°F): noticeably humid and sticky.
- Above about 21°C (70°F): oppressive, sweat evaporates slowly.
Practical tips
The dew point is a steadier measure of how humid it actually feels than relative humidity alone, because relative humidity changes as the air warms and cools through the day even when the moisture content stays the same.
- Use the dew point to judge condensation risk on windows, pipes, and cold walls.
- Outdoor air at or below your indoor dew point can be used to ventilate and dry a space.
Caveats and common mistakes
Relative humidity must be greater than zero for the formula to work, and the Magnus approximation is most accurate across ordinary near-surface weather conditions rather than extreme cold or very high altitudes.
These figures are general weather and comfort guidance, not medical advice. If heat or humidity is affecting your health, follow local health authorities.
Formula
γ = (17.27·T)/(237.7+T) + ln(RH/100); Td = 237.7·γ/(17.27−γ)