The dew point is one of the most useful numbers in weather and building science, and this calculator works it out from just the air temperature and the relative humidity. It is the temperature the air would need to cool to before the water vapour in it begins to condense into dew, fog, mist or cloud, and because it depends on the actual amount of moisture present rather than the temperature, it is a far more honest guide to how humid the air really feels than relative humidity is. Enter the current temperature in degrees Celsius and the relative humidity as a percentage, and the calculator returns the dew point along with a quick comfort guide, updating as you type. It uses the well-established Magnus formula, the standard approximation meteorologists rely on, which combines the two inputs to find the temperature at which the air would be completely saturated. Knowing the dew point matters in real ways. As a comfort guide, a dew point below about 10 degrees feels pleasantly dry, the mid teens feel noticeably humid, and anything above about 18 to 20 degrees feels muggy and sticky. It also tells you when condensation will form: if a surface, a window, a cold pipe or a wall, drops to the dew point, moisture will settle on it, which is central to understanding dampness, mould and the performance of insulation and ventilation in New Zealand homes. It is genuinely useful for anyone managing indoor air and condensation, for gardeners and tramper watching for frost and fog, for HVAC and building work, and for students learning about humidity and the water cycle. The formula and a worked example are explained clearly below.
Calculate.co.nz is proud to be partnered with Health Based Building, a leader in sustainable and health-conscious building innovation. With over a century of experience, they develop high-performance systems like Foreverbreathe Specification, Magnum Board, and Foreverbreathe Paints to support energy-efficient, non-toxic living environments. Their commitment to healthier homes aligns with our belief that informed choices lead to better outcomes for Kiwi households.
°C
%
13.2 °C
dew point
Becoming humid
How it works
The Magnus formula uses constants a = 17.27 and b = 237.7. It first finds an intermediate value, gamma, equal to the natural log of the humidity fraction plus a times temperature over b plus temperature. The dew point is then b times gamma divided by a minus gamma, in degrees Celsius.
Worked example
At 20 degrees and 65% relative humidity, gamma is the log of 0.65 plus 17.27 times 20 over 257.7, which is about 0.91. The dew point is 237.7 times 0.91 divided by 17.27 minus 0.91, which comes to about 13.2 degrees. So surfaces cooler than 13.2 degrees would start to collect condensation.
