About this tool
Dew Point Calculator is a free, in-browser tool that finds the dew point — the temperature at which air becomes saturated and water begins to condense — from the air temperature and relative humidity you enter. It also rates how the resulting dew point feels and echoes your humidity. Everything is computed locally; nothing is uploaded and no live weather is fetched.
It uses the Magnus-Tetens approximation with constants a = 17.625 and b = 243.04 °C. First it computes γ = ln(RH / 100) + a·T / (b + T), then the dew point Dp = b·γ / (a − γ), with the temperature in Celsius. Fahrenheit inputs are converted to Celsius first and the answer is converted back if you selected °F.
The humidity is a value you type in, not a live reading — enter the temperature and relative-humidity percentage from a thermometer or forecast. A comfort label follows the usual dew-point scale: dry below 10 °C, comfortable 10–16 °C, humid 16–21 °C and muggy or oppressive above 21 °C.
Frequently asked questions
What is the dew point?
It is the temperature to which air must cool, at constant pressure, for its water vapour to reach saturation and start condensing as dew, fog or droplets. A higher dew point means more moisture in the air and a muggier feel.
Which formula is used?
The Magnus-Tetens approximation: γ = ln(RH/100) + 17.625·T/(243.04 + T), then dew point = 243.04·γ / (17.625 − γ), with temperature in Celsius. It is accurate to a few tenths of a degree over normal weather conditions.
Does this use live weather data?
No. The temperature and relative humidity are values you enter yourself. The tool never connects to a weather service — it simply applies the formula to your inputs, entirely in your browser.
Why does relative humidity have to be above zero?
The formula takes the natural log of RH/100, which is undefined at zero and negative for values under 100%. At exactly 0% humidity there is no moisture to condense, so the tool requires a value between just above 0 and 100.
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