Dew point calculator
Dew point from temperature and humidity, with the coefficients named so you can reproduce it — and why it tells you how a day will feel when relative humidity cannot. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Dew point tells you what humidity cannot
Relative humidity is relative — to the temperature. 70% humidity at 5 °C and 70% at 30 °C are wildly different amounts of water in the air, because warm air can hold far more of it.
Dew point is absolute. It is the temperature the air would have to be cooled to before water condensed out, and it does not change when the temperature does. That is why forecasters use it, and why it predicts how sticky a day will feel when humidity does not.
Roughly: below 10 °C is dry, 16 is noticeable, 18 is sticky, 21 is uncomfortable and above 24 is miserable. Those thresholds are conventional rather than derived, and they are stated here as conventions.
It can never be higher than the air temperature
At 100% humidity the dew point is the air temperature — that is what saturation means. Below saturation it is lower. There is no arrangement of real weather in which it is higher, and a calculator returning one has an error in it.
This page computes the two independently and shows the spread between them, so the constraint is visible rather than assumed.
Which Magnus coefficients
The Magnus formula has a pair of constants, and more than one pair is in use. The older Magnus-Tetens values are a = 17.27 and b = 237.7; this page uses the Alduchov-Eskridge refinement, a = 17.625 and b = 243.04, which is more accurate over a wider range.
The difference is small — a few hundredths of a degree in ordinary conditions — but it is a difference, and a page that does not say which pair it used has left you unable to reproduce it.