The dew point calculator works out actual vapour pressure and absolute humidity from the values you supply. Enter air temperature, critical pressure, critical temperature, relative humidity, and the calculator applies the standard relationship straight away, so you get the answer without working through the algebra yourself.
What Is the Dew Point Calculator?
It is built around actual vapour pressure, absolute humidity, sat vap pressure, and it works the same way whether you are checking a homework answer, confirming a colleague's figure, or working something out for yourself. Everything runs in your browser, so nothing you enter is uploaded anywhere.
How the Dew Point Calculation Works
Nothing is hidden: each output is worked out from the values above it, which makes it easy to see where a surprising answer came from.
The relationship between air temperature, critical pressure, critical temperature, relative humidity and actual vapour pressure, absolute humidity, sat vap pressure, temp water, dew point, α(t,rh), tau is fixed, so the same figures always give the same answer. That makes it a good cross-check: run the numbers a second way and see whether the result agrees.
Getting a Result You Can Trust
Start with a set of figures you are confident about. If one of the air temperature or critical pressure or critical temperature or relative humidity values looks wrong, the result will be wrong too, because everything downstream is built on top of it. Work through the outputs one at a time and check that each one is the size you would expect before relying on the last figure.
Frequently Asked Questions
What do I need to work out actual vapour pressure?
Enter air temperature, critical pressure, critical temperature, relative humidity. The calculator then works through the relationship and reports actual vapour pressure, absolute humidity, sat vap pressure.
What does the dew point actually calculate?
It takes air temperature and critical pressure and critical temperature and relative humidity and converts them into actual vapour pressure, absolute humidity, sat vap pressure using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter air temperature?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.