The formula
How to calculate dew point
Dew point is the temperature air would need to cool to for it to become fully saturated with the moisture it already holds, at which point dew, fog or condensation starts to form. It is a direct measure of how much moisture is actually in the air, unlike relative humidity, which changes with temperature even when the moisture content does not.
The Magnus-Tetens formula used here is an approximation of the physical relationship between temperature and the saturation vapour pressure of water, accurate to within about 0.4 °C across the ordinary range of weather conditions — close enough for forecasting and comfort purposes, though not for laboratory precision work.
What to enter:
- Air temperature (°C)
- Relative humidity (%)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Where more decimal places matter than the fields above display, the underlying calculation is not rounded until the final figure is shown — the precision used internally is higher than what is printed, which matters for anyone chaining this result into a further calculation of their own.
Why dew point matters
A dew point calculation gets used both to check work already done by hand and to explore how a formula behaves without redoing the algebra every time an input changes — this page exists for both, since the underlying arithmetic is the same either way.
Beyond a single check, the same calculation is worth rerunning whenever a measured input changes — a new reading, a corrected value, an updated assumption — since the result here always reflects exactly what is currently in the fields above rather than a value calculated once and then left stale.
This calculation sits in a long tradition of being done first by hand with tables and slide rules, then with a scientific calculator, and now with a page like this one — the underlying mathematics has not changed at any point in that history, only the speed and convenience of getting from the inputs to the answer. Understanding the formula itself, shown above, is still worth doing even when a tool computes it instantly, since it is what makes the result trustworthy rather than just fast.
Where a calculation like this one is part of a larger piece of work, it is generally worth running it with a round, easy-to-check set of numbers first — inputs of exactly 1, 10 or 100 — purely to confirm the formula is being applied correctly, before switching to the real measured values the actual result depends on.
Worked example
Take the figures the calculator starts with:
- Air temperature: 20 °C
- Relative humidity: 50 %
That gives:
- Dew point: 9.3 °C
The figures above are the calculator's own default values, shown purely so the working is visible rather than hidden — the same steps apply exactly to your own numbers, entered in the fields at the top of this page.
Reading the result
Dew point is widely considered a better gauge of how muggy the air feels than relative humidity: below about 10 °C is dry and comfortable, 10–16 °C is comfortable, 16–20 °C is somewhat humid, 20–24 °C is humid and sticky, and above 24 °C is oppressive — regardless of what the air temperature itself happens to be.
Where this goes wrong. Dew point can never be higher than the air temperature — physically, air cannot be cooled to saturation at a temperature above the one it is already at. The closer dew point sits to air temperature, the closer the air already is to saturated, which is also why a small drop in temperature on a humid evening quickly produces dew, fog or mist.
A result that is wrong by an exact factor of ten, a hundred or a similar round number is almost always a units error rather than a mistake in the formula itself — checking each input against the unit stated beside it is the fastest way to track it down.
Relative humidity is relative to how much moisture the air *could* hold at its current temperature, and warm air can hold far more than cold air — so 50% relative humidity means something very different at 5 °C than at 30 °C. Dew point measures the actual moisture content directly, so it stays comparable across different air temperatures.
Fog tends to form when the air temperature cools to within about 2–3 °C of the dew point, since that is close enough to saturation for water vapour to condense into visible droplets — which is why calm, clear nights that let the ground (and the air just above it) cool quickly are the classic conditions for morning fog.
The headline figure is dew point. With 20 °C air temperature and 50 % relative humidity, that comes to 9.3 °C. Change any field and the figure moves with it.
Generally, no more than the least precise input justifies — a result reported to six decimal places from inputs measured to two significant figures is implying a precision the calculation does not actually have. The calculator shows full precision so you can round appropriately for your own use.
Yes — the equation shown in the formula section above is the standard form used in textbooks and reference material for this calculation, not a simplified or approximate version.
Yes, in the sense that it applies the correct standard formula and returns an accurate result for the inputs given — but check your own course or publication's requirements for how results should be rounded, presented and referenced, since those conventions vary and are not something a calculator can know on your behalf.