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Dew Point Calculator

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The dew point is the temperature air has to be cooled to before water starts coming out of it, and it is a better description of how humid a day feels than relative humidity is — because it does not move when the temperature does. Enter a temperature and a humidity and this gives you the dew point, the wet bulb, how much water the air is actually carrying, and the surface temperature at which that water starts appearing on your windows.

What do you know?
°C
%

What a hygrometer or a weather app reports.

hPa

Sea level is 1013.25 hPa. Only affects the mixing ratio and the wet bulb, not the dew point.

How to use the dew point calculator

  1. 1Choose whether you have a relative humidity reading, a dew point, or a wet bulb measurement.
  2. 2Enter the air temperature — the dry bulb reading — and pick Celsius or Fahrenheit.
  3. 3Enter the second figure. Leave the pressure at 1013.25 hPa unless you are working at altitude.
  4. 4Read the dew point, and below it the temperature at which a cold surface will condense or grow mould.

Examples

A muggy afternoon

Input
32 °C at 60% humidity
Result
Dew point 23.2 °C — oppressive, and evaporative cooling is losing its effectiveness

The same 60% at 18 °C gives a dew point of 10.3 °C, which is comfortable. Relative humidity alone tells you almost nothing.

Why the windows are wet

Input
21 °C indoors at 55% humidity
Result
Dew point 11.6 °C, so any glass colder than that grows water

Single glazing on a cold night sits well below 11.6 °C, which is why it streams while the room feels dry.

Mould before water

Input
20 °C at 65% humidity
Result
Condensation at 13.2 °C, but the surface reaches 80% humidity at 16.4 °C

Three degrees of difference, and mould germinates in the gap without a drop of visible water.

About the dew point calculator

Where the numbers come from

Everything here rests on saturation vapour pressure — how much water vapour air can hold at a given temperature — and that quantity has no closed-form expression. Every practical formula is a fit to measurements, and the fits disagree slightly, so the honest thing is to name the one in use. This tool uses the Alduchov and Eskridge (1996) Magnus coefficients, which their paper gives as accurate to better than 0.4% from -40 °C to 50 °C. The older Magnus constants that circulate in textbooks are more than 1% out at the cold end, enough to shift a computed dew point by a few tenths of a degree.

The dew point itself is that formula run backwards: find the temperature at which the air's current vapour pressure would be the saturation pressure. Absolute humidity comes from the ideal gas law applied to the vapour alone, and the mixing ratio from the ratio of the molar masses of water and dry air. The wet bulb uses Stull's 2011 empirical fit, which is why it carries a warning outside the range that paper covers — near sea level, -20 °C to 50 °C, humidity between 5% and 99%.

The building problem, which is not the weather problem

Most people who need a dew point are not forecasting; they are trying to work out why a wall is damp. The chain is short: warm indoor air holds a lot of water, it meets a cold surface, and the water comes out where they touch. The dew point is the exact temperature at which that happens, so it turns a vague question about damp into a comparison of two numbers.

The part usually missed is that mould does not wait for liquid water. Surface humidity of around 80% is enough for most moulds to germinate, and a surface reaches 80% several degrees before it reaches 100%. That is why this tool reports both temperatures. A wall that never visibly sweats can still be growing something, and a survey that only looks for condensation will not find it.

The two levers are the same in either case. Raise the surface temperature — insulate, or stop putting furniture against the cold outside wall — or lower the dew point by getting the moisture out. Extractor fans in kitchens and bathrooms remove it at the source, which is far more effective than trying to dry a whole house afterwards.

What this cannot do

It cannot tell you the temperature of your wall, which is the other half of every condensation question and needs an infrared thermometer or a surface probe. It also assumes the air is well mixed at one temperature and humidity, which a room with a cold corner is not.

It is arithmetic on figures you supply, not a measurement. A cheap hygrometer can be several percent out, and several percent of relative humidity moves the dew point by more than a degree — so if the answer is going to inform spending money, calibrate the instrument before trusting the third digit.

Frequently asked questions

Why is dew point a better measure of humidity than relative humidity?
Because it does not depend on the temperature. Relative humidity is a ratio — how much water the air holds against how much it could hold — and the denominator roughly doubles every eleven degrees, so the same air can read 90% at dawn and 45% by noon without a single molecule of water moving. The dew point is the absolute quantity, so it stays put through the day and comparing two places by it actually means something.
What dew point counts as comfortable?
Below about 13 °C most people find the air pleasant, 16 to 18 °C starts feeling sticky, and above 21 °C it becomes genuinely unpleasant because sweat stops evaporating fast enough to cool you. These bands are the descriptions used in weather forecasting rather than a measurement of anyone's comfort, and acclimatisation shifts them by several degrees — a dew point that is oppressive in Manchester is an ordinary July in Chennai.
How do I use this to stop condensation on my walls?
Compare the dew point this gives you with the temperature of the coldest surface in the room. Condensation forms wherever a surface is below the dew point, so the two fixes are raising the surface temperature — insulation, or not blocking a radiator — and lowering the dew point by ventilating or dehumidifying. The tool also gives the temperature at which the surface reaches 80% humidity, which is where mould starts and is several degrees warmer than where water appears.
What is the wet bulb temperature and why does it matter?
It is what a thermometer reads with a wet wick over it and air moving past, so it is the lowest temperature evaporation can reach at that humidity. It sets the limit for evaporative coolers, cooling towers and the human body: sustained wet bulb readings around 35 °C are survivable by no amount of shade or water, because sweat cannot shed heat into air that is already that close to saturated.
Why does the dew point differ from the frost point below freezing?
Because ice and supercooled water hold different amounts of vapour at the same temperature. Below 0 °C this tool reports both: the dew point is the conventional figure that forecasts quote, and the frost point — always slightly higher — is where frost actually forms on a surface. At -10 °C the gap is about a degree, which is enough to decide whether a windscreen frosts overnight.

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