Dew Point Calculator

This calculator estimates the temperature to which air must be cooled to become saturated with water vapor and form dew. Provide any two of air temperature, relative humidity and dew point, and it works out the third, along with the vapour pressure, the absolute humidity and the moisture concentration by volume and by weight. Air at 20 °C and 65 percent humidity has a dew point of 13.2 °C, which is 55.8 °F or 286.4 K.

What humidity is

Humidity is the amount of water vapor, the gaseous phase of water, in the air. It is what governs dew, frost, fog and precipitation. The maximum amount of water vapor air can hold depends on temperature: the warmer the air, the more it can carry before it saturates, and the relationship is steep rather than gentle. Warming air from 20 °C to 30 °C raises its capacity by a factor of 1.82.

Humidity is usually discussed as either absolute or relative, and this calculator returns both.

Absolute humidity is the water content of the air by mass, normally in grams per cubic meter. It is the total mass of water vapor divided by the volume of air. For a fixed volume with the same amount of vapor in it, absolute humidity does not change when the temperature changes. In the open atmosphere the volume is not fixed, so absolute humidity does shift as temperature and pressure move the air about.

Relative humidity compares the current absolute humidity against the maximum for that temperature and states it as a percentage. It depends on both temperature and pressure. For the same amount of water vapor, cool air shows a higher relative humidity than warm air, because the denominator has shrunk.

Relative humidity is the figure quoted in weather reports, and it is a good indicator of precipitation, dew, frost, fog and apparent temperature. Apparent temperature is what the body perceives. In summer, higher relative humidity means a higher apparent temperature, because humid air slows the evaporation of sweat and evaporation is how the body sheds heat. A relative humidity of 100 percent means the air is saturated: under the current conditions the vapor content cannot rise any further, and it is also the point at which dew begins to form.

What dew point is

Dew point is the temperature at which a given volume of air at a certain atmospheric pressure becomes saturated with water vapor, causing condensation and the formation of dew. Dew is the condensed water seen on grass and flowers early in the morning, when the ground has cooled overnight past that temperature.

Dew point rises with the amount of water vapor present, so humid air has a higher dew point than dry air. The higher the relative humidity, the closer the dew point sits to the current air temperature, and at 100 percent relative humidity the dew point equals the air temperature exactly. Where the dew point falls below freezing, 0 °C or 32 °F, the vapor turns directly to frost rather than passing through liquid dew, and the same quantity is then called the frost point.

Perception varies between people, and there is some acclimatisation to higher dew points, but high dew points are generally uncomfortable because the humidity prevents sweat evaporating properly and the body struggles to cool down. Low dew points bring their own problems: dry skin, irritation and dried-out airways. The US Occupational Safety and Health Administration recommends indoor air between 68 and 76 °F, which is roughly 20 to 24 °C, with relative humidity of 20 to 60 percent.

Dew point is also used in general aviation to work out the likelihood of carburetor icing and fog. In some cases it is measured directly with a dew point meter, which passes air over a polished metal mirror while cooling it. The temperature at which dew appears on the mirror is the dew point.

Why dew point beats relative humidity for comparing days

Relative humidity is a ratio, so it moves when either half of it moves. A morning at 20 °C and 50 percent humidity and an afternoon at 30 °C and 50 percent humidity have the same percentage and are not remotely the same air: the dew points are 9.3 °C and 18.4 °C, more than nine degrees apart. All that happened is the air warmed and its capacity grew, so the same reading now describes far more moisture.

Dew point does not have that problem. It is a direct statement of how much water vapor is present, and it barely moves through the day unless an actual air mass change occurs. That is why meteorologists watch it during heat waves, and why it is the better number for deciding whether tonight will be uncomfortable.

Dew point	How it feels
Below 10 °C (50 °F)	Dry. Comfortable, and low enough that static and dry skin become the complaint instead.
10 to 13 °C (50 to 55 °F)	Pleasant for most people.
13 to 16 °C (55 to 60 °F)	Noticeable. Starting to feel humid.
16 to 18 °C (61 to 64 °F)	Sticky. Uncomfortable for anything active.
18 to 21 °C (64 to 70 °F)	Oppressive. Sleep is affected without air conditioning.
Above 21 °C (70 °F)	Severe. Heat stress becomes a real risk with exertion.

How the calculation works

Everything on this page runs through saturation vapour pressure, the pressure water vapor exerts when air at a given temperature is holding all it can. This calculator uses the Buck equation, which is accurate to roughly 0.05 percent between 0 and 50 °C. At 0 °C saturation pressure is 611 Pa, at 20 °C it is 2,338 Pa, at 30 °C it is 4,245 Pa and at 40 °C it is 7,382 Pa. That steep climb is the whole reason warm air holds so much more water.

Actual vapour pressure is then the saturation pressure multiplied by the relative humidity, so 2,338 Pa at 65 percent gives 1,520 Pa. The dew point is the temperature whose saturation pressure equals that 1,520 Pa, which is 13.2 °C. Working the other way, relative humidity is the saturation pressure at the dew point divided by the saturation pressure at the air temperature.

The remaining outputs follow from the vapour pressure. Absolute humidity comes from the ideal gas law using the specific gas constant for water vapor, 461.5 J/(kg·K). Moisture volume concentration is the vapour pressure as a share of atmospheric pressure at 101,325 Pa, expressed in parts per million. Moisture weight concentration converts that to a share by mass using the ratio of molar masses, 18.015 for water against 28.97 for dry air, which is 0.6219 and is why the weight figure always comes out lower than the volume figure.

Where dew point matters in practice

Condensation on windows is the everyday case. Any surface colder than the dew point of the room will collect water. A room at 21 °C and 50 percent humidity has a dew point of 10.2 °C, so single glazing on a cold night will run with condensation while double glazing usually will not. The fix is either warmer glass or drier air, and only one of those is cheap.

The same reasoning drives mould. Mould needs a surface relative humidity around 80 percent rather than liquid water, so a cold corner behind furniture can grow it while the middle of the room stays fine. Lowering the indoor dew point through ventilation is more effective than heating the room.

In air conditioning, dehumidification only happens when the coil runs below the dew point of the air passing over it. An oversized unit that cools the room quickly and shuts off never runs long enough to wring the water out, which is why an oversized system can leave a house cold and clammy.

Aviation uses the gap between temperature and dew point, called the spread. A small spread means fog or low cloud is likely, and the base of cumulus cloud sits roughly 400 feet up for each degree Celsius of spread. Compressed air systems, paint shops and anywhere storing electronics track dew point for the same reason: it predicts when water will appear on something.

Reading the results on this page

Fill any two boxes and the third is solved. If all three are filled, the air temperature and relative humidity are used and the dew point box is ignored, which the calculator says explicitly rather than silently. The panel gives the answer in all three temperature scales, then the vapour pressure, the saturation pressure, the absolute humidity and the two moisture concentrations. The steps card shows the saturation pressure, the vapour pressure derived from it, and which quantity was searched for.