Dew Point Calculator: Temperature, Humidity, and Condensation Explained

On a humid morning, windows fog, glasses mist when you step outside, and a cold drink sweats on the table. All of those are the same phenomenon: water vapour in the air condensing on a surface that is colder than the dew point. The dew point is the temperature to which air must be cooled for its water vapour to start condensing, and it is a far better measure of mugginess than relative humidity alone, because it reflects the actual amount of moisture in the air.

The Dew Point Calculator computes the dew point from two inputs you can measure: air temperature in Celsius and relative humidity as a percentage. It uses the Magnus formula, a well-established approximation that is accurate enough for everyday and building purposes. The result helps with comfort, condensation risk, painting and construction decisions, and understanding weather forecasts, where dew point is routinely reported alongside temperature.

How it works

The Magnus formula works in two stages. First, it combines temperature and relative humidity into an intermediate value. With temperature T in Celsius and relative humidity RH as a percentage, the intermediate value, often called alpha, equals (17.27 multiplied by T, divided by 237.7 plus T), plus the natural logarithm of RH divided by 100. The constants 17.27 and 237.7 are empirical values that fit the formula to the physics of water vapour over the usual temperature range.

Second, the dew point is recovered from alpha: dew point equals 237.7 multiplied by alpha, divided by 17.27 minus alpha. The result is in Celsius. The formula is an approximation, but over the range of temperatures people normally encounter, roughly minus 40 to plus 50 degrees Celsius, it agrees with more complex formulations to within a fraction of a degree, which is ample for practical decisions about condensation and comfort.

The relationship between the inputs is worth grasping intuitively. At a fixed temperature, higher relative humidity means more moisture and a higher dew point, closer to the air temperature. At a fixed relative humidity, higher temperature also raises the dew point, because warm air at the same relative humidity holds more absolute moisture. That is why a dew point of 20 degrees feels oppressive in summer but would be impossible in a cold winter room without a great deal of added moisture.

Worked example

Take air at 25 degrees Celsius with 60 percent relative humidity, a typical warm indoor or outdoor condition. First compute the temperature term: 17.27 multiplied by 25 is 431.75, divided by 237.7 plus 25, which is 262.7, giving 1.6435. The humidity term is the natural logarithm of 0.60, which is minus 0.5108. Adding them gives alpha equal to 1.6435 minus 0.5108, which is 1.1327.

Now recover the dew point: 237.7 multiplied by 1.1327 is 269.24, divided by 17.27 minus 1.1327, which is 16.1373, giving 16.68 degrees Celsius, rounded to 16.7 degrees Celsius. Any surface colder than about 16.7 degrees in that air will collect condensation. That explains why a cold water pipe, a poorly insulated wall, or a window on a cool night fogs up even when the room itself feels warm and dry enough.

  • Temperature term: (17.27 multiplied by 25) divided by (237.7 plus 25) equals 1.6435.

  • Humidity term: natural log of 0.60 equals minus 0.5108; alpha equals 1.1327.

  • Dew point: (237.7 multiplied by 1.1327) divided by (17.27 minus 1.1327) equals 16.68, rounded to 16.7 degrees Celsius.

Common mistakes to avoid

The most common mistake is confusing dew point with relative humidity. Relative humidity is a percentage that depends on temperature, so 60 percent at 25 degrees and 60 percent at 10 degrees describe very different amounts of moisture. Dew point is an absolute measure in degrees, and it is comparable across temperatures: a dew point of 16.7 degrees means the same moisture content regardless of the air temperature it was computed from. A second mistake is entering humidity as a decimal, 0.60, when the tool expects a percentage, 60, or vice versa, which produces a wildly wrong answer.

A third mistake is assuming condensation only happens outdoors or in cold weather. Indoor condensation is common in kitchens, bathrooms, and unventilated rooms, where moisture from cooking, showering, and breathing raises the dew point while walls or windows stay cool. A fourth mistake is treating the dew point as a comfort threshold on its own: comfort also depends on air movement, clothing, activity, and radiant temperature, so use dew point as one input among several.

What it does not do

This calculator uses the Magnus approximation for water vapour over liquid water. It is accurate for everyday purposes but less exact at extreme temperatures, over ice rather than water, and at very low humidities. It also assumes the air is well mixed and at the entered temperature and humidity; in a real room, temperature and humidity vary by location, so the dew point near a cold wall may differ from the dew point in the centre of the room.

The result predicts condensation on a surface at or below the dew point, but real condensation also depends on surface properties, air movement, and time. A surface slightly below the dew point may take time to show moisture, and a surface with a hygroscopic coating may show dampness earlier. For building moisture problems, persistent condensation, or mould, seek professional assessment rather than relying on a single calculation.

A short practical routine

A practical routine for a home is to check the dew point when condensation appears, rather than guessing at the cause. Measure temperature and humidity near the affected surface, compute the dew point, and compare it with the surface temperature if you can measure it. If surfaces are below the dew point, the remedies are ventilation to lower indoor humidity, heating to raise surface temperatures, or insulation to keep surfaces warmer, and the calculation tells you which lever matters most.

For outdoor comfort, note the dew point in weather forecasts: dew points below about 13 degrees generally feel comfortable, 16 to 18 degrees feel humid, and above 21 degrees feel oppressive to most people. For related measurements, use the Weight Converter for unit checks, while the guide on Area Calculator: Measure Any Shape From Its Dimensions covers the room measurements that feed ventilation estimates.

Start with the Dew Point Calculator, check units with the Weight Converter, and measure rooms with the Area Calculator.