WIN-J · TEMPERATURE LAB
Dew point and condensation condition calculator
Calculate dew point from air temperature and relative humidity, then compare an optional surface temperature. Review liquid-water model limits and unrounded temperature differences, and export the results as CSV.
Calculation results
Values below freezing also use liquid water as the reference. Frost point is not calculated.
Compare the relationship between the inputs and calculated values.
How to use
- Select Celsius or Fahrenheit and enter the air temperature in that unit.
- Enter relative humidity above 0% and at most 100%. Zero humidity does not have a finite dew point in this calculation.
- If known, enter the measured temperature of a window, wall or other surface in the same unit. Otherwise leave it blank.
- Calculate dew point, the air–dew-point spread and the surface–dew-point difference.
- With a surface temperature, also review the relative humidity for saturation at that surface. Copy the results or download CSV.
Worked example
At 20°C air temperature and 60% relative humidity, dew point is about 12.0°C. A 10°C surface is about 2.0°C below the unrounded dew point. If no surface temperature is entered, only dew point is calculated; no surface condition is compared.
Methods and limits
Uses the WMO Magnus approximation over liquid water with a=17.62 and b=243.12°C: γ=ln(RH/100)+aT/(b+T), Td=bγ/(a−γ), with T and Td in Celsius. Air and optional surface temperatures must be −45 to 60°C (−49 to 140°F), humidity above 0% and at most 100%, and the computed dew point must stay within the same model range. The surface saturation threshold is 100×es(surface)/es(air). Values above 100% are not attained within the ordinary humidity range for that temperature pair. Comparisons use unrounded values. Below-freezing values still refer to liquid water; frost point is not calculated. Sensor error, ventilation, heat flow, materials and changes over time are not modelled. This does not predict moisture amount, formation time or mould.
Frequently asked questions
How is dew point different from surface temperature?
Dew point is the temperature at which air with the given water-vapour content becomes saturated relative to liquid water. Surface temperature is separately measured on a window, wall or other object. The tool compares these temperatures under the entered conditions.
Does a surface below dew point guarantee visible droplets?
The calculation compares a saturation-temperature condition over liquid water. Real surfaces depend on airflow, materials, time and measurement error; this alone cannot determine when or how much moisture appears. Below freezing, frost-point calculations are a separate matter.
Can I use very dry or below-freezing conditions?
Humidity must be above zero. Even if the input temperature is valid, very dry air can produce a dew point below −45°C, which is outside the model. Supported subzero results remain dew points over liquid water, not frost points.
Why can equal displayed temperatures have a different comparison?
Displayed numbers are rounded, but the comparison uses unrounded values. A small difference can be hidden by the display rounding. Consider your sensor’s accuracy as well.
Does switching Celsius to Fahrenheit convert the inputs?
No. Changing the unit reinterprets the existing numbers in that unit. Enter both air and surface temperatures in the newly selected unit. Use the unit converter for simple temperature conversion.