EngineerHub.orgDrawing No. EH–TH–011 // Thermal Engineering & HVAC
Dew Point Calculator
Reviewed August 2026
Calculate dew point from air temperature and relative humidity in °C or °F using the Magnus formula, with an instant result and worked example.
Drawing No. EH–TH–011 // Thermal Engineering & HVAC
Reviewed August 2026
Calculate dew point from air temperature and relative humidity in °C or °F using the Magnus formula, with an instant result and worked example.
Condensation may begin on surfaces at or below the calculated dew-point temperature.
Room air at 22 °C and 55 % relative humidity.
Td = C·γ / (B − γ), γ = ln(RH/100) + B·T/(C + T)
With B = 17.62 and C = 243.12 °C: γ = ln(0.55) + (17.62 × 22)/(243.12 + 22) = −0.598 + 1.462 = 0.864. So Td = (243.12 × 0.864)/(17.62 − 0.864) = 12.5 °C, a dew-point depression of 9.5 K.
Practically: any surface in that room colder than 12.5 °C will condense — a single-glazed window on a cold night, an uninsulated cold-water pipe, or a thermal bridge at a wall junction. Raising the room temperature without removing moisture does not help the pipe; it only lowers the relative humidity of the air.
Points that come up most often with this calculation.
Heating adds no moisture, so the absolute humidity is unchanged and the dew point stays put — only the relative humidity falls. To move the dew point you must remove water vapour, by ventilating with drier air or by dehumidifying.
It is the gap between the air temperature and the dew point. A small depression means surfaces only slightly cooler than the room will condense, which is the usual cause of mould in corners and around window reveals.
The Magnus coefficients used here are for saturation over liquid water. Below 0 °C the physically correct quantity is the frost point, which uses a different saturation relationship and reads a little higher than this calculator will show.
Within roughly ±0.1 K over the −45 to 60 °C range this page accepts, which is well inside the accuracy of any ordinary humidity sensor.
Include the inputs and assumptions used.