I would like to ask how to calculate the relative humidity that a heated indoor area would have using only outside temperature and outside relative humidity.

I found the Relative Humidity Calculator at https://www.lenntech.com/calculators/humidity/relative-humidity.htm. I tried to understand the source code, but I am confused with the formulas (absMoisture1, RH2) used for the calculation of relative humidity.

Please, have you ever face up to the similar problem? Many thanks in advance.

  • $\begingroup$ It appears you are trying to find the relative humidity for air inside a building/room given relative humidity & temperature of the air outside the building. If so, this question is more of an engineering question, not an earth science question. Calculating relative humidity is an involved process that requires the measurement of dry, wet bulb temperatures & barometric pressure. You then have to do a series of calculations to find saturated vapour pressures, moisture content of air, enthalpies of water vapour & liquid water. One book that I used a long time ago was: $\endgroup$
    – Fred
    Commented Jan 23, 2018 at 15:19
  • $\begingroup$ Environmental Engineering in South African Mines, The Mine Ventilation Society of South Africa, 1989, pp 451-455 $\endgroup$
    – Fred
    Commented Jan 23, 2018 at 15:20
  • $\begingroup$ Are you indeed trying to find the relative humidity inside based upon the values outside? If so it will not be usefully possible because the air masses are different inside versus outside. You are only able to convert to a different measure of humidity in the same location if given one of the other forms of humidity measure (e.g. dew point, RH, wet bulb, mixing ratio, etc) plus air temperature. $\endgroup$ Commented Jan 23, 2018 at 19:03
  • 2
    $\begingroup$ You can not calculate the humidity in a typical building because of the moisture additions from respiration , etc ( eg. i lose hundreds of gallons a year from my aquariums ) , It could be done for some sealed vessel or chamber containing no water sources .) $\endgroup$ Commented Jan 24, 2018 at 20:48
  • $\begingroup$ @Fred: Thank you very much for your response and for the recommendation of the book. $\endgroup$
    – klarkson
    Commented Jan 27, 2018 at 6:46

1 Answer 1


You can easily achieve that assuming that there is no sources of water vapour inside, so the absolute humidity (mass of water per volume of air), remains constant. This excerpt from my undergrad tesis will explain the formula for absolute humidityenter image description here

Once you know the absolute humidity, you can adapt the same formula to calculate the relative humidity of a parcel of outside air (of now known content of water), that is taken from outside to inside a building where its temperature risen to a new value. If we add subscripts "e" to exterior temperature and relative humidity and "i" for interior ones. Most of the constants can be dropped and the interior relative humidity would be given by enter image description here

  • $\begingroup$ I am looking for relative humidity inside but I think that the equation would help me. Thank you very much. $\endgroup$
    – klarkson
    Commented Jan 27, 2018 at 6:48
  • $\begingroup$ @klarkson I've improved the answer for that case, have a look at the changes. If it solves your problem please accept the answer. $\endgroup$ Commented Jan 27, 2018 at 16:40
  • $\begingroup$ can you please tell me above equation of relative indoor humidity is from which source $\endgroup$ Commented Jun 16, 2021 at 22:22
  • $\begingroup$ @VanshitaSharma I'm sorry but in my undergrad dissertation I did not include the reference. But it is a standard formula you will find in many sources. If you find a reliable source I can add as refernce for this please share it here, so I can add it to the answer. $\endgroup$ Commented Jun 17, 2021 at 22:51

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