<p>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 humidity<a href="https://i.sstatic.net/YOFyA.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YOFyA.png" alt="enter image description here"></a></p>

<p>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
<a href="https://i.sstatic.net/XEQzF.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XEQzF.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2018-01-27T06:48:06.067Z
I am looking for relative humidity inside but I think that the equation would help me. Thank you very much.
klarkson
CC BY-SA 3.0
2018-01-27T16:40:43.763Z
@klarkson I've improved the answer for that case, have a look at the changes. If it solves your problem please accept the answer.
Camilo Rada
CC BY-SA 4.0
2021-06-16T22:22:31.887Z
can you please tell me above equation of relative indoor humidity is from which source
Vanshita Sharma
CC BY-SA 4.0
2021-06-17T22:51:12.103Z
@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.
Camilo Rada
CC BY-SA 3.0
2018-01-26T22:22:11.763Z
2018-01-27T16:37:32.917Z
2018-01-27T16:37:32.917Z
Camilo Rada
<p>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.</p>

<p>I found the Relative Humidity Calculator at <a href="https://www.lenntech.com/calculators/humidity/relative-humidity.htm" rel="nofollow noreferrer">https://www.lenntech.com/calculators/humidity/relative-humidity.htm</a>.
I tried to understand the source code, but I am confused with the formulas (absMoisture1, RH2) used for the calculation of relative humidity. </p>

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

CC BY-SA 3.0
2018-01-23T15:19:34.407Z
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:
Fred
CC BY-SA 3.0
2018-01-23T15:20:53.827Z
*Environmental Engineering in South African Mines*, The Mine Ventilation Society of South Africa, 1989, pp 451-455
Fred
CC BY-SA 3.0
2018-01-23T19:03:41.450Z
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.
JeopardyTempest
CC BY-SA 3.0
2018-01-24T20:48:36.920Z
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 .)
blacksmith37
CC BY-SA 3.0
2018-01-27T06:46:26.673Z
@Fred: Thank you very much for your response and for the recommendation of the book.
klarkson
CC BY-SA 3.0
2018-01-27T06:47:04.127Z
@JeopardyTempest: Yes, I try to find relative humidity inside based upon the values outside. Thank you very much for your advice.
klarkson
CC BY-SA 3.0
2018-01-27T06:47:36.853Z
@blacksmith37: Yes, I will take a look at other factors that influence the humidity.
klarkson
CC BY-SA 4.0
2021-06-20T09:35:17.903Z
Actually sir i searched everywhere but i didn't find any relevant source for this. It would be a great help if you provide me the source or anything about this formula. can you please tell me where you get this formula?
Vanshita Sharma
<meteorology><atmosphere-modelling><humidity>
How to calculate humidity using outside temperature and outside relative humidity?
klarkson
# How to calculate humidity using outside temperature and outside relative humidity?

(Asked by: username_0 on 2018-01-23)

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.

**Tags:** meteorology, atmosphere-modelling, humidity

> **Comments:**
> **username_1**: 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:  
> **username_1**: *Environmental Engineering in South African Mines*, The Mine Ventilation Society of South Africa, 1989, pp 451-455  
> **username_2**: 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.  
> **username_3**: 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 .)  
> **username_0**: @username_1: Thank you very much for your response and for the recommendation of the book.  
> **username_0**: @username_2: Yes, I try to find relative humidity inside based upon the values outside. Thank you very much for your advice.  
> **username_0**: @username_3: Yes, I will take a look at other factors that influence the humidity.  
> **username_4**: Actually sir i searched everywhere but i didn't find any relevant source for this. It would be a great help if you provide me the source or anything about this formula. can you please tell me where you get this formula?  

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## 1 answers

### Answer by username_5 on 2018-01-26. Score: 3 | Accepted answer

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 humidity[![enter image description here](https://i.sstatic.net/YOFyA.png)](https://i.sstatic.net/YOFyA.png)

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](https://i.sstatic.net/XEQzF.png)](https://i.sstatic.net/XEQzF.png)

> **Comments:**
> **username_0**: I am looking for relative humidity inside but I think that the equation would help me. Thank you very much.  
> **username_5**: @username_0 I've improved the answer for that case, have a look at the changes. If it solves your problem please accept the answer.  
> **username_4**: can you please tell me above equation of relative indoor humidity is from which source  
> **username_5**: @username_4 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.  

---

