In addition to its main function, which is the place for washing and cleaning, the historic bath also had other ancillary functions, including a place for the formation of social interactions. According to historical evidence, the construction of public baths from the Achaemenid period was typical in Iran, and with the arrival of Islam in Iran and finding a religious aspect, the importance of this type of building increased; many different baths were built next to the bazaar complex and mosques of different cities. Considering the distribution of baths in the Islamic period throughout Iran and the relative stability of Islamic religious beliefs about bathing, the functional uniformity of the interior spaces of this building can be justified. Still, with a simple look at the dimensions of their constituent spaces, we can see the difference in their proportions. To discover the reasons for these differences, the present study uses library documents and descriptive-analytical research methods to explain the physical proportions in historic baths located in hot and dry or cold and mountainous areas. It analyzes the proportions formed under the influence of climatic conditions in them. As a result, similar proportions can be seen in the body of baths located in the same climate. Given the uniform performance of interiors in the construction of bathing houses, it seems that the architect has tried to overcome the challenges related to energy and climate adaptation by creating proportions specific to each climate in the main spaces and corridors.