[SDKJapan]

The sources and dates of the information I have seen are a bit sketchy, so do not hold me to this. However, here is one explanation from a hotel industry blog(Japanese) that had more detail than others.

The law that governs room occupancy today is the Fire Service Law.(Appended Table 1 (5) of Fire Service Act Enforcement Order (under Supplementary Provisions)) (Fire Service

Law Enforcement Regulations.; Article 1-3)

Here is how the capacity is calculated.

– For Western-style accommodation rooms, the number corresponding to the number of beds in the accommodation room.

– For Japanese-style accommodation rooms, the number obtained by dividing the floor area of the accommodation room by 6 square meters (3 square meters for simple lodgings and those that mainly accommodate groups of guests).