Space needed for a piece of furniture is calculated from the design and shape of the furniture. The architect studies the piece of furniture, designs,how it will be placed inside the room and draws it in plan, and elevation.
Architects have studied ergonomics in detail so they easily know the average dimensions of a human being and the average dimensions of furniture.
For example, a working table is 900mm high. These dimensions have come up after architects have done a lot of research over hundreds of years and have come up with time-saver standards for most dimensions.
Different furniture has different space requirements. A lounge chair will require a different space from a working chair.
In a working chair, the person sitting on the chair will have to be mostly upright on his back as he performs the task on his desk. His desk will correspond with the chair and will be in the level of 900mm height which is the most comfortable for a person in the upright sitting position.
After in depth study of ergonomics, furniture is designed according to the uses.
The size varies slightly according to continents-Asians are much smaller than Europeans and Africans so their furniture is slightly smaller than ours her in Kenya.
Invite an architect to study your space requirements and the use of the furniture so that he can come up with the best egonomic and aesthetic design for your furniture requirements.
Design software such as ArchiCAD has an inbuilt furniture space calculator that has derived the information from the architectural ergonomic research.