Churchchairs
Actual chair dimensions are determined by measurements of the human body or anthropometric measurements. The two most relevant anthropometric measurement for chair design is the popliteal height and buttock popliteal length.
For someone seated, the popliteal height is the distance from the underside of the foot to the underside of the thigh at the knees. It is sometimes called the "stool height." The term "sitting height" is reserved for the height to the top of the head when seated. For American men, the median popliteal height is 16.3 inches (410 mm) and for American women it is 15.0 inches (380 mm) [1]. The popliteal height, after adjusting for heels, clothing and other issues is used to determine the height of the chair seat. Mass-produced chairs are typically 17 inches (430 mm) high.


Girl sitting on a chair.
For someone seated, the buttock popliteal length is the horizontal distance from the back most part of the buttocks to the back of the lower leg. This anthropometric measurement is used to determine the seat depth. Mass-produced chairs are typically 15-17 inches deep.
Additional anthropometric measurements may be relevant to designing a chair. Hip breadth is used for chair width and armrest width. Elbow rest height is used to determine the height of the armrests. The buttock-knee length is used to determine "leg room" between rows of chairs. "Seat pitch" is the distance between rows of seats. In some airplanes and stadiums the leg room (the seat pitch less the thickness of the seat at thigh level) is so small that it is sometimes insufficient for the average person.
For adjustable chairs, such as an office chair, the aforementioned principles are applied in adjusting the chair to the individual occupant.
Caster wheels are attached to the feet of chairs to give more mobility
Gas springs are attached to the body of the chair in order to give height adjustment and more comfort to the user