Chair design considers intended usage,
ergonomics (how comfortable it is for the occupant), as well as non-ergonomic functional requirements such as size, stacking ability, folding ability, weight,
durability, stain resistance, and artistic design.
Seat height Ergonomic design distributes the weight of the occupant to various parts of the
body. This is done by having an easily adjustable seat height. A seat that is higher results in dangling feet and increased pressure on the underside of the knees ("
popliteal fold"). It may also result in no weight on the feet which means more weight elsewhere. A lower seat may shift too much weight to the "seat bones" ("
ischial tuberosities").
Gas springs are attached to the body of the chair in order to give height adjustment and more comfort to the user. Some chairs have foot rests. Around 15% of women and 2% of men need foot rests, even at the chair height. A
stool or other simple chair may have a simple straight or curved bar near the bottom for the sitter to place their feet on. Actual chair dimensions are determined by measurements of the human body or
anthropometric measurements. The two most relevant anthropometric measurements for chair design are 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 and for American women it is . 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 high. Researchers such as
Mary Blade and
Galen Cranz found that sitting on the edge of a high stool with feet on the floor is less harmful for the lower back than sitting up straight on a conventional chair.
Reclining angle , China: with a fairly low seat and the back inclined at about 45 degrees from the vertical Different types of chairs can have a variety of seating positions, depending on the intended task. Typically, chairs intended for people completing work or dining can only recline very slightly (otherwise the occupant is too far away from the desk or table).
Dental chairs are necessarily reclined. Research has shown that the best seated posture is a reclined posture of 100°–110°. In order to recline, the back-rest may be independently adjustable. A reclining seat and back will reduce the load on the occupant's back muscles. In general, if the occupant is supposed to sit for a long time, weight needs to be taken off the seat area and thus "easy" chairs intended for long periods of sitting are generally at least slightly reclined.
Back and head support The back of the chair will support some of the weight of the occupant, reducing the weight on other parts of the body. Some back-rests support only the
lumbar region, while shoulder height back-rests support the entire back and
shoulders.
Headrests support the head as well and are important in vehicles for preventing "
whiplash" neck injuries in rear-end collisions where the head is jerked back suddenly.
Reclining chairs typically have at least shoulder-height back-rests to shift weight to the shoulders.
Padding There may be cases where padding is not desirable, such as chairs that are intended primarily for outdoor use. Where padding is not desirable, contouring may be used instead. A contoured seat pan attempts to distribute weight without padding. By matching the shape of the occupant's
buttocks, weight is distributed and maximum pressure is reduced.
Armrests A chair may or may not have armrests; chairs with armrests are termed "armchairs". In
French, a distinction is made between
fauteuil and
chaise, the terms for chairs with and without armrests, respectively. In Germany, an armchair was once called a
Krankensessel, or sick-chair, because it was intended for people who were too ill to stand or sit without extra support. If present, armrests will support part of the body weight through the arms if the arms are resting on the armrests.
Elbow rest height is used to determine the height of the armrests. Armrests should support the forearm and not the sensitive elbow area. Hence, in some chair designs, the armrest is not continuous to the chair back but is missing in the elbow area. Armrests further have the function of making entry and exit from the chair easier (but from the side it becomes more difficult).
Seat size and legroom 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 deep. Additional anthropometric measurements may be relevant to designing a chair. Hip breadth is used for chair width and armrest width. 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. ==Types of chairs==