In ergonomic terms, the chair is known as the Base of Support (BoS) and should fit the individual comfortably in all planes, being neither too low or high, narrow nor wide, too deep, or shallow.
So, to set up your workspace, you need to factor in a few simple measurements:
- your height <165cm, 165-180cm, >180cm
- your physique petite/ slim, average, large/broad, XXXL
- Standard desk is 720-740mm high.
Basic ergo rules for correct set-up of office chair: (accompanying generic line drawing)
Sitting fully back into the chair,
- Feet flat on the floor (footrest supplied if necessary*)
- Thighs parallel to floor, or hips slightly higher than knees, (never the other way round)
- 3 finger breadth distance between back of knee to front edge of chair
- Forearms parallel to floor, resting on armrests.
- Armrests level with the desk, to be considered as a continuation of the desk surface.
- Upper arms essentially vertical, with elbows resting naturally on armrests.
- Lumbar support (internal or external) meeting your natural waist.
- Chair back support up to lower shoulder blade level.
If you tick all above, your chair should feel comfortable and supporting.
Now check with a measuring tape and most likely you will find the seat pan 9where you sit) is at 48-51cm height. Armrests will be 72-76cm. Max point of the lumbar support will be approx. armrest height. It is a good idea to record these 3 levels, seat pan height, armrest height & desktop and keep for future reference in case of changes to your work situation.
Issues:
Your height:
Tall:
If you are tall, firstly you may benefit from a taller desk, say 76cm. This can be achieved by desktop risers, pieces of plastic or wood placed under the desk legs to increase desktop height. The height of the seatpan and armrests should be higher than the average ranges suggested above. Tall people >180 will have longer legs and find they can fit 4,5, or even more fingers between the front edge of the seat and back of their knees. In this case you need to ensure you get a chair with either a deep seat pan (500-560mm deep) or choose a chair with a sliding seat component. Usually done by pressing a button under the front edge of the seat, this slides the seat forward, making it deeper, so more of the thigh is supported.
My Recommendation:
For all taller than 180cm I recommend purchase & use of a chair with sliding seat. In my experience correct use of a sliding seat is the #1 comfort buster for tall people.
Small:
If you are less than 165cm in height, a standard office chair often fits fine with the following adaptations. At 165cm or less you usually have short limb and trunk dimensions. To have feet on the floor and accommodate shorter lower legs, chair seat pan height may be as low as 45-46cm. While this is comfortable for the lower back and legs, the chair becomes too low to work comfortably at the desk. The forearms will have to come diagonally upwards to use the keyboard and mouse, transferring discomfort to the shoulders and wrist. Easiest way to address this is to place a footrest under the desk. Even a shoe box will do. The chair seat can now be raised to meet the desk, making forearms and thighs both parallel to the floor. A simple way to adjust this is to add the height of the box or footrest to seat height and raise the seat accordingly. Now the seat may be 54-56cm high, compensating for your small frame.
My Recommendation:
Use a footrest/ shoe box and raise the seat by the same height as the box.
Physique
The standard chair fits the standard physique: not too small or tall, not too thin, or large.
Slim /Petite
For a physically slight person the width of a standard chair may swamp you. If your elbows cannot not naturally reach or land on the armrests because they are too far out to the side and/or there is more than 4 cm extra on each side of the chair seat, this chair is too big and unsuitable for you. It will never provide a comfortable BoS.
My Recommendation:
Look for a chair with a seatpan width of 460-490mm. You will find this in the detail of chair specification or dimensions (scroll down the manufacturer’s page)
Large
You should not feel wedged in between the armrests. Taking account of both the with and size of your thighs will assist in choosing the optimal chair.
NB: Many desktops are 2 -3cm deep. Some can have a bar or strut across the front of the desk to support the desk. This strut can range in depth from 2-12 cm. If you have large thighs this type of desk (or dining table if WFH) is unsuitable, as the depth of the thighs in sitting brings them right up under the bar, constricting movement. You need to request a different desk style in this instance.
My Recommendation:
Seat pan width of 520-560mm may prove necessary for comfort.
XXXL
Manufacturers design standard office chairs for 8–10-hour usage for body weights of up to 100kg. A large individual may require a non-standard or ‘Bariatric’ chair both in respect of seat size and component manufacture capacity. A heavy person sitting in an inappropriate chair every working day is at risk that the parts may fail. There have been many instances of the chair suddenly breaking, with accompanying risk of injury and humiliation should the chair collapse under you.
My Recommendation:
Be sensible and ensure the chair you have is guaranteed for weights >100kg.
