A guide for individuals & Facilities Managers
What specs matter?
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.
The Health & Safety Authority’s (HSA) 2005 DSE guidelines (Display Screen Equipment, Visual Display Units, monitors or laptops) states that an office chair must have the following characteristics:
- Must be stable, comfortable and have easy freedom of movement. This has become interpreted as requiring to be on castors.
- Seat height adjustable
- Adjustable seatback height and tilt. Often taken to mean have a moveable lumbar support component)
To choose an office chair, essentially there are four guiding principles:
- Desk height & type
- The individual’s height,
- Their physique
- Usage
Desk Height
The standard office desk is 720-740mm high. In the office setting the desktop should be 120-140cm wide x 80cm deep, as in HSA 2007 Regulations. However, work practice change in the decade and one half since means that these desk top dimensions may not always be necessary. As computer processors have got smaller or disappeared off desks, and with many workplaces encouraging paperless working, the ongoing requirement for such large desks is under review. In the unprecedented move to home working since Covid-19 struck, many modern homes cannot accommodate this size desk. Recent personal clarification from the HAS allows individual interpretation of desk size, recognising home space constraints.
However, irrespective of desk size, what remains unchanged is that desk height of 720-740mm remains ideal. Both the individual’s height & their desk height are central components of correct individualised chair set-up.
The individual’s height
Ergonomics classifies people into three height bands. Average band is men and women of 165-180cm height, usually 50-60% of any work cohort. Most office equipment is designed and manufactured for this average group. Thus, any standard office chair should be suitable for people in this height range, except for individuals of petite or large physique.
Small:
Individuals of <165 height are often assigned the same standard chair, but with the addition of a footrest as required in the HAS 2007 Regulations ‘A footrest shall be made available to any individual who requires one’. A person of less than 165cm will have short limb and trunk dimensions. To get their feet flat on the floor, as required in the same HSA regulations, a smaller person may lower the seat to achieve the guideline of thighs parallel to floor. Now the chair becomes too low to work comfortably at a standard desk of 72-74mm height.
The forearms will have to come diagonally upwards to use the keyboard and mouse. To compensate and address this, simply using a footrest under the desk reduces the distance between the new floor level and desktop. The chair seat can now be raised, by the same amount as the height of the footrest so that forearms and thighs now become parallel to the floor. This may bring the seat to 52-54 cm high.
My Recommendation:
Small workers will require a footrest irrespective of where they work, so in a fulltime WFH or a Blended work contract they will need a footrest in each work setting.
Tall:
Tall individuals are often deeply uncomfortable in a standard chair, where seatpan depth is insufficient. With longer thighs, the chair may end at mid-thigh level, not providing sufficient support to the lower back and thighs. Prolonged sitting in too shallow a chair can lead to Sciatic nerve or hamstring muscle discomfort, poor low back posture and ultimately contribute to development of low back pain.
Taller persons require a chair with longer or deeper seatpan. This can be achieved by seeking out a deeper seat pan in chair dimensions, or via use of a ‘sliding seat mechanism’. Usually located via a button near the underside front edge, activating this allows the seat to slide forward, increasing the distance from the chair back to the front edge of the seat.
My Recommendation:
Sliding seat chairs should be supplied for all individuals >180cm height.
Physique
The standard chair fits the standard physique: not too small or tall, not too thin, or large.
Petite/ Slim
For a physically slight individual, the width of a standard chair may swamp them. Often the elbows cannot not naturally find the armrests, which are positioned too far out on each side. In sitting, if there is more than 4 cm extra on each side of the chair seat, this chair is too big and unsuitable.
My Recommendation:
Seek a chair with a seatpan width of 460-490mm, as specified in manufacturer’s dimensions. Also note the distance between armrests as some armrests are inappropriately designed to sit far out from the chair seat. If this is the case, it is worth avoiding this chair for this group of slim individuals.
Large / XXXL
The larger individual should not feel wedged in between the armrests. Here a seat pan width of 520-560mm will prove more comfortable.
Standard office chairs are usually designed 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 every day in an inappropriate chair is at risk that the parts may fail even breaking without warning from excessive use, causing humiliation to the individual and risk of injury, even compensation claims.
Usage & Chair Purchase Cost
Usage is about the individual and the environment.
As stated above, a heavy person is one individual for whom a separate chair spec is required.
Another issue relates to whether the chair is used for a standard working day by.
- one person in a single office,
- one person at an open plan workstation,
- same chair across open plan workspace where identical chairs are inadvertently moved around,
- is in a shift-based environment where the chair is potentially occupied 24/7
- where a chair purchase is expected to last 6-10 years with normal office usage,
Or by a
- single individual doing fulltime of blended working in their own home.
The more densely the chair is used, the more it is moved around, the sturdier the required design.
More than any other factor, anticipated usage defines level of expenditure required. The more ‘abuse’ a chair will receive, the more solid the construction needs to be. Purchse criteria needs to address number of separate parts, ease & cost of replacement of broken armrests, backs, and hydraulic systems.
A chair seat unable to keep an adjusted height is the most common finding in my experience. When a chair back cannot be correctly adjusted, the chair’s BoS ability is compromised, making all other chair measurements less accurate and ultimately unsuitable for use.
As laid down by The Health & Safety Authority, office chairs must comply with EN 1335, having the following:
Must be stable yet able to move freely in the office environment, A star / 5 castor? Soft castors are to be recommended for hard surface flooring to prevent the chair rolling away,
A height adjustable seat, a tiltable & height adjustable back rest.
Armrests are not considered mandatory under the 2007 regs, but in the last decade scientific ergonomic & musculoskeletal research has demonstrated the efficacy of armrest for a keyboarding population especially in injury prevention. In my clinical opinion adjustable armrests should be automatically supplied with each office chair.
For optimal use, armrests should be height adjustable, short so they do not extend into the front one third of the seat pan, padded for prolonged comfortable usage. These three attributes are described as 3D armrests. 4D armrests add the capacity to swivel the armrests in towards the body, a nice addition when using a short keyboard, or for a slim individual.
Basic ergo rules for correct set-up of office chair: (accompanying generic line drawing required)
- Sitting fully back into the chair,
- Feet flat on the floor (footrest supplied if required)
- Thighs parallel to floor, or hips slightly higher than knees (never the other way round).
- 3 finger breadth distance from 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 the person’s natural waist.
- Chair back support up to lower shoulder blade level.
Criteria definition:
| Height | Physique | Usage |
| Small <165 cm
|
Slim/petite | Office environment, multiple users/ 6-10 years lifespan
|
| Average 165-180cm
|
Medium | Individual,
Own office
|
| Tall
>180 |
Heavy | Individual
Home
|
| XXL > 100kg
|
