Of all the components of an office chair, the seat pan is one of the most critical to get right for optimal ergonomics. After all, the weight of our entire body bares down on the chair’s seat; even small changes in the seat’s length, width, height, or contours can have a dramatic effect on our comfort levels.
This article is part of our “Choosing an Ergonomic Office Chairs” Series:
- What is Seat Pan?
- Syncro Tilt and Other Tilt Mechanisms in an Office Chair Explained
- Types of Lumbar Support for Office Chairs (and What to Choose)
- Armrests on Ergonomic Office Chairs- What You Need to Know
What is Seat Pan?
Seat Pan refers to the flat surface area of the chair we sit on. It’s attached to the base of the chair that usually come with various levers for common adjustments such as chair height and backrest recline angle.
An ergonomically designed seat pan will usually feature a sloped (aka waterfall) edge and slightly hollow center to stabilize the pelvic area and distribute your body weight more evenly.
What to Look for in an Ergonomic Seat Pan
The BIFMA G1 2013 ergonomic guidelines serves as a great starting point when determining the ideal specs for the most ergonomic seat pan. It uses a large civilian dataset that is more reflective of the current general population than other guidelines such as csa-z412.
As an ergonomist, I would never say a seat pan should be exactly 16” deep or 14” tall, as that would ignore the fundamental fact that no two bodies are alike, and what works for one person will fail miserably for another. However, guidelines produced by BIFMA or the OSHA are based on what works for most people. If your body is on the much larger or smaller size, use the baseline suggestions and then extrapolate the extra adjustments needed to fit your body.
The following are the recommended specs in a seat pan on an ergonomic chair:
| BIFMA G1 2013 | OSHA | Herman Miller Embody | Herman Miller Aeron | |
| Seat Pan Height | 14.8” to 20.2” | 15” to 22” | x ✔ | ✔ ✔ |
| Seat Pan Depth | <16” | 15” to 17” | x ✔ | ✔ ✔ |
| Seat Pan Width | 19.2” | >18” | ✔ ✔ | ✔ ✔ |
| Seat Pan Angle | 0 to 4 degrees back | -5 degrees (forward tilt) to 5 degrees back | ✔ x | ✔ ✔ |
| Seat Pan Contouring | Waterfall sloped edge | Waterfall sloped edge | ✔ ✔ | ✔ ✔ |
Like everything, the devil’s in the details. Let’s look closer at each of the important items to check off when it comes to seat pan selection.
Seat Pan Height
The height of the seat pan is measured from the back of your knees to the ground. Proper seat pan height is critical to prevent undue pressure from building up in your thighs and hips. If the seat is too high, circulation is cut off from the underside of your thighs as your legs dangle, while if it’s too low, increased pressure is applied to the hip and sitting bones.
Choose an ergonomic chair with an adjustable seat pan in the range of 15” to 22” to accommodate most people. Adjust the seat pan so your two feet are firmly touching the ground, with your knees bent at a 90 degree angle.
Seat Pan Depth
Seat Pan depth is measured as the horizontal distance between your buttock and the back of your knees. Proper seat pan depth ensures your body is properly aligned in the seat and against the backrest of the chair. If the seat pan depth is too short, a good part of your knees will be unsupported as you sit. On the other hand, a long seat pan will prevent your back from fully resting on the backrest of the chair, all the while applying excess pressure on the ends of your thighs.
Look for a seat pan depth of between 17” to 21” to fit most people. Ergonomic chairs with a 2” adjustable seat depth are ideal to accommodate different sitting positions and people. In the end, the ideal seat pan depth is where there is a 2-3 fingers gap between the edge of the seat and your inner thighs.
Seat Pan Width
Seat pan width simply refers to the width of the seat on a chair. The proper seat pan width ensures that you’re able to enter and exit a chair easily, and just as importantly, that the armrests (if any) are spaced appropriately so that you’re not extending or contracting your arms to rest on them.
BIFMA guidelines suggest a seat pan width of 19.2” to fit most people. OSHA simply suggests that the seat width should be at least the width of your thighs, which you can easily measure yourself. If the chair you are purchasing comes with armrests (which it should), also measure the distance elbow to elbow and compare it with the horizontal distance between the armrests on the chair to make sure they match.
Seat Pan Angle
The angle of the seat pan is important to make sure your back can effortlessly make good contact with the chair’s backrest. The seat pan angle should not be confused with backrest angle, though in chairs with synchronous tilt, the two reclines in tandem.
The ideal resting angle for the seat pan should be between 0 degrees and 4 degrees as suggested by BIFMA. OSHA also recommends a chair that can forward tilt up to 5 degrees, which lessens the strain on your thighs and waist as you lean forward to perform certain tasks. Currently very few chairs on the market support forward tilt, with the Herman Miller Aeron and some of its alternatives among the elusive list of chairs that do.
Seat Pan Contouring
Last but not least, we arrive at the contouring in a seat pan. Look for a seat that slopes downwards at the edge to reduce pressure build up around the soft tissues of the thighs.
Proper seat contouring all around the seat pan also makes a huge difference. Half of your body weight when sitting down is supported by an 8% area under the sit bones. A seat with a slightly hollow center can more evenly distribute this weight across your entire buttock.
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Elizabeth Mina says
I wonder if your ergonomic study of chairs includes accommodating people with sitting disabilities, a little-acknowledged category of people who have conditions such as coccydynia, pudendal nerve disorders, or even sciatica. Although many such problems with painful sitting are temporary–for example, a broken tailbone–others are long-term or even life-long. As a sufferer myself, I find the new trends in chair design that purport to be scientifically-designed to reduce musculoskeletal disorders by relieving stress on muscles, nerves, ligaments, tendons, et. al., actually MAKE THINGS WORSE for those who have physical conditions that cause pain upon sitting. These include hardening the seat pan foam (this exacerbates pressure points, particularly for sufferers who weight less than average) and contouring the seat pan like a hammock with wings on the side and a depression in the middle (this concentrates all pressure on the base of the spine). The best kind of seat for someone with pain points is perfectly flat and actually skews towards the spongy side…this allows the user the most movement to find the position with the least pain. The analogy is running shoes made to accommodate people with differently-working hips, legs, and feet: either supportive (rock hard, keeping the foot in one position) or neutral (super cushiony, allowing for free movement). The problem today is chair designers are making only the supportive types of chairs, completely ignoring the group of bodies that need the “neutral” type akin to the bench seats of old cars before bucket seats came along. So, although I have had my sitting disorder for nearly 40 years, all of a sudden I have found myself unable to sit AT ALL in certain auditoriums with new hammocked seats, in church pews with hard rounded tops, in cars with super hard shaped seats.
CPT Ralph T Vandagriff, USAR Retired says
George, a very well written article. You impressed me. Where may I find chairs that accommodate OSHA standards for 6’5″ 350 lb employees? I am coming off an incident of a blood clot behind my knee and I am currently off work. I am need your help please. Herman-Miller and X-Chairs are really expensive but wide seat pans with adjustable seat pan depth and adjustable lumbar support are not available.
Any help would be greatly appreciated by this Vet.
Jon Muller says
Hi:
At your body size you should be looking at a big and tall office chair with a seat height of 20″ or more, and pan of at least 20″ as well. The 1st and 3rd chairs on this list are worth trying.