Your chair should support your lower back while its seat supports your thighs without pressing behind your knees. Lumbar support and seat depth work together: if the seat is too deep, you may have to sit forward to avoid pressure at the knees and lose contact with the backrest; if it is too short, your thighs may not be adequately supported. The right fit depends on your body, workstation, and tasks—not a single universal measurement.
How lumbar support and seat depth work together
Lumbar support is the part of a chair backrest that supports the lower back. It should meet your lower back in a comfortable position, without painful pressure or a concentrated pressure point. OSHA recommends lumbar support that can be adjusted in height to fit the user.
Seat depth is the distance from the backrest to the front edge of the seat. The seat needs to support your thighs, but the front edge should leave space behind your knees. If the pan is too deep, reaching the backrest may mean the seat edge presses into the area behind your knees. You may then sit forward, away from the lumbar support. OSHA’s workstation evaluation checklist specifically cautions against a seat so deep that the user cannot use the lumbar support.
As the Canadian Centre for Occupational Health and Safety (CCOHS) puts it: “A chair becomes ergonomic only when it specifically suits a worker’s size (body dimensions), their particular workstation, and the tasks that must be performed there.”
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Check whether your chair fits
- Sit back against the backrest. Settle into your usual working position so the backrest can support your lower back. Do not judge the fit while perched at the seat’s front edge.
- Check lumbar contact. The support should meet your lower back without painful pressure. If it does not, see whether its height or position can be adjusted.
- Check the front edge. Notice whether the seat presses behind your knees when you sit back. If it does, shorten the seat depth if the chair allows it, or try a chair with a shallower seat.
- Check thigh support. The seat should support your thighs, not leave them inadequately supported. A shorter seat is not automatically a better fit if it sacrifices that support.
- Check the whole workstation. Adjust the chair in relation to your desk, keyboard, and monitor. If the chair needs to be high enough for the work surface but your feet cannot rest flat, a footrest may help.
Judge the fit by these checks together: sitting back should let you use the lumbar support, the seat should support your thighs, and its front edge should not press behind your knees.
What to look for when choosing a chair
Look for a chair that lets you position lumbar support for your lower back and adjust seat depth to balance thigh support with clearance behind the knees. It also needs to suit your desk and the tasks you do. OSHA advises trying different chairs for comfort and fit before buying; a product label or one measurement cannot establish that a chair will fit you.
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CCOHS guidance, on a page last revised March 10, 2014 and confirmed current February 4, 2025, says adjustable chairs typically accommodate around 90 percent of the population. It notes that people near the shortest and tallest 5 percent may require custom-made chairs. This is a broad statement about adjustable chairs, not a guarantee that any particular model will fit an individual.
When a cushion or footrest may help
Lumbar cushion
OSHA describes a removable back-support cushion as a temporary option when a chair lacks lumbar support. It may provide support, but it does not change seat depth. If the seat presses behind your knees or prevents you from sitting back, a cushion does not correct that mismatch.
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Footrest
A footrest may be appropriate when chair height has to be raised to fit the work surface and your feet then cannot rest flat. It addresses foot support in that setup; it is not a fix for a seat that is too deep or too short.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose fit over a one-size-fits-all measurement
Official guidance emphasizes adjusting the chair to the person and workstation rather than prescribing one ideal lumbar-support projection or seat depth for everyone. Prioritize a chair that lets you sit back with comfortable lower-back support while its seat supports your thighs and leaves clearance behind your knees.
Quick Recap
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