Most office chair advice is folk wisdom. You will read that you should fit "two to three fingers" between your knee and the seat edge, that lumbar support should sit "in the small of your back", that you should sit "with your feet flat". None of it is wrong, and none of it is checkable.
There is a better source. OSHA publishes workstation guidance that states the criteria in measurable terms, and ANSI/BIFMA G1 sets the dimensional requirements that office chairs sold in the United States are commonly designed against. Using those, you can decide whether a specific chair will work for you in about five minutes, without sitting in it.
The two measurements that decide everything
Everything else in this article depends on two numbers you can take at home.
Sit on a hard chair, shoes on, feet flat, knees roughly at a right angle. Measure from the floor to the crease behind your knee. That is your popliteal height.
Then measure from that same crease, horizontally, to the back of your hip. That is your buttock-to-knee length, and it is what seat depth has to match.
Write both down. If you are shopping for a chair you cannot sit in first, these two numbers eliminate most of the market, and they are the only measurements where being shorter or taller than average changes the answer.
Seat height: the knee, not the foot
The common instruction is "feet flat on the floor". OSHA's criterion is more precise: the seat height is right when the entire sole of the foot rests on the floor and the back of the knee sits slightly higher than the seat.
That is a real test, and it is why a chair that is too high is worse than it looks. When the seat is above your popliteal height, the front edge lifts the back of your knee, pressure builds behind the leg, and most people slide forward to escape it. Slide forward far enough and the backrest is no longer touching you, which is the support you paid for.
A worked example. If your popliteal height is 42 cm, you need a chair whose seat adjusts down to 42 cm or below. A chair with a minimum seat height of 45 cm cannot be made to fit you, no matter how it is adjusted, and no amount of lumbar support will compensate.
If the chair is otherwise right but sits a little high, OSHA's own fix is a footrest, which raises the knee slightly and takes the pressure off the back of the leg. That is a legitimate solution rather than a workaround, and it is far cheaper than replacing the chair.
Seat depth: a standard, not a finger count
The finger-count heuristic exists because seat depth is the measurement people get wrong most often, and it is the one that hurts.
OSHA's criterion: the seat pan should support most of the thigh without contact between the back of your knee and the front edge of the seat pan. Your buttock-to-knee length is what that has to match.
Two failure modes, and they are not symmetrical:
A seat that is too deep presses the front edge into the back of your knee. Circulation suffers, and you move forward to relieve it, which costs you the backrest. A seat that is too shallow leaves your thighs carrying weight with no support under most of their length, which concentrates pressure instead of spreading it.
The standards answer to this is depth adjustability, which OSHA lists as a requirement for chairs shared between users. On some chairs the seat pan slides forward and back; on others the backrest moves. Either achieves the same thing, and it is the single most useful adjustment if you are shorter than the chair was designed for.
What the standards say about the rest
Once height and depth are settled, the remaining criteria are easier to check against a specification sheet than by sitting.
Recline. OSHA calls for a backrest that reclines at least 15 degrees from vertical and either locks or has adjustable tension. A backrest that only holds one angle cannot be adjusted to your posture.
Seat edge. The front edge should be rounded, which OSHA describes as a "waterfall" edge. A square edge concentrates pressure on the same spot behind the knee.
Base. Five legs, not four. OSHA is explicit that chairs with four or fewer legs are prone to tipping. Casters should match the floor: hard casters for carpet, soft for hard flooring.
Armrests. Optional, and often better removed than badly adjusted. The test is that your shoulders stay relaxed and your elbows stay close to your body while your forearms rest lightly. Armrests that are too high lift the shoulders; too wide and you reach sideways to use them.
Backrest height adjustment. Lumbar support needs to move up and down, because where it belongs depends on where your pelvis ends up once your feet are supported.
How to check a chair you cannot sit in
Read the specification for the seat height range and compare it against your popliteal height. That single number rules chairs in or out.
Then look for seat depth adjustment and, if you are shorter than average, treat its absence as a reason to keep looking.
Then check the three items that are visible in a photograph: a five-leg base, a rounded seat edge, and a backrest that reclines. If a listing does not state the seat height range at all, that is information about the seller, and it is usually enough to move on.
Sources and further reading
- OSHA, Computer Workstations eTool: Chairs. The source for the seat height criterion, the seat pan criterion, the 15 degree recline, the waterfall edge, and the five-leg base.
- OSHA, Computer Workstations eTool: Monitors. Useful for the matching question of how far the screen should sit from your eyes.
- ANSI/BIFMA G1, Ergonomics Guideline for Furniture Used in Office Work Spaces. The dimensional standard that office chairs are commonly designed against. The full document is paid, so look for a manufacturer's summary when comparing chairs.