Seat height and seat depth by your height

Two numbers decide whether a chair fits you, and neither one is your height. They are your popliteal height - floor to the underside of your thigh, just behind the knee, when you are seated - and your buttock-popliteal length, which is how deep the seat can be before its front edge presses into the back of your knee.

Height is a proxy for both, and a reasonable one. The table below converts it, using published percentile data rather than a rule of thumb. But if you can measure the two real numbers, do - the section after the table takes about a minute and it beats any average.

Seat height includes a 1-inch shoe allowance. The band is plus or minus 0.8 inch, because two people the same height can differ by about that much in leg-to-torso proportion - which is also why a chair with a 4-inch adjustment range suits far more people than one with a 2-inch range, whatever its mid-point.

1. Popliteal height. Sit on a hard, flat surface with your feet flat on the floor, knees at a right angle. Measure from the floor to the underside of your thigh right behind the knee. Wear the shoes you work in. That figure, as measured, is your seat height - no allowance needed, because the shoes are already on.
2. Buttock-popliteal length. Sitting the same way, measure from the back of your buttock to the back of your knee. Your maximum seat depth is that figure minus 2 inches - the clearance that keeps the seat's front edge off the back of your knee.
3. Check the chair against both. A chair whose published seat-height range contains your figure with an inch to spare at each end, and whose seat depth is at or below your maximum, fits you. One that fails either is the wrong chair however good it is.

This is the common case, not the rare one. Most task chairs start at 17.5 to 19 inches, and our table says a 5 ft 2 in user wants something closer to 16. If the chair will not go low enough, you have three options and only one of them is good:

• Add a footrest. This is the real fix, and it is the single most useful cross-link on this site: a footrest restores the knee angle and gets your weight off the front edge of the seat, without touching anything else in your setup. It is also cheaper than a chair.
• Lower the desk instead, if it adjusts. Works, but only if you can then still fit your knees under it.
• Buy a chair that goes lower. The right answer if you are buying anyway - see chairs for petite users, where the seat-height floor is the ranking criterion rather than an afterthought.

The mirror problem, and harder to fix: you cannot make a gas cylinder taller. A 6 ft 3 in user wants about 19.5 to 21 inches of seat height, and plenty of chairs stop at 21.7. That leaves no adjustment headroom, so you sit permanently at the top of the range with the cylinder fully extended.

Two things worth knowing. Replacement gas cylinders in taller lengths are sold separately and fit most chairs with a standard taper - not something manufacturers advertise. And a drafting chair is simply a task chair on a longer cylinder with a footrest ring added, which is why it is the usual answer for a tall user at a tall desk. Our chairs for tall people roundup ranks on the seat-height ceiling rather than on how the marketing describes the back.

Below is the percentile table every figure on this site derives from. We show it rather than describing it, because a table you can check is worth more than an assurance that we checked it.

The interpolation between those points is linear. We use interpolation rather than a single ratio of stature on purpose: the popliteal-to-stature ratio drifts from about 0.245 at the short end to 0.263 at the tall end, so a single multiplier systematically under-predicts for tall people - the group already least well served by the market.