A chair that fits is the difference between finishing the day with a straight back and finishing it with a dull ache across the shoulders. Most home office chairs have five or six adjustments, and most of us set them once — badly — on the day the chair arrived, then never touch them again. Sorting it out takes about fifteen minutes, a mirror and a tape measure.
Order matters. Seat height changes everything above it, so that comes first. Work down through the steps, then look at the whole thing side-on before you decide you're finished.
Gather these first, so you're not up and down like a yo-yo mid-adjustment:
The mirror view is the one that matters. From the front, almost any sitting position looks acceptable. From the side you can spot a collapsed lower back, a shrugged shoulder or a dangling foot in about two seconds. If you'd rather not move furniture, prop your phone against a stack of books and take a short video instead.
One thing before you touch anything: on thick carpet, castors sink and every measurement you take will be a centimetre or two out. Put something rigid underneath and start again.
Sit down properly — bottom at the back of the seat, back against the backrest — and raise or lower the chair until your feet are flat on the floor and your thighs are roughly parallel to it. Your knees should be level with your hips, or a fraction lower.
Use the tape measure to note where you land: floor to the top of the seat cushion at the front edge. Most people finish somewhere between 40cm and 48cm, but treat that as a sanity check rather than a target. Bodies vary more than chairs do.
Two failure modes to watch for. If your feet dangle, or your thighs slope downwards towards your knees, you're too high. If your knees rise above your hips, you're too low. If the chair won't go low enough, a footrest under the desk fixes the dangling — a sturdy box file works at a pinch — and keeps your thighs supported.
You want a gap of roughly two to three fingers, so about 5cm to 7cm, between the back of your knee and the front edge of the seat. If that edge presses into the back of your thigh, the seat is too deep and you'll end up perched forward with nothing supporting your back. If there's a fist's worth of space, it's too shallow and your thighs are taking weight they shouldn't.
Some chairs slide the seat pan with a lever under the front; others move the whole backrest in and out. Either way, measure with the tape from the front edge to the back of your calf, then sit and check with your hand.
The support should sit in the inward curve of your lower back — roughly level with your belt line, or a touch above it. Too low and it tips your hips forward. Too high and it digs under your ribs. Adjust the height first, then the depth, so the support meets your back without pushing you forwards out of the chair.
If your chair has no lumbar adjustment, a rolled-up towel or a small cushion behind that curve does the same job. Check it in the mirror: you're aiming for a gentle S in the spine, not a straight line and not a slump.
Start with the backrest close to upright, then release it a notch or two. A backrest angled slightly open — around 100 to 110 degrees from your thighs — takes load off the lower back without tipping you into a full recline. In the mirror, your ear should sit above your shoulder, and your shoulder above your hip.
Drop your shoulders. With elbows at roughly 90 degrees and forearms parallel to the floor, the armrests should meet the underside of your forearms lightly. If setting them correctly forces your shoulders up towards your ears, lower them or take them off.
Check the width too. Your elbows should fall naturally at your sides, not pushed out or squeezed in, or you'll be sitting crooked by mid-afternoon. Some chairs adjust for width; on others, a spanner and ten minutes will remove the pads entirely. That's usually a better trade than a permanent shrug.
Turn the tension knob, usually under the front of the seat, until you can lean back with a gentle push and come upright again without hauling yourself forward. If the chair has a lock, leaving it just off the locked position lets it move a little as you work. Lock it out for detailed typing if you prefer, but change position regularly either way.
Sit the way you actually sit, not the way you'd sit for a photograph. Then check the mirror against this list:
Photo: Matheus Bertelli / Pexels
