“Sit up straight” is advice about your body. It’s aimed at the wrong target.
Posture, it turns out, is mostly not a behavior — it’s an output. Put a human in front of a monitor that’s four inches too low and they will end up craned forward, every time, regardless of intentions. The body negotiates with the equipment all day long, and the equipment always wins. So the professionals — the ergonomists at Cornell, the OSHA workstation people, the physical therapists who see the results — don’t start with the body. They start with the setup.
This is the blueprint for that setup: the measurable relationships between you, your chair, your desk, your keyboard, and your monitor. It’s the cornerstone of our Body pillar, and nearly everything else on this site assumes it’s in place. Budget thirty minutes and bring a tape measure.
One framing note before the numbers: the target is neutral posture — the position where your joints sit near the middle of their range and your skeleton, rather than your muscle effort, carries the load. Ears over shoulders, shoulders over hips, wrists straight, feet supported. Not rigid, not “at attention.” Stacked.
Start from the chair — and start from your elbows
Everything keys off one measurement: your seated elbow height. Sit, let your shoulders hang completely relaxed, and bend your elbows to roughly 90 degrees. The height of your elbows in that position is the anchor for the entire workstation.
Seat height: Adjust the chair so your feet rest flat on the floor with your knees around 90 degrees and your thighs roughly parallel to the floor (hips very slightly above knees is fine, and many people find it easier on the low back). If the chair must be raised for the desk’s sake and your feet dangle — footrest. A ream of paper or a sturdy box works while you shop; dangling feet quietly load the backs of your thighs and pull you into a slump.
Seat depth: Sitting fully back against the backrest, you want roughly two to three fingers’ gap between the seat-pan edge and the backs of your knees. If the pan is too deep you’ll perch forward and lose the backrest entirely — the most common silent failure in “good” chairs.
Backrest: The lumbar support belongs in the inward curve of your low back — for most people, roughly at belt height. Recline slightly; a backrest angle a bit beyond vertical (in the neighborhood of 100–110 degrees) takes measurable load off the lumbar discs compared to bolt-upright sitting. The chair is supposed to hold you. Let it. (The lumbar mechanics get a full treatment in the low-back piece.)
Armrests should just kiss your relaxed elbows — supporting, never lifting — and must never force your arms outward or stop you from pulling close to the desk. If they do, drop them or remove them. Shoulders hunched by armrests are worse than no armrests. (Buying the chair itself is a different question — the chair-buying guide covers what the spec sheets won’t.)
The keyboard and mouse: where most pain starts
Here’s the uncomfortable fact about standard desks: at a typical 29–30 inches tall, they were sized for handwriting, not typing. For most seated adults, that puts the keyboard above relaxed elbow height, which cocks the wrists back (extension) and shrugs the shoulders — the two postures most implicated in the wrist and forearm trouble we cover in Before the Brace.
The Cornell University ergonomics group (CUErgo) is unusually direct about the fix: get the keyboard at or slightly below elbow height, with a flat or slightly negative tilt — the keyboard sloping gently away from you — so the wrists stay straight. In practice that means one of three moves: a height-adjustable keyboard tray with a negative-tilt setting (Cornell’s preferred arrangement), a lower desk, or a higher chair plus footrest. And fold those little keyboard feet down; propping the near edge up (positive tilt) is exactly backwards.
The mouse lives at the same height, immediately beside the keyboard. Distance is the killer here: a mouse parked out past the numpad keeps your arm in a constant low-grade reach all day. If you don’t use the numpad, consider a compact (“tenkeyless”) keyboard — it lets the mouse tuck a few crucial inches closer to your midline. For tablet users, your stylus surface obeys the same rule; the full illustrator’s version is in Drawing Tablet Ergonomics.
The monitor: distance, height, angle
Three numbers:
Distance — about an arm’s length. Sitting back, your fingertips should just about reach the screen; call it 20–30 inches (50–75 cm) for a typical monitor, a bit farther for very large ones. Too close forces continuous close-focus effort; too far invites the forward crane. If you’re leaning in to read, don’t move your spine — enlarge the text. (Your eyes get their own article: Eye Strain Is a Setup Problem.)
Height — top of the screen at or slightly below eye level. Your relaxed gaze naturally falls a bit downward; the working area of the screen should sit in that zone. The head weighs 10–12 pounds and every inch it drifts forward multiplies the load on your neck — the “tech neck” arithmetic (the cervical-spine deep dive runs those numbers). Laptops fail this requirement by design: screen and keyboard can’t both be in the right place. The only real fix is separation — a laptop riser (or a stack of books) plus an external keyboard and mouse, or an external monitor. For anyone working full days on a bare laptop, this is the single highest-value purchase in ergonomics.
Angle — screen tilted slightly upward (10–20 degrees) so it faces your eyes squarely, positioned to avoid catching reflections from windows or overhead lights. Glare belongs to the lighting article, but the short version: screens perpendicular to windows, never facing or backing them.
Dual monitors: put your primary screen dead center and the secondary angled beside it — or, if you use both equally, center the seam. What you’re avoiding is a day spent rotated 20 degrees toward your “main” screen with your spine in a slow twist. (When a second screen helps and when it quietly hurts: the dual-monitor dilemma.)
Now the caveat that makes it all work
Dial in everything above and you have a workstation that fits you the way a tailored jacket does. Here’s what it still can’t do: make eight motionless hours safe. The research on sedentary time is blunt — a perfect posture held all day is still a static load, and the musculoskeletal system’s actual requirement is variation. The blueprint sets your default; Sit, Stand, Move covers the rhythm that keeps the default from becoming a cast.
Set it up once, measure it honestly, and then — this is the good part — stop thinking about it. That’s what a setup that fits is for: it takes posture off your list of things to manage with willpower, and gives the attention back to the work.
Sources & further reading: Cornell University Ergonomics Web (CUErgo) computer workstation guidelines and neutral-posture typing tutorial; OSHA Computer Workstations eTool (workstation components and evaluation checklist).