If your lower back hurts by mid-afternoon and feels better the moment you stand up, the cause is almost never weak muscles and almost never your chair's price tag. It is one of three numbers, and you can find which one in about ten minutes.
The reason this is worth narrowing down: the three causes have three different fixes, and the most common one — buying a lumbar cushion — fixes only one of them.
First, rule the screen out
Before touching the chair, look at where your screen is. A screen set too low pulls your whole torso forward, and once you are leaning in, your lower back is holding your upper body's weight with no support behind it. This is the most misdiagnosed cause of desk-related back pain, because the pain shows up in the back while the problem is up at eye level.
Two-minute test: raise your screen — a stack of books will do — so the top edge is level with your eyes. Sit for twenty minutes. If the ache eases, you never had a chair problem.
Full numbers for that in what height your monitor should be.
The three real causes
1. Seat too high — pelvis rolls backward
Raise a chair too far and the seat presses into the backs of your thighs. Your pelvis responds by rotating backward, which flattens the natural inward curve of your lower back and puts the load on the passive structures — ligaments and disc — instead of muscle.
How to tell: you keep sliding forward onto the front of the seat, and when you stand up your lower back feels stiff and takes a moment to straighten. This is the "sink" pattern.
Fix: lower the seat until you can slide two fingers under your thigh at the front edge, then re-check your elbows. If the desk is now too high for your forearms, you have found a real conflict and the desk has to come down — see chair height.
2. Seat too deep — backrest abandoned
If the seat pan is too long front-to-back for your legs, you cannot sit back against the backrest and keep your knees comfortable. You choose the knees, and spend the day with an unsupported lumbar curve.
How to tell: there is a visible gap between your lower back and the backrest when you are sitting normally, and you have to make a deliberate effort to lean back.
Fix: most chairs with a seat-depth slider solve this outright. Without one, a small cushion behind your lower back lets you sit further back while still reaching — but note that this is treating the depth problem, not the lumbar problem, and it is a workaround.
3. Lumbar support in the wrong place
Nearly every adjustable chair has lumbar support, and a large share of people have it set too low — down near the seat rather than at the inward curve of the lower back. In the wrong position it does nothing, or pushes your pelvis forward into an awkward tilt.
How to tell: the support feels like it is pressing on your tailbone or doing nothing at all.
Fix: the correct height is level with or slightly above your belt line. Adjust it while sitting, and give it a day — a correctly placed support feels like pressure at first and like relief by the afternoon.
The two adjustments that fix most cases
If you only do two things:
Set the seat so your hips sit slightly above your knees. Not level, not below. A hip angle around 90–100° keeps the pelvis from rolling backward, which is the mechanism behind most sitting-related back pain.
Slide all the way back into the backrest. Sounds obvious; almost nobody does it. Your buttocks should touch the backrest's back edge. Every inch you sit forward is an inch your lower back has to hold on its own.
Then recline slightly — 100–110° rather than a rigid 90°. Measured disc pressure is lower in a supported recline than bolt upright, which surprises most people who were told to sit straight.
What does not help
- A more expensive chair. Price buys adjustability and durability. If the three numbers above are wrong, an expensive chair reproduces them more comfortably.
- A lumbar cushion used as the first move. It fixes cause 2 and nothing else. Diagnose first.
- Sitting perfectly still. Your discs have no blood supply and exchange fluid through movement. Unchanging posture — however good — is itself a risk factor. Change position every 20–30 minutes.
- A standing desk as a way out. Standing trades one loading pattern for another and has its own failure mode. Useful as a way to change position; see how long to stand.
When it is not the desk
Most desk-related back pain responds to the above within a few days. Worth seeing someone if:
- Pain radiates down a leg, or you get numbness or tingling below the knee.
- It wakes you at night or is worse in the morning than the evening.
- It has not shifted at all after two weeks of correcting the setup.
- There was a specific incident that started it.
That pattern is not a furniture problem, and no amount of adjustment will fix it. This page is about setup, not diagnosis.
FAQ
Why does my lower back hurt only when I sit, and not when I stand?
Standing keeps your pelvis in a neutral tilt with your spine stacked. Sitting — especially with the pelvis rolled backward — reverses the lumbar curve and shifts load onto passive structures. If standing relieves it within a minute or two, the mechanism is almost certainly positional rather than structural.
Should my chair recline?
Yes, slightly. A supported recline of 100–110° transfers load to the backrest and measurably lowers disc pressure compared with upright sitting. Locking the chair bolt upright is one of the more common mistakes.
Is a standing desk the answer?
It is a tool for changing position, not a cure. Standing all day brings its own problems — locked knees, a forward-tipped pelvis, foot and calf fatigue. The useful pattern is alternating, roughly 30 minutes sitting to 15–30 standing.
How long until it feels better?
Positional pain usually eases within a few days of correcting the setup. If there is no change after two weeks, the cause is likely not the three numbers on this page.
Does a footrest help back pain?
Indirectly, yes — if your feet do not reach the floor, you slide forward to stabilise, and that slide is what removes your backrest. A footrest lets you sit back. It does nothing for people whose feet already rest flat.
Related
- Which of the five numbers is wrong? — if the pain is not in your lower back, or the three causes here did not fix it.
- Ergonomic desk setup: the complete guide — the full sequence, plus a symptom-to-cause chart for pain that is not in your lower back.
- What height should your chair be? — cause 1 in detail, including where the 25%-of-height rule comes from.
- How to sit at a desk correctly — the four patterns, and why "sit up straight" lasts four minutes.
- Workspace checkup — scores desk, chair, and screen together and tells you which single change fixes the most.
- How to set up a standing desk — if you are alternating, the standing number is a different calculation.
Get your exact numbers
Enter your height and see the desk, chair, and monitor heights that fit you — about 30 seconds.