24 inches. That's the seat height I picked for my Varier kneeling chair. I measured my desk, subtracted my elbow height, and ordered. Turned out the angle of the shin support matters more than the seat height. My posture didn't magically fix itself. My knees just hurt for two weeks.
What I Got Wrong
I treated the Varier like a regular chair. Sat down, tried to sit up straight, waited for the magic. That's not how it works. The whole point is that it tilts your hips forward, so your spine aligns naturally. But if the shin support is too low or too high, you're just kneeling at a bad angle. My shin support was set for someone with shorter legs. My knees were bent at like 100 degrees. That's not a chair, that's a squat.
What to Check Before You Order
Measure your lower leg. From the floor to the back of your knee. That number should be close to the distance from the floor to the top of the shin support. My leg is 21 inches. The shin support was at 18. Yeah. That's a 3-inch mistake. Also, check the seat tilt. If the seat is too flat, you're not getting the pelvic tilt. You're just sitting on a pillow with a weird stick. Adjust it until you feel a stretch in your lower back. If you don't feel that, it's set wrong.
The Real Test: 10 Minutes
Sit in it for 10 minutes. If your knees or shins ache after 5, something's off. If your lower back feels worse, something's off. The chair should feel like a workout for your core, but not a endurance test. I spent 20 minutes the first day and limped for the rest of it. Don't do that. Build up slow, like 15 minutes at a time. And for the love of everything, don't expect it to fix your posture overnight. It's a tool, not a magic bullet. You still have to actually sit up.
What I'd Do Differently
I would have gone to a store that had one and tried it. Or at least watched a video of someone adjusting it. The manual is useless. It's just a picture of a human in an ideal posture. No one sits like that. I also would have checked the return policy before ordering. Shipping a 30-pound chair back is a pain. I kept mine, but only because I fixed the shin support. Oh, and if you have a desk that's over 30 inches high, you'll need a footrest or a higher chair. Don't ignore that. My desk is 31 inches, so I'm raising it a bit. Otherwise, you're just kneeling at a weird angle again.
Final Word
If you're buying a Varier kneeling chair to fix your posture, measure your leg first. Or you'll be the one kneeling in regret. Which is worse.
Related: For more on office chair setups, see Office Chair at Work: The First Week Setup and the One Decision That. Or if you're considering a different type of chair, check out What $200 vs $500 Gets You in a Heavy Duty Computer Chair.