Quick Answer: A sinking office chair is almost never a broken chair — it’s a worn gas cylinder, and it’s a $15-$30 part you can swap in about 15 minutes without tools. Confirm the diagnosis (the seat drops under weight without the lever being touched), measure your current cylinder’s diameter and length, and buy a generic Class 3 or Class 4 replacement, which fits the large majority of office and gaming chairs. Only a small number of brands, mainly some Herman Miller and Steelcase models, use a proprietary cylinder size that needs an OEM part instead.
A chair that slowly sinks throughout the day, or drops the moment you sit down, is one of the most common office-chair complaints and one of the cheapest to fix. The part responsible is the pneumatic gas cylinder — a pressurized cylinder that sits between the base and the seat mechanism and holds your seat height once you release the lever. When its internal seal wears out, it loses the ability to hold pressure and the seat gradually settles under your weight instead of staying put.
Confirm it’s actually the cylinder
- Height creep with the lever untouched. The seat sinks gradually while you’re sitting still, without you pressing the height lever. This is the single clearest cylinder symptom.
- Instant drop on sitting. If the seat drops to its lowest position the moment you sit down and won’t hold any height, the seal has failed completely rather than just weakened.
- Rule out the tilt mechanism. Wobbling side to side, an uneven tilt, or the seat leaning to one side are usually the tilt-tension mechanism or a loose seat plate, not the cylinder — replacing the cylinder won’t fix those.
- Rule out the base. Squeaking or grinding that happens only when rolling, not when sitting, points to the casters or base, not the cylinder.
What to measure before you buy
Most generic replacement cylinders are sold by class (Class 3 is standard for office chairs, Class 4 for taller/heavier-duty chairs) and by extended length. Before ordering, measure your current cylinder’s outer diameter at the top and its fully extended length so the replacement fits both the seat mechanism above and the base below. If your chair is a Herman Miller Aeron, a Steelcase Leap, or another chair known for a proprietary mechanism, check the manufacturer’s own parts page first — a generic cylinder may not seat correctly in those mounts.
Generic Class 3 replacement cylinder
- Measure your current cylinder's top diameter and extended length before ordering — most listings state a compatible diameter range.
- Class 4 cylinders cost a few dollars more and are worth it for chairs rated over roughly 300 lb.
Replacing chairs for a whole team instead of just one? A free Amazon Prime trial gets parts to your door in two days, which matters if half the office is sinking at once.
The swap, step by step
| Step | What to do |
|---|---|
| 1. Remove the seat | Flip the chair over and unscrew the seat mechanism from the seat pan (usually 4 bolts), or leave the seat attached and work from underneath. |
| 2. Pull the old cylinder | Use a claw-style cylinder puller under the seat mechanism to pop the cylinder loose from the mounting collar, or pry against the base with a flathead screwdriver if you don't have a puller. |
| 3. Separate cylinder from base | Pull the cylinder straight up out of the star base. It's friction-fit, not threaded, so steady upward force is all it takes. |
| 4. Press the new cylinder into the base | Push the new cylinder into the base's center hole until it seats fully. A rubber mallet on the top helps if it's snug. |
| 5. Press the seat mechanism onto the cylinder | Set the seat mechanism onto the top of the cylinder and press down with your body weight until it clicks into place. |
| 6. Reattach and test | Bolt the seat back on if removed, then test the height lever through its full range before trusting it with a full workday. |
By the numbers: A generic Class 3 cylinder runs $15-$30 on Amazon versus a new mid-range office chair starting well above $150, and the swap itself typically takes under 15 minutes with no tools beyond an optional cylinder puller. It’s one of the highest cost-to-effort ratios of any office chair repair.
When to replace the whole chair instead
A cylinder swap makes sense when the rest of the chair — the base, casters, tilt mechanism, and upholstery — is still in good shape and the sinking is the only real problem. Skip the repair and shop new if the chair is also showing other age-related failures (cracked base, torn mesh, a tilt mechanism that’s loose regardless of tension setting), or if it’s a proprietary-mount chair where sourcing the right part turns out to be more expensive or harder to find than expected. If you’re shopping new, our best ergonomic office chair guide and best office chair under $200 roundup both cover solid current picks.
The bottom line
A sinking chair is a worn gas cylinder in the overwhelming majority of cases, not a chair that needs replacing. Measure your current cylinder, confirm it’s a standard Class 3 or 4 mount rather than a proprietary Herman Miller or Steelcase fitting, and a $15-$30 generic replacement will have the chair holding height again in about 15 minutes.