I have purchased 4 of these: 3 yrs ago I bought 2 smallish ones and, this past week, 2 large. The cushion is long-lasting and nearly indestructible (be sure to follow manufacturer's care recommendations - Do NOT - put it in the washing machine, as I did with one of them. The cushion turned into an amorphous mass of gel when I foolishly did that.
I have one on my living room armchair - it's been there for 3 yrs. I did not put one on my office chair and recently pressure from the hard plastic edges of the chair started to cause sciatica - excruciating pain! That's why I ordered this larger cushion. The cushion gave me practically immediate relief.
The smaller cushion is the shape of and fits a small kitchen chair. This larger one is exactly the size advertised, namely 18" x 16.5" x 1.75" thick.
The Fomi is more expensive than many advertised as "quality" "gel" cushions. But remember that old saying - "you get what you pay for..."