The actual diameter of the ball is about 85cm

First off, the box it came in had me a little nervous... Looked like something designed for kindergartens and preschoolers... However, once I took it out, I realized the thickness of the material, and the size of the thing. I'm super glad I had an adaptor to an air compressor which still took a while to inflate the ball, but little effort... It's enormous.*One negative out of the box was that it seemed to have a waxy feeling to it, and a very odd smell. Almost like that oily, waxy smell some chocolates can get... But slightly more chemical in aroma.As far as strength, and relative size... I spend a lot of time at my desk, it tightens me up after workouts, and I'm not crazy about being so stationary for so many hours at a time, so that's what I purchased this for... To mix between my chair and the ball. I'm 6'5, my feet just touch the floor, working my legs... Sometimes I switch back to the chair to give my legs a break. I'm heavier than most humans, around 270, and thus far, the ball has held up for the time short time I've owned it, hopefully it lasts a while. So the only negative, at this early stage, is that while that smell, and oily feeling, has decreased a little each day, it still, lingers on.Lastly, I've used the ball to stretch out on too, and it just hits my body different than the other two sized balls I own, so while my office looks like a playroom at this point... I'm very happy with my decision to buy each different sized ball.

Quality is good, durable, probably the best one available. This is what my hospital's PT department uses. The odor was pretty bad, though. Can finally store it inside after gassing off overnights on the porch for more than a month.

I ordered this ball because this brand was recommended by my physical therapist and I had used it during sessions. However, once I blew it up I realized it was not the same ball. It looks similar, but blows up to a smaller size than the Gymnic 'Plus' version of the 55cm ball. It is a good ball, it is sturdily made and would serve its purpose. I would recommend the Plus version however, it blows up a bit larger and has a softer feel to it, not the 'sticky' rubber feel of this one. If you're looking for a cheaper version though this one is a good one.