I’m having difficulty with a geometry type of question. I currently own a pilates ball that is 30 inches in circumfrence/9.54 inches in diameter, and I’d like to buy a smaller one. They don’t sell smaller ones in stores near me, though, just online, so I’d be buying one sight unseen.
These balls are given helpful measurements like 6", 7.5", or 9". I hunted for quite a while trying to figure out what this measurement referred to and was unhappy to discover that it’s not diameter or circumference. Instead they’re measuring the length of the ball when it’s not inflated :mad:
I can’t disinflate the ball I have to compare the lengths, because the stopper won’t come out.
What does the length tell me about the size of the ball? It can’t be as simple as multiplying the length by two to get the circumference because they stretch as you inflate them. Or can it?
Any suggestions? I did poorly in geometry, so I’m at a loss here.
Well, if it’s 9" across, uninflated, that would inflate to an 18" circumference, without accounting for any stretching.
Go to one of those stores. They don’t have the size you want, but you can get the info you need from one of the sizes they do have. They’re likely to have an inflated one on display, and uninflated ones for sale. Use those numbers (take a tape measure, estimate, or they might even have the specs on the package) to check your assumptions and calculate a stretching correction factor, and you’re all set.
I agree with Hunter Hawk. I think it is safe to assume the uninflated dimension will be close to the inflated diameter. Yes, the ball will stretch, but it is not like blowing up a balloon where there would be gross error in this assumption. - Jinx
Do you know the manufacturer or distributer that sold you your ball? If so, get their uninflated number, calculate the ratio of the inflation difference, and apply to other balls? Not exactly right, but my best idea atm.
spheres can actually be pulled quite a bit longer(without stretching) when uninflated. A deflated sphere made of a very loose material will hang with a length of approximately half the circumference. As Robot Arm said, a 9" length uninflated would be ~18" circumference.
I would expect them to inflate slightly larger in diameter as they are in length. They didn’t give a diameter probably because deflated they are not a circle and they didn’t think to put it in dimensions for the inflated ball. I used to inflate small play balls for kids and they inflated about an inch or two larger than the deflated size. I was told to inflate some balls once to the 9 inch size that were really 6 inch balls. It exploded and was deafening.
Uninflated lying flat it’s 9" across. Inflated the diameter is less.
Take a rubberband and lie it flat and measure it’s length. If its 2" then the circumference of a circle made with the same rubber band is 4". The diameter would be ~1.27"
I think you guys are missing that the ball is not inflated. Think of it like a t-shirt that has been flattened out on a table or something. If it is 9 inches wide in the front, it is also 9 inches wide in the back, meaning it is 18 inches “around”. When you wear it, the diameter shrinks, but the circumference stays the same.
…crap. I was misreading it. elfkin477, if you’re just looking for something basic, another option would be to go to a local athletic-goods store and see if they have any small kickballs.
Yeah, yeah. But inflatable balls don’t deflate nice and flat like a t-shirt. It’ll deflate and still have a curve to it. In fact, it’ll be like a half-sphere. The 9" deflated ball is the same size as elfkin’s 9" inflated ball.
While this is true of a rubberband or a cylinder, a sphere will never really lay flat. You will have a lot of material bunched up in the center. The measurement across while uninflated is more or less the same as the diameter assuming the material doesn’t stretch much when inflated.