# How do I compare the size of these balls? **URL:** \ **Category:** Factual Questions\ **Created:** [May 6, 2009, 6:45am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522 "2009-05-06T06:45:16Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![elfkin477](https://avatars.discourse-cdn.com/v4/letter/e/a9a28c/32.png) [@elfkin477](https://boards.straightdope.com/u/elfkin477)\ **Post date:** [May 6, 2009, 6:45am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/1 "2009-05-06T06:45:16Z")
Now that I’ve got your attention… 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. ---
**Author:** ![Hunter\_Hawk](https://avatars.discourse-cdn.com/v4/letter/h/50afbb/32.png) [@Hunter\_Hawk](https://boards.straightdope.com/u/Hunter_Hawk)\ **Post date:** [May 6, 2009, 6:59am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/2 "2009-05-06T06:59:02Z")
> [@elfkin477](#): > > 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: Am I misreading this? Why wouldn’t this just be the diameter? ETA: And speaking as a Pilates student, what do you do with those balls? We seldom use small inflatable balls in our studio. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [May 6, 2009, 7:45am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/3 "2009-05-06T07:45:19Z")
> [@Hunter\_Hawk](#): > > Am I misreading this? Why wouldn’t this just be the diameter? No. It’s impossible to answer the OP, without knowing how much they’re inflated. That changes everything. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 6, 2009, 7:49am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/4 "2009-05-06T07:49:21Z")
Well, if it’s 9" across, uninflated, that would inflate to an 18" circumference, without accounting for any stretching. > [@elfkin477](#): > > 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. 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. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [May 6, 2009, 9:30am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/5 "2009-05-06T09:30:06Z")
> [@Robot\_Arm](#): > > Well, if it’s 9" across, uninflated, that would inflate to an 18" circumference, without accounting for any stretching. How did you draw this conclusion? ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [May 6, 2009, 9:32am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/6 "2009-05-06T09:32:06Z")
> [@Hunter\_Hawk](#): > > Am I misreading this? Why wouldn’t this just be the diameter? 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 ---
**Author:** ![spayced](https://avatars.discourse-cdn.com/v4/letter/s/9fc29f/32.png) [@spayced](https://boards.straightdope.com/u/spayced)\ **Post date:** [May 6, 2009, 11:54am UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/7 "2009-05-06T11:54:50Z")
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. ---
**Author:** ![CutterJohn](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@CutterJohn](https://boards.straightdope.com/u/CutterJohn)\ **Post date:** [May 6, 2009, 12:16pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/8 "2009-05-06T12:16:14Z")
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. However, this depends on what the material is. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [May 6, 2009, 1:38pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/9 "2009-05-06T13:38:09Z")
_Well, if it’s 9" across, uninflated, that would inflate to an 18" circumference, without accounting for any stretching._ > [@Jinx](#): > > How did you draw this conclusion? Flat circle 9" across on one side, 9" across on the other. 9 + 9 = 18. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [May 6, 2009, 1:49pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/10 "2009-05-06T13:49:02Z")
Why don’t you link to the balls in question. 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. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [May 6, 2009, 1:55pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/11 "2009-05-06T13:55:19Z")
> [@Hampshire](#): > > _Well, if it’s 9" across, uninflated, that would inflate to an 18" circumference, without accounting for any stretching._ > > Flat circle 9" across on one side, 9" across on the other. 9 + 9 = 18. Circumference is PI\*Dia or about 28 inches in your example. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [May 6, 2009, 2:27pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/12 "2009-05-06T14:27:02Z")
> [@Harmonious\_Discord](#): > > Circumference is PI\*Dia or about 28 inches in your example. 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" ---
**Author:** ![Antigen](https://avatars.discourse-cdn.com/v4/letter/a/848f3c/32.png) [@Antigen](https://boards.straightdope.com/u/Antigen)\ **Post date:** [May 6, 2009, 2:31pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/13 "2009-05-06T14:31:21Z")
> [@Hampshire](#): > > Flat circle 9" across on one side, 9" across on the other. 9 + 9 = 18. Um… I came back, brought pi. circumference = pi x diameter Play with this site a little to figure it out: > **[Interactive Circumference of a Circle: Explore diameter, radius, Π and...](https://www.mathwarehouse.com/geometry/circle/interactive-circumference.php)** > > Interactive Circumference of a circle. Explore diameter, radius, pie and circumference _Damn, I’ve been scooped._ ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [May 6, 2009, 2:52pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/14 "2009-05-06T14:52:14Z")
> [@Antigen](#): > > Um… I came back, brought pi. > > circumference = pi x diameter > > Play with this site a little to figure it out: > [Interactive Circumference of a Circle: Explore diameter, radius, Π and Circumference.](http://www.mathwarehouse.com/geometry/circle/interactive-circumference.php) > > _Damn, I’ve been scooped._ Repeat- The measurement across of a deflated sphere is not the same as it’s diameter when inflated. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [May 6, 2009, 2:52pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/15 "2009-05-06T14:52:21Z")
> [@Antigen](#): > > Um… I came back, brought pi. 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. ---
**Author:** ![Hunter\_Hawk](https://avatars.discourse-cdn.com/v4/letter/h/50afbb/32.png) [@Hunter\_Hawk](https://boards.straightdope.com/u/Hunter_Hawk)\ **Post date:** [May 6, 2009, 3:18pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/16 "2009-05-06T15:18:34Z")
…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. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [May 6, 2009, 3:23pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/17 "2009-05-06T15:23:48Z")
> [@DrCube](#): > > 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. 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. ---
**Author:** ![Anthony\_N](https://avatars.discourse-cdn.com/v4/letter/a/858c86/32.png) [@Anthony\_N](https://boards.straightdope.com/u/Anthony_N)\ **Post date:** [May 6, 2009, 3:24pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/18 "2009-05-06T15:24:50Z")
> [@Hampshire](#): > > 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" 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. ---
**Author:** ![acetylene](https://avatars.discourse-cdn.com/v4/letter/a/f19dbf/32.png) [@acetylene](https://boards.straightdope.com/u/acetylene)\ **Post date:** [May 6, 2009, 3:36pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/19 "2009-05-06T15:36:27Z")
Elfkin, are you talking about the large balls that [you can lay on or sit on](http://shopping.yahoo.com/p:Perfect%2010%20Fitness%20-%20Pilates%2C%20Ball%20%26%20Floor%20Exercise%20Yoga%20%26%20Meditation:1809940363;_ylt=AoUl5uchWnTeMRjrgYH_G4kbFt0A?clink=dmps/pilates_balls/ctx=mid:3,pid:1809940363,pdid:3,pos:3,spc:14489115,date:20090506,srch:kw,x:), or are you talking about the smaller balls that you [put between your knees](http://www.fitnessgiant.com/baba.html)? ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [May 6, 2009, 3:44pm UTC](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522/20 "2009-05-06T15:44:24Z")
> [@acetylene](#): > > the smaller balls that you [put between your knees](http://www.fitnessgiant.com/baba.html)? snerk [Next page](https://boards.straightdope.com/t/how-do-i-compare-the-size-of-these-balls/495522.md?page=2)