# How to find out how big a ball is?

Ok, This is probably a really simple question but. I need to know how I can find out how big a ball is. For example, a tennis ball is 2 1/2 inches big, but how do you find that?

Though, for reference, the explanation and answer to this question needs to be as simple as it can possibly get. I have a learning disability that heavily affects my mathematics capabilities and severe Dyscalculia. This explanation here probably makes me sound pretentious, but a lot of people don't understand or they throw too many numbers at me and get frustrated when I haven't said anything before hand. I'm sorry if it does, but I'm covering all my bases lol.

Any help on how to figure this question out is very much appreciated!

• Do you mean like, where do I look it up? en.wikipedia.org/wiki/Tennis_ball#Standardization Or measure it with a ruler? Jun 19, 2015 at 19:44
• There's more than one way to measure a ball, so $2.5''$ is actually ambiguous Jun 19, 2015 at 19:45
• @ Robert Israel...I'm not sure? I thought all tennis balls were the same...the tennis ball was just an example.... Jun 19, 2015 at 20:08
• @ GFauxPas I honestly don't know...I mean...I just need to know the size...what other ways are there to measure the size of it? Again, I don't do well with math here, I really don't. Jun 19, 2015 at 20:09

One easy way to measure the "size" of a ball is how far it is from side to side. Your tennis ball example is a good one: a typical tennis ball is about 2.5 inches form side to side. And one way to measure that size is to place the ball on a table in the sunlight when the sun is nearly overhead -- close to noon.

The ball's shadow is then almost a circle, and you can measure the width of the circle with a ruler, and that'll be the width of the ball as well.

Another way to measure this is to put the ball between two (large) books and hold the books parallel; it's then easier to measure the distance between the books with a ruler.

Finally, you can wrap a string around the middle of the ball -- the very widest part, like wrapping around the equator of the earth -- and mark the string with a pen so that between the two pen-marks is exactly one trip around the ball's middle. Suppose that this comes out to be 42 inches (which might happen for a kid's kickball, for instance). If you divide the length (42 inches) by the number 3.14, you get

42 / 3.14 = 13.38, more or less

and that tells you that the ball is a little more than 13 inches across.

The number 3.14 is special -- it works no matter how large your ball is: you divide the "length around the ball" by 3.14 and you get the length across the ball. (The actual number is a tiny bit bigger than 3.14, but the difference only matters when you'll doing very precise things; 3.14 works for almost all practical purposes.)

I hope this helps a bit.

• This helps, yes. I mean...it's really hard for me to understand it, but the verbal explanation of it and examples add just enough that I get what you are saying. I have plenty of string around here, so I can do the measuring thing and use the equation. The other two option are a little harder for me to do, considering my window in my room only gets noon sun on my wall and I don't have that many heavy books lol. I can try it though when and if I screw up the equation lol. Thank you though, for your help. Jun 19, 2015 at 20:07

You can use calipers to measure the diameter. Or a vice.

A somewhat more complicated approach: Put tennis ball in a measuring glass. Hold it down while you fill it with water until the water is just over the ball. Note the indicated volume, call this $$v_1$$. Remove the ball then check the volume again, and call this new volume $$v_2$$. The volume of the tennis ball is the change in volume,

$$V=(v_1-v_2)$$. The cube of the radius, $$r^3=0.2387\cdot V$$ or diameter=$$2\cdot(0.2387\cdot V)^\frac{1}{3}$$

Make a cylinder of paper that is large enough to enclose the ball. Put the ball on a table and close the cylinder around the ball. Mark where the paper's (verticle) edge meets the other side of the ball while the cylinder is perpendicular to the table. This allows you to measure the circumference. Then the division by $$\pi$$ ($$3.1418$$) gives the diameter, which is the width of the ball. The old approximation for $$\pi$$ is $$\frac{22}7$$, so you can mulitply by $$7$$ and divide by $$22$$ to get the number you are looking for.