This is the last thing I expected to have trouble on

Density.

Anyway, I've got everything done except now I need to plot the graphs according to density, and it just so happens that I feel like I'm getting ridiculous numbers.

Example:

I have a sphere with a circumference of 39 centimeters, which I then changed to 0.39m

Therefore it's diameter is $\displaystyle \frac{0.39m}{\pi}$ (I know latex doesn't work, but you guys can understand it.)

Therefore it's radius is $\displaystyle \frac{0.39m}{2\pi}$

Which is about

: $\displaystyle 0.0621$ (My actual work is done to around 10 decimal places, just so everything works)

I cube that, then multiply by pi, and finally multiply by 4/3 to get the volume, which comes out to : $\displaystyle V=0.001$

Now I happen to have a ball that size with a weight of 687g, or 0.687kg

Then : $\displaystyle Density (\rho?)=\frac{0.687}{0.001}=687$

I don't know why, but that seems like too large a number, is it? (I'm not good with SI units)