# Measuring for a hollow constructed sphere

Hello all. I have made a few hollow constructed silver spheres
before, but never before had to make one with a precise diameter. I
am wondering if anyone knows of a formula I could use to calculate
the size of discs I need to start out with, in order to get a sphere
of a specified outside diameter.

Thanks!
Diane

there was a thread in October this year… “Formula of disk size
for dome making”.

In my reply I suggested the disc be 28% larger diameter than the
finished dome diameter. There were other thoughts.

Hi Diane,

In Alan Revere’s book Professional Goldsmithing on page 204 there is
a formula for just this very thing. It is called “Calculating the
diameter of a disk for a hemisphere” and the formula is:

(outside diameter-wall thickness) x 3.14 x 0.9

The outside diameter is the diameter of the sphere that you need to
make and you will of course need 2 discs. I was wondering why
multiply the whole thing by .9 so I emailed Alan to ask him and he
told me that the disc will stretch about 10% from the dapping and so
he is taking that much off in the beginning. I did try and it worked
for me so I am sure that it will work for you as well.

Good luck,
Lona

Lona Well, I am going to try it again as the whole formula that I
typed in the first post didn’t come through.

It should be: (outside diameter-wall thickness) x 3.14 x 0.9 / 2