I'm working on a project and I need to measure the volume of a balloon. In fact, I need to measure its radius. I want you to give me advice and ways of measuring it.
If you assume that the balloon is spherical you can fill a bucket completely with water, gather the volume it displaced and measure it. Then you use the formula $V=\frac43\pi r^3$ to get the radius.
To do this i am assuming that the compression of the balloon is negligible when it is underwater.
Fill the Balloon with water instead of Air, and use the Water volume displacement method in the prior answer.
A Displacement cylinder that is either rectangular, or, a Cylinder is far easier to calculate volume change, with and without the water balloon.
A short piece of smooth walled large diameter plastic water pipe, with a homemade flat bottom would work well. Plywood disc glued into the cylinder with silicone sealer ( adhesive ), and varnished to make it waterproof inside.
A 6 inch by 12 inch concrete testing cylinder would also work, as the diameter is then known.
Mike Clark Golden, Colorado, USA