In article ,
CRNG wrote:

100' of 1/2" copper holds 4.07 gallons.

$ printf '%f\n' $(( (3.1415926 * (.5 * .5) * (100 * 12)) / 231 ))
4.079990

FWIW

It's length * pi r^2, not d^2. 1.02 gal.

Is the above "1/2" diameter the internal or external diameter of the
tube? I would think it's the internal diameter so the radius should
be less than 0.25".

You're right, it is internal diameter. Or at least close to it. 1/2"
nominal copper has an OD of 0.625". Type K has a wall thickness of
0.049", and type L has a wall thickness of 0.040". The ID is either
0.527" or 0.545". So it's either 1.13 or 1.21 gal.