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".
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