1 gallon (US, liquid) = 231 in^3
100 ft of 1/2 in tubing = 1200 in of 1/2 in tubing
1/2 in tubing means diameter = 1/2 in, which means radius = 1/4 in
Area of a circle (tubing cross-section) = Pi * r^2 = Pi * (1/16) = Pi/16
100 ft of 1/2 in tubing = 1200 * Pi/16 in^3 = about 235.619 in^3
Tubing / gallon = 231 in^3 / 235.619 in^3 = about 0.9804
Answer: 100 ft of 1/2 in tubing holds about 0.9804 gallons of water.