For General.A (gallon) is a fluid measurement, and a (cubic foot) is a volume measurement. So to measure a gallon, you need to measure the square in. of container (use container that measures the same square in. from bottom to top), then pore in one gallon of any liquid and measure the hight it comes; multiply this by square in. of container and you have your cubic inches in that gallon. For Water.One gallon equals 0.134 cubic feet.
This is granted that the water remain at constant temp., and at a point to have the stated volume.