First determine the contents of the tank in cubic feet. To do this, square the diameter in feet and multiply by.7854: then multiply by the length in feet. Thus the contents of a circular tank 7 ft. in diameter by 18 ft. high will be 7 x 7 x .7854 x 18 = 38.5 x 18 -093 cub. ft. (approx.). Then 6.23 gal. of water occupy 1 cub. ft., so that the contents of the tank will be about 693 x 6.23 = 4,317 gal. A quicker way is to reckon that a tank lft. in diameter and 1ft. high will hold 4.9 gal. Then, since the capacity will vary with the square of the diameter and with the length of the tank, it will be about 4.9 x 7 x 7 x 18 = 4,320 gal.