You need to measure the diameter of the tank as well.
The formula to calulate the volume of the remaining fluid inside the cylindrical tank is:
π(d/2)Â²h, where π = 3.14, d - diameter of the cylindrical tank, and h - the height of the "remaining fluid inside the tank".
The following calculator allows you to enter the measurement in inches and calculate the volume of a cylinder in various units, including US gallons:
http://www.aqua-calc.com/calc_volcyl.php