The classic way to convey the error that exists in a measurement is a formula that shows the obtained measurement (X) as a sum of a true score plus some error. One version of this formula is: X = T + e. With this formula in mind, obtaining more precise measures depends on reducing error.
  Mega is the prefix meaning one million, therefore there are one million (1,000,000) gallons in a mega gallon.
There are 7.48 gallons in a cubic foot, so multiplying the volume in cubic feet by 7.48 results in the volume in gallons. If the round tank is spherical rather than cylindrical, the radius of the interior of the tank should be determined in feet. The volume in gallons is the product that results from multiplying the radius cubed, pi, 4/3 and 7.48.