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.
A gallon is a measure of volume primarily used in the United States but also commonly used in many other English speaking countries. Most other countries use liters - the metric unit of measure for volume. The US gallon is used in the United States and is equal to exactly 231 cubic inches or 3.785411784 liters. The Imperial gallon or UK gallon is used in the United Kingdom and is equal to approximately 277.42 cubic inches. Its exact value is defined as 4.54609 liters. One imperial gallon is approximately 1.2 US gallons.