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.
The imperial gallon is defined as 4.546 liters, and the US gallon equals to 3.785 liters. What is more, the dry gallon is also in use in the USA. It is a bit larger than the liquid gallon and is less common.