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 (UK) gallon was originally defined as the volume of 10 lbs of water, and is now defined as exactly 4.54609 L, which is 10.02 lbs of water at its densest (2.20456 lbs / L at 4 C or 39 F [2]).