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.
You can view more details on each measurement unit: mega litres or gallon [US]. The SI derived unit for volume is the cubic meter. 1 cubic meter is equal to 0.001 mega litres, or 264.172051242 gallon [US].