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.
Thanks for writing to Ask Dr. Math, Davis! The weight will vary some with the temperature of the water. To begin, we know that by definitions, 1 gallon = 231 in^3*(2.54 cm/in)^4/(1 cm^3/ml), = 3785.411784 ml, exactly.