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.
If the conversion is from U.S. fluid gallons to ounces then there are 128 ounces in a gallon. The commonly converted numbers and the results can be found in the conversion table at the end of the page but for anything else please use the converter below. 1 Gallon [Fluid, US] = 128 Ounces [Fluid, US].