text: what are gallons compared to in the metric system?   a us liquid gallon = 231 cubic inches = 3,785.411 784 cubic centimeters or 3.785 litres.  an imperial gallon is approximately 277.4193 cubic inches and exactly “4.54609 cubic decimeters” (i.e., litres).  the imperial gallon is therefore 20% bigger than a us one so it is important to specify which one. better still, don't use either and stick to international units.  litres or even liters have been legal in the us since 1875. can we please flush gallons down the pan.