Jet A-1 type fuel, in terms of Imperial weights and measures, measures 0.15 gallons per pound. Thus, one gallon of Jet A-1 type fuel weighs 6.66 pounds when calculating 6.66 lbs multiplied by 0.15.
Jet A-1 fuel is the most common type of jet fuel sold and used for civilian and commercial purposes. It is sold by the pound rather than by the gallon for ease of flight load calculations. Knowing the precise weight of a plane at any given moment during flight is crucial for proper performance and safety. Knowing how many pounds a full fuel load weighs is more useful to a pilot than knowing how much physical space it requires.