Fuel: JP-4 is a common type of Jet Fuel in the US Armed Forces. How much does it weigh per gallon, though? Well, that's not as dumb a question as you might think. Even the manufacturers of JP-4 can't decide how much it weighs. They vary wildly between 6.3 lbs. per gallon to 6.7 lbs. per gallon. What's a Toolkit user to do? Use ATF's numbers, of course! Jane's/EA decided to settle on one number: 6.6 lbs. per gallon. So if you want to make that 600 gallon fuel tank, just do the math: 600 x 6.6 = 3960 lbs. of fuel.