That depends That depends on how much you have! A tanker full of the stuff weighs more than a gallon of it. What you probably meant to ask is how much it weighs per unit volume. In other words, you wish to know its density. The specific gravity of gasoline is about 0.72. That is, it's about 72 percent the density of water. Since a gallon of water weighs 8.34 pounds per gallon, we can easily calculate the density of gas as 6.00 pounds per gallon. By the way, I say "about" above because the density of gasoline varies with temperature.