What weighs more, a gallon of water or a gallon of butane?

The water. Butane is lighter fluid.

Did You Know?

Butane is actually about 38% lighter than water when comparing equal volumes. A gallon of water weighs approximately 8.34 pounds, while a gallon of butane weighs only about 5 pounds at room temperature. This density difference is why butane makes such effective portable fuel for camping stoves and lighters.