A gallon of water weighs about 8 pounds.
A gallon is eight cups (1/2 quart), so it will weigh a little more than 2 pounds, which makes sense because you have to add the weight of the container and then subtract the perfect fluid weight. But it’s not like people are just lifting gallons out of containers with nothing in them, so how much does a gallon weigh? It depends on what you’re holding it in. A wet towel weighs less than an empty bucket – that’s why gravity matters to physics!