How much does a gallon of ice weigh?

8.35 pounds.

Because ice is a solid, technically you cannot measure ice using gallon as the units. However, a gallon of water that has turned to ice will have the same weight before and after freezing. This will not be true if the temperature is lowered even further or the ice is under pressure.

At 4 degrees C, just before freezing, a gallon of water weighs 3.79 kg or 8.35 pounds.

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All the above do not take into account that water expands in volume as it freezes (that is why Ice floats) thus if you started with a gallon of water, you would have more than a gallon of ice.

The density of ice is 0.9169, the density of water is 1.0

Therefore a gallon of Ice will weigh 7.5598405 pounds or 3470.4665 grams, while a gallon of water weighs 8.245 pounds or 3785 grams.