"a pint's a pound, the whole world round" -- so gallons * 8 (8 pints to a gallon) and have the weight in pounds.

I've heard the pint's a pound phrase before, but with completely different meaning. it's used to remember how many ounces in a cup of liquid or in a pound of weight. there are 16 oz in a pound and 16 oz in a pint (or 8 oz in a cup).

have you tested the pint/pound method of measuring the weight of a liquid? is it accurate? have to be careful about that. water's pretty dense. it most likely wouldn't work for cooking oil or alcohol even if it did work for water.

EDIT
i checked the math, and a galon of water does weigh 8 pounds. that also means that one oz of water weighs one oz. that's very useful information to have. Thanks Helen!