Weight of Imperial Gallon of Water.
1 gallon [UK] = 1.201 gallon [US, liquid]
One US gallon weighs 8.34 pounds, so an imperial gallon weighs about 20 percent more, or almost exactly 10 pounds.
Weight of a Gallon of Salt Water.
See the web links below this answer for an automatic calculator that will give you the density of water at whatever temperature and salinity you want. Once you know...