How many gallons does a 40 lb LPG gas bottle hold?

a cylinders capacity is rated by the amount of "water pounds" it holds. this is marked on the cylinder, usually on the neck ring. cylinders are specially constructed containers that can be transported across federal and state roadways legally. water weighs about 8 pounds per gallon and propane weights 4.2 pounds per gallon. about 1/2 as much as water. 40 lbs water times .42 propane is 16.8 pounds propane in the cylinder. cylinders are filled by weight, not by gallons. 16.8 divided by 4.2 equals 4 gallons propane the cylinder holds. this is usually the basic bbq tank on you gas grill.