Pretty easy to figure. About how much water will it displace? Water weighs 7.5# to the gallon. When its heavier than the water it displaces, it sinks.
RossC's 7.5 pounds per gallon is a mistake taken from "7.5 gallons per cubic foot". Don't feel bad Ross...I know this cause I have made the mistake myself.
There are 8.33 pounds per gallon of water, give or take some depending on the temperature of the water (if we get picky...lol).
