Mars has a mean density of 3,933 kilograms per cubic meter, or 3.94 grams per cubic centimeter, according to NASA. By contrast, Earth has an average density of 5.51 grams per cubic centimeter. Mars is therefore only around 71 percent as dense as Earth.
Mars' lightweight composition has some curious implications. Though Mars is overall much heavier than Mercury, its large volume relative to its mass results in a weaker surface gravity on Mars than would be felt on the surface of Mercury, which is a far smaller planet. Though Mars is the least dense rocky planet, it is considerably denser than any of the gas giants.