Density of Jupiter

We have

Mass is 320 times the mass of the earth.
Radius is 11 times the radius of the earth.

or approximately

Mass is 300 times the mass of the earth.
Radius is 10 times the radius of the earth.

The density is then 300/(10³) = 300/1000 = 0.3 times the density of the earth. More exactly, we get 0.24 times the density of the earth (5500 kg/m³) or 1300 kg/m³.

That's really light. Jupiter is certainly not made of rock. It can't even be made of water, even though 1000 kg/m³ is the density of water. That's because Jupiter is so big that whatever it is made from would be much compressed compared to its density in a laboratory on Earth.

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Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu