If I have a given gravity and the density of a planet is that of Earth, how do I determine its size (mass and radius) in Earth units? I know that g = m/r2 and that rho (density) = m/v; that v = 4/3*pi*r3, so m = rho*v, giving; g = rho*4/3*pi*r. But, this falls apart. I'm sure it's because I'm putting it into Earth terms. I mean, if I were to put in for Earth, I'd have; 1 = 1*4/3*pi*1 And, that ain't it. So, if I presume a density equal to that of Earth and I'm given a gravity, how do I find the radius? What nice, neat little formula will give that? Is there one? Thanx!