Radius vs. mass iso-contours from a suite of evolutionary calculations. The solid red lines show R/ R _J vs. M/ M _J at fixed values of log (age[yr]), labeled on the left of each curve. The dashed blue curves are for fixed entropy, with each curve labeled by S/( N _A k _B) on the right. The dotted black curves are for fixed luminosity, with each curve labeled by log ( L[erg s ⁻¹]) above M = 1  M _J. The green curve at the bottom is the T = 0 M– R relation from Zapolsky & Salpeter (1969) for a solar mixture of H and He.