Recent developments in quantitative and diffusion MRI, have made it possible to estimate the axonal g-ratio in human white-matter in-vivo. g-ratio is the ratio between the inner and outer radii of the myelin sheath wrapped around the axon. We suggest a simplified measurement of g-ratio incorporating proton density mapping, and implement it in the Corpus-Callosum of 100 subjects (ages 8-80). We find the g-ratio values agree with previously results. Furthermore, g-ratio values are stable over the lifespan and between the sexes. These results converge with theoretical evidence suggesting g-ratio has an optimal value for white-matter function.