Although neutrinos are Standard Model particles, they are listed separately because they are ultra-relativistic and hence behave like radiation rather than like matter. The density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or 4.08 × 10 kg/m. To convert this density to mass we must multiply by volume, a value based on the radius of the "observable universe". Since the universe has been expanding for 13.8 billion years, the comoving distance (radius) is now about 46.6 billion light-years. Thus, volume (⁠ 4 / 3 ⁠ πr) equals 3.58 × 10 m and the mass of ordinary matter equals density (4.08 × 10 kg/m) times volume (3.58 × 10 m) or 1.46 × 10 kg.