Copper is a fairly common element, with an estimated concentration of 50–70 ppm (0.005–0.007 percent) in earth's crust (1 kg of copper per 15–20 tons of crustal rock).[26] A concentration of 60 ppm would multiply out to 1.66 quadrillion tonnes over the 2.77e22 kg mass of the crust,[27] or over 90 million years' worth at the 2013 production rate of 18.3 MT per year.