Abstract: What would the atomic weight of osmium have to change to in order for iridium to become the densest metal [1]? Iridium becomes denser than osmium when ρ(Ir)>ρ(Os). The density of iridium is ρ(Ir)=22.562±0.011 g/cm and the density of osmium is ρ(Os)=22.589±0.005 g/cm. Comparing two numbers that have associated uncertainties is not a trivial task, but in the most simplistic approach the density of osmium has to be less than 22.562 g/cm in order for iridium to become the “densest metal” at normal conditions. The density and the atomic weight are directly related: