The time required for the evaporation in a vacuum of a given amount of tungsten from a round tungsten filament heated by constant current is computed and experimentally verified. Tables of time vs normalized wire diameter for initial temperatures between 2000° and 2950°K, in steps of 50°K are presented. The results are valid for any initial wire diameter. The increasing electric field is also computed and tabulated. From the results, the ultimate life of the heated filament can be obtained. For example, at an initial temperature of 2400°K, it is found that when the mass of the wire has diminished 10%, the filament has attained about 98% of its life. The life of the wire under constant‐current conditions is shown to be substantially less than that under constant‐temperature conditions.