This paper presents a method to quantify the tritium activity in a drum by measurement of its helium-3 leak. A model of helium exchanges with the atmosphere has been developed. It takes into account the diffusion phenomena and the influence of atmospheric pressure changes. The validation has been achieved with a pilot and the comparison between theoretical and experimental data has highlighted a very good agreement. Drum's helium-3 leak equilibrates after six months and then equals the helium-3 production in the drum and so to the total tritium activity. The measurement technique is also described. It's based on quantitative helium trace level determinations with an adapted leak detector. After a drum's confinement period of 5 hours and a cryogenic treatment of gaseous samples, the method allows to detect a 5 GBq activity of tritium in a drum.