Methods for accurately and conveniently calculating the inventory of hydrogen storage in a stationary storage vessel using the external ambient temperature and the internal pressure are disclosed in the present invention. To account for the delay in the heat transfer from the ambient air to the storage vessel and then to the hydrogen gas a first order filter can be used on the external ambient temperature. Following the application of the first order filter on the external ambient temperature, an empirical equation is used to calculate the mass of hydrogen in the stationary storage vessel.