The equations are developed for the calculation of leak flow rates in various leak regimes. Leaks due to pressure-driven convection and due to permeation through metals are considered. For convective leaks. the conditions under which the flow transitions from laminar to turbulent and from subsonic to choked (sonic) flow are discussed. Equations are presented to calculate leak rates for subsonic laminar and turbulent flows. as well as choked (sonic) flow rates. Given the advantages of using noncombustible gases for leak testing and measurement. equations are also developed for calculating the equivalent leak rate of helium when it is used as a surrogate for the combustible gases hydrogen and methane in each of these flow regimes. Equations are derived for the permeation rate of hydrogen through several common metals. Tabulated data is presented for the permeation rates of hydrogen through pure iron and two types of stainless steel over a pressure range from 5000 to 15,000 psi and a temperature range of -40-100 degrees C. The results clearly show the sensitivity of flux to temperature, with over an order of magnitude increase in flux as the temperature is increased from ambient to 373 K (100 degrees C). Permeation rates are also found to vary significantly with material. For example. permeation rates for construction steel (as estimated from pure iron) are about three orders of magnitude higher than 403 stainless steel and nearly five orders of magnitude higher than type 316L stainless steel for a given temperature and pressure. Under many combinations of pressure and temperature, leak rates for Fe exceed the permissible gaseous hydrogen leak rates. while rates for 316L stainless steel are well below permissible permeation rates at all combinations of temperature and pressure considered. Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.