Abstract The problem addressed here is the calculation of the mass of hydrocarbons generated by the active source rock pod of a petroleum system. This calculation method follows a four-step sequence: (1) the source rock is identified and its boundaries defined, (2) the mass of organic carbon in the source rocks is calculated (equation 1), (3) the mass of hydrocarbons generated per gram of organic carbon is estimated (equation 2), and (4) the total mass of hydrocarbons generated is determined by multiplication of these data (equation 3). The key idea of the calculation is use of the hydrogen index to quantify the fractional conversion of kerogen to hydrocarbons. The hydrogen index, derived from Rock-Eval pyrolysis, represents the potential of a source rock to generate additional hydrocarbons. The difference between the original hydrogen index prior to any petroleum generation and the present-day hydrogen index thus approximates the decrease in generation potential and is equated here to hydrocarbons generated.