eCalc is the name of a software tool for high-quality emission forecasting. eCalc allows integration of subsurface and operational knowledge and calculates emission forecasts directly relating drainage strategy to operational strategies and equipment. Emissions directly relating to reservoir drainage (CO2 in particular) vary with the reservoir production rates and pressures, and the process assumptions. Emissions from drilling, flaring and mobile vessels may be included for integrated bookkeeping. eCalc is a holistic asset prognosis tool for calculation of the total emissions allowing break-down into single emission sources. eCalc has integrated energy functions to be used for modeling the energy usage of various process equipment such as pumps, compressors, generators, and turbines. The energy functions are models that relate input variables to energy consumption. The input variables are typically rate and pressures. For the compressor models, it uses compressor curves and thermodynamical models to calculate the shaft work needed for the predicted compression work through a single or a series of compressors. Also, it has built in models for future low-pressure projects, for less mature tie-in projects and new builds where vendor data is not available. In addition, there is also support for energy functions based on tables generated by external software (e.g. Unisim, Hysys). The flexibility of the software allows evaluating varying subsurface drainage strategies combined with process modifications in an integrated calculation. The tabulated energy functions, or likewise thermodynamical calculations, allow for a fast and easy to use calculation tool that can be used in multi-realization systems like the fast model update environment. eCalc is batch callable in Unix and can thus be easily integrated in workflows. eCalc also allows integration with external software, like for example numerical optimizers. eCalc uses process knowledge to mimic anti-surge and minimum flow without needing iterative solvers. Depending on where the operating point is relative to the chart, eCalc implements control strategies to automatically mimic process control. Eg if the operating point is in the surge domain, eCalc adds mass rate to the rate input to move to a point on the control line. A forward model results which is at the core of the calculations. It is not intended to replace the commercial process simulation software but allows subsurface personnel access to simplified process calculations and therefore opens the possibility for a better cross-disciplinary working environment. The integrated tool makes mutual information exchange between operations, subsurface and sustainability disciplines easy. The engineering approach with real physical models, capacities and data provides a high-quality emission forecast suitable for the long term. eCalc has been implemented on nearly all Norwegian installations (Espelid et al, 2022) and three generic examples are included in this paper.