Key performance indicators, herein after referred as KPIs, are calculated within a manufacturing execution system. The calculation is performed by defining a custom KPI set at engineering time. In the case that the custom KPI definition is valid, a function code for calculating each custom KPI belonging to the custom KPI set is, automatically generated, through a software framework with support for code generation, at engineering time. At a runtime, a calculation is requested of a given custom KPI belonging to the custom KPI set. In the case that the calculation request is valid, at runtime, the calculation function code of the given custom KPI is executed.