This paper describes two metrics to measure the complexity involved in assembly and disassembly tasks for open architecture products during its use phase. The approach proposed is based on the summary of all tasks required to assemble and disassemble a predefined set of modules to generate several product variants, which comprise different working levels or functionalities. The aim of the method is to provide a useful tool to designers in the analysis of product complexity regarding use and further phases in which the assembly and disassembly of modules are required. The benefits and usefulness of the metrics are oriented to enhance the sustainability performance of products through the measurement of complexity in modular systems for the decision-making during the design stage. The reduction of complexity involves significant benefits in all lifecycle phases of product, especially when the user or customer is responsible for many related tasks (maintenance, upgrading, reconfiguration and final disposal of modules). The metrics and their calculation process are illustrated using two case study products.