Assessing breast cancer risk from imaging remains a subjective process, in which radiologists employ computer aided detection (CAD) systems or qualitative visual assessment to estimate breast percent density (PD). More advanced machine learning (ML) models have become the most promising way to quantify breast cancer risk for early, accurate, and equitable diagnoses, but training such models in medical research is often restricted to small, single-institution data. Since patient demographics and imaging characteristics may vary considerably across imaging sites, models trained on single-institution data tend not to generalize well. In response to this problem, MammoDL is proposed, an open-source software tool that leverages UNet architecture to accurately estimate breast PD and complexity from digital mammography (DM). With the Open Federated Learning (OpenFL) library, this solution enables secure training on datasets across multiple institutions. MammoDL is a leaner, more flexible model than its predecessors, boasting improved generalization due to federation-enabled training on larger, more representative datasets.