Data centers have become an increasingly significant contributor to the global carbon footprint. In 2021, the global data center industry was responsible for around 1% of the worldwide greenhouse gas emissions. With more resource-intensive workloads, such as Large Language Models, gaining popularity, this percentage is expected to increase further. Therefore, it is crucial for data center service providers to become aware of and accountable for the sustainability impact of their design and operational choices. However, reducing the carbon footprint of data centers has been a challenging process due to the lack of comprehensive metrics, carbon-aware design tools, and guidelines for carbon-aware optimization. In this work, we propose FootPrinter, a first-of-its-kind tool that supports data center designers and operators in assessing the environmental impact of their data center. FootPrinter uses coarse-grained operational data, grid energy mix information, and discrete event simulation to determine the data center’s operational carbon footprint and evaluate the impact of infrastructural or operational changes. FootPrinter can simulate days of operations of a regional data center on a commodity laptop in a few seconds, returning the estimated footprint with marginal error. By making this project open source, we hope to engage the community in the development of methodologies and tools for systematically assessing and exploring the sustainability of data centers.