The Low Carbon Clinical Trials Working Group has developed a method to quantify the carbon footprint of clinical trials, aiming to reduce their environmental impact. The group created a process map defining core trial activities and corresponding emission factors to convert activity data into greenhouse gas emissions. This method was applied to two Cancer Research UK-funded trials: CASPS, an international phase 2 trial with 47 participants, and PRIMETIME, a UK-based phase 3 trial with 1962 patients. The carbon footprint of CASPS was estimated at 72 tonnes CO₂e, primarily due to clinical trials unit emissions and staff travel. PRIMETIME produced 89 tonnes CO₂e, mainly from in-person participant assessments. The group has also developed guidance to help trialists determine their carbon footprint, identify carbon hotspots, and explore alternative trial designs to reduce emissions. The guidance is being refined and tested on more trials to improve its usability and applicability. The ultimate goal is to support sustainable clinical trials and align with broader climate change mitigation efforts.The Low Carbon Clinical Trials Working Group has developed a method to quantify the carbon footprint of clinical trials, aiming to reduce their environmental impact. The group created a process map defining core trial activities and corresponding emission factors to convert activity data into greenhouse gas emissions. This method was applied to two Cancer Research UK-funded trials: CASPS, an international phase 2 trial with 47 participants, and PRIMETIME, a UK-based phase 3 trial with 1962 patients. The carbon footprint of CASPS was estimated at 72 tonnes CO₂e, primarily due to clinical trials unit emissions and staff travel. PRIMETIME produced 89 tonnes CO₂e, mainly from in-person participant assessments. The group has also developed guidance to help trialists determine their carbon footprint, identify carbon hotspots, and explore alternative trial designs to reduce emissions. The guidance is being refined and tested on more trials to improve its usability and applicability. The ultimate goal is to support sustainable clinical trials and align with broader climate change mitigation efforts.