One unit of blood typically refers to approximately 450 milliliters (ml) to 500 ml of whole blood. This volume represents a standard blood donation and is often used as a unit of measure for transfusions or blood products. However, it's important to note that blood transfusions can involve specific blood components, such as packed red blood cells, platelets, or plasma, which may have different volumes and purposes. The specific volume of blood or blood component required for a transfusion depends on the patient's condition and the medical professional's assessment.