Mean Corpuscular Volume (MCV) Explained by a Doctor in 4 Minutes! MCV is the average amount of space a red blood cell occupies. Red blood cells are made by the bone marrow using a standard recipe. They aren't clones, but are usually about the same size, between 80-100 femtoliters. When the recipe goes wrong (lack of iron, B12, folate, or other ingredients) the red blood cells come out all different shapes and sizes! Since the main job of red blood cells is to carry oxygen, they need to be wel...