Calcium supplements often include Vitamin D because it aids in intestinal absorption of calcium. That is actually one of the main roles of Vitamin D. You can find Vitamin D by itself, in anywhere from 400 to 5000 units per pill. Taking high doses of Vitamin D can cause high calcium (due to more being absorbed in the intestines), and taking extra calcium would just exacerbate that. If someone has low calcium levels, I recommend taking both calcium and Vitamin D. Low Vitamin D can cause low calcium levels.