Elemental calcium is a term used in dietary supplement ingredient lists to refer to the amount of calcium in a product. Calcium pills contain calcium in a variety of molecules, such as carbonate, citrate, citrate-maleate, orotate, etc. Each pill supplies a different amount of elemental calcium when absorbed, and hence differs in its effectiveness. For example, calcium carbonate is 40% elemental calcium by weight; citrate is roughly 20% calcium, etc. Thus a 500 mg pill of calcium carbonate contains 200 mg of calcium and the container will indicate each pill has 200 mg of elemental calcium. This is the calcium that is actually available to the body and, as such, is the amount that one needs to consider towards his/her daily requirement. Hence, one should read the label carefully to determine the number of pills to be taken. Other factors may influence the choice of molecule prescribed. Calcium carbonate is by far the best, since it is cheap and allows more calcium intake per dose, but leads to indigestion, flatulence and other intestinal problems in quite a few patients. It also requires that the patient take the tablet after a full meal and avoid taking antacids concomitantly, which is difficult to achieve in patients taking multiple medications. Citrate or citrate-maleate is the molecule of choice in such cases; these need to be taken before meals and have less/no interaction with antacids.
-  Women, Hormones and the Menstrual Cycle
-  The Washington Manual of Surgery
-  Calcium supplements
- The National Institutes of Health web page on dietary supplements