I just looked it up, calcium carbonate contains 40% elemental Calcium, and calcium citrate has only 21% elemental calcium, citrate is also more expensive, but cause less constipating than carbonate.

The daily recommended amount Calcium is 1200-1500 mg per day, and Vitamin D is 2000 units per day. For best absorption, take it in divided dose.

Also, Calcium and iron block each other's absorption, so don't take them at the same time!