If you are unable to meet your calcium needs through your diet, then any calcium supplement can be useful. The highest amount of calcium by weight is calcium carbonate, at 42%. It is absorbed best with food. The next most popular and highest amount of calcium is calcium citrate at 21%. When it is taken is not an issue. Look for calcium supplements that also include vitamin D and magnesium, as they are involved with proper calcium metabolism. Make sure you stay close to 1000mgs/day including diet.