Various calcium compounds are used in additives such as calcium carbonate, calcium phosphate, and calcium citrate. Elemental calcium represents the actual amount or percentage of calcium in a compound. 

To check the amount of available elemental calcium, it is important to read the calcium supplement label. You can locate various calcium supplements at this website

So a supplement with a daily value of 25% for calcium contains 250 mg of elemental calcium. All you need to take to reach this elemental calcium level. If you choose food supplements, don't consume more than 2500 mg of elemental calcium per day (a combination of diet and food supplements)!

Calcium supplements are currently not regulated by the FDA. However, when choosing a calcium supplement, check the USP initial label. This is a guarantee that the product meets US Pharmacopoeia’s voluntary standards for quality, purity, and tablet disintegration. 

The body easily absorbs most branded calcium products. If in doubt, look for the USP symbol or try a simple test on a sample tablet. Put in a glass of warm water or clear vinegar. Stir occasionally. 

Within 30 minutes a tablet should be dissolved in water then supplements will also dissolve in your stomach. Chewing supplements and liquid calcium are good alternatives to ensure proper intake. Food supplements are best absorbed if consumed in an increase of fewer than 500 mg throughout the day.