Allschoolabs Nutrition-Windmill Calcium as Carbonate 600 mg Compare to Caltrate 600amp;reg; and Save60 Tablets Windmill Calcium Carbonate 600 mg formula provides you with 600 mg of calcium from carbonate per tablet. If you compare the Windmill Calcium Carbonate formula to the Caltrate 600amp;reg; formula, you will find out that the two formulas are almost identical; however, Windmill does not use artificial colors, flavors, or preservatives.