Allschoolabs Nutrition Center ? Windmill Oyster Shell Calcium with Vitamin DCompare to Os-Cal 500 + D and Save120 Tablets Windmill Calcium as Carbonate 600 mg with Vitamin D formula provides you with 600 mg of calcium carbonate plus 400 IU of vitamin D per tablet. If you compare the Windmill Calcium 600 with Vitamin D formula to the Caltrate 600 + Vitamin Damp;reg; formula, you will find out that the two formulas are almost identical; however, Windmill does not use artificial colors, flavors, or preservatives.