Calcium is one of the essential minerals in poultry nutrition. It is estimated that each egg contains 2.2g of calcium, present mainly in the eggshell.

Many researches have been done on the form of calcium carbonate that is used in the diets of laying hens. Limestone and oyster shells are considered as insoluble forms of calcium.

Due to the slow process of separating calcium from sources, it remains in the digestive system for a long time. Limestone is more soluble due to its particle size and physical structure, so it can remain in digestive system for a short time.

Calcium is supplied from remnant of food in digestive system and some of the calcium that are needed to make egg shells, are provided at this time. Calcium supplementation influences eggshell quality. Calcium has several functions in bird metabolism and it is the main component of the eggshell.