Calcium (Ca) is a metallic element with atomic number 20. In the Earth's crust it is one of the most abundant metals. The most common calcium isotope is ⁴⁰Ca (20 protons, 20 neutrons).

Limestone, which is a common sedimentary rock, is made of calcium in the form of calcium carbonate (CaCO₃).

In our bodies, calcium is probably best known for being a crucial building block in our bones and teeth.