Calcium (Ca) has 24 known isotopes with mass numbers varying between 34 and 57 but has no known nuclear isomers. Five of these isotopes are stable, Ca-40, Ca-42, Ca-43, Ca-44 and Ca-46. Ca-40 and Ca-46 are radioisotopes with extremely long half-lives because no disintegration has been observed at this time. The Ca-48 radioisotope has such a long half-life (43 × 10¹⁸ years, nearly three billion times the age of the universe) that it is considered practical in practice.

The most abundant isotope is Ca-40, accounting for nearly 97% of the existing calcium, Ca-44 is 2%, the other three isotopes share the remaining 1%. The standard atomic mass of calcium is thus 40.078 (4) u, close to the isotopic mass of Ca-40.

Calcium also has a cosmogenic isotope, radioactive Ca-41, with a half-life of 102,000 years.

The most stable of the artificial radioisotopes is Ca-45, with a half-life of 163 days. All other isotopes have a half-life of less than 5 days, and most less than 1 minute. The least stable is Ca-34 with a half-life of less than 35 nanoseconds.

Calcium 45 is the most available calcium isotope. It emits beta radiation and is used as a tracer. It is used in particular in the study of calcium metabolism.