Calcium (₂₀Ca) has 26 known isotopes, ranging from ³⁵Ca to ⁶⁰Ca. There are five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one isotope (⁴⁸Ca) with such a long half-life that for all practical purposes it can be considered stable. The most abundant isotope, ⁴⁰Ca, as well as the rare ⁴⁶Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a cosmogenic isotope, radioactive ⁴¹Ca, which has a half-life of 99,400 years. Unlike cosmogenic isotopes that are produced in the atmosphere, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper metre of the soil column, where the cosmogenic neutron flux is still sufficiently strong. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar system anomalies. The most stable artificial radioisotopes are ⁴⁵Ca with a half-life of 163 days and ⁴⁷Ca with a half-life of 4.5 days. All other calcium isotopes have half-lives measured in minutes or less.

⁴⁰Ca comprises about 97% of naturally occurring calcium. ⁴⁰Ca is also one of the daughter products of ⁴⁰K decay, along with ⁴⁰Ar. While K–Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K–Ca age dating.