Non-SI unit of energy. There are three differently defined calories: thermochemical calorie (cal th = 4.184 J), international steam table calorie (cal IT = 4.1868 J), 15 °C calorie (cal 15 = 4.1855 J), however only the symbol cal is usually used.
Green Book, 2nd ed., p. 112
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 964