Calcium isotopes in bones, urine, blood, soils, roots, and leaves can vary significantly. Ab initio calculations show that isotopic fractionation between different forms of calcium in water can be up to 3000 ppm. The difference in hydration degree of Ca 2+ explains isotopic fractionation between soil solutions and plant roots, while Ca-oxalates control the fractionation between roots and leaves. In blood and urine, heavy calcium binds with citrate and oxalates, causing isotopic fractionation. These findings help understand how calcium moves in living organisms and may aid in interpreting future calcium isotopic measurements.