Key message: Xylem functionality is the primary determinant of the Ca relations of developing sweet cherry. Calcium influx is positively related to transpiration and decreases as xylem functionality is progressively lost during development. Abstract: Fruit calcium (Ca) may be associated with susceptibility to rain cracking in sweet cherry but little background information is available on the fruits’ Ca relations. The objectives of the study were to identify the key determinants of the Ca content in developing sweet cherry fruit. The masses of Ca and K in the flesh (mg per fruit) increased continuously through to maturity. However, their dry mass ratios [mg per g dry mass (DM)] decreased continuously. The decrease in Ca/dry mass ratio was to about 30% of the transition stage II/III value, and for K to about 55%. These decreases occurred in all cultivars investigated. Moreover, the Ca mass per fruit and the Ca/dry mass ratio were normally distributed and wide ranging; the maximum recorded values were about twice the minimum ones. Within a fruit, the Ca/dry mass ratio was two to three times higher in the stem end than in the stylar scar end. Fruit held under low relative humidity (26.2%) contained more Ca per fruit and had a higher Ca/dry mass ratio compared with fruit held under high humidity (91.6%). There was a positive relationship between Ca mass per fruit and cumulative transpiration. Our results indicate xylem functionality is the primary determinant of the Ca relations of sweet cherry fruit. © 2020, The Author(s).