When formulating diets for pigs, it is more accurate to use values for standardized or true nutrient digestibility than values for apparent nutrient digestibility because the former are additive in mixed diets. No values for standardized or true total tract digestibility of calcium in pigs have been reported. The true total tract digestibility (TTTD) of a nutrient is calculated by correcting apparently total tract digestibility (ATTD) by total endogenous losses, which may be estimated using a regression procedure. To our knowledge, no measurements of the endogenous loss of calcium in pigs have been reported. An experiment was, therefore, performed to measure endogenous loss of calcium and to determine TTTD of calcium in growing pigs, and to investigate if the addition of microbial phytase to the diets affects TTTD of calcium. In addition, calcium retention was measured in pigs fed diets containing varying levels of calcium with or without microbial phytase.