Several experiments were conducted to estimate Ca digestibility in different feed ingredients in the presence or absence of microbial phytase to allow formulation of diets for pigs to be based on standardized total tract digestible (STTD) Ca instead of total Ca. Thus, 4 experiments aimed at determining Ca requirements expressed as a ratio between STTD Ca and STTD P in pigs from 11 to 130 kg. A follow-up study was later conducted to validate those data and to evaluate the effect of using ratios that maximize growth performance on bone development because maximum bone ash requires more Ca than maximum growth performance. However, data indicate that STTD Ca to STTD P ratios to maximize Ca retention are greater than to maximize bone ash synthesis. The use of STTD Ca to STTD P ratios in diet formulation may result in a reduction in excess dietary Ca, which is beneficial because excess dietary Ca is detrimental to P digestibility and growth performance of pigs. Therefore, the objective of this experiment was to test the hypothesis that formulating diets for growing pigs based on a ratio between STTD Ca and STTD P instead of total Ca and STTD P does not decrease Ca retention, but increases P utilization.