It is crucial to understand the requirements and availability of dietary phosphorus to be able to limit its use. There are big environmental and economically reasons to reduce the use of dietary P but it must be done while not impacting growth. As a prerequisite to understanding dietary P one should understand the fate of ingested P, its interaction with dietary calcium, and exogenous phytase supply. Understanding these criteria researchers were able to build a mathematical model based on in vitro and in vivo literature data. The model has 3 sections which successfully occur: 1. P solubilisation and phytic P hydrolysis, 2. P absorption and 3. P insolubilisation. To better understand key factors of P utilization parameterisation of equations was used in both pigs and chickens. Sensitivity analysis indicated that P solubilisation/insolubilisation and especially phytic P solubilisation prior to hydrolysis by phytase were sensitive to pH and dietary Ca supply. Behavioral analysis found that the model was able to predict the lower plant phytase in both pigs and chickens. The model correctly predicted apparent digestibility in pigs. The model offers the potential to be combined with a metabolic model that integrates regulation of P and Ca metabolism.