Abstract: Two experiments determined the quantitative relationships between dietary calcium, NPP (Nonphytate Phosphorus), vitamin D and phytase in diets for broiler chicks to 20 days of age. In experiment one, the NPP requirement was approximately 0.40% when the diet contained 0.90% calcium and 3000 ICU of vitamin D/kg. This was based on a linear response in chick weight, feed intake, bone ash and blood calcium and phosphorus to increasing dietary NPP. When the diet contained 0.70% calcium, bone ash and blood phosphorus had significant responses to added dietary NPP but other parameters had no significant responses. The NPP requirement was approximately 0.30% of the diet. Data from these experiments indicate the calcium: NPP ratio is approximately 2.25. Adding phytase at 773 or 1545 units per kg of diet gave responses of approximately 0.05% NPP per 773 units. In experiment two, vitamin D additions were 500, 2,500 or 12,500 ICU kg^-1 of diet that contained 0.80% calcium. Between 500 and 2,500 ICU kg^-1, the response was equal to approximately 0.013% NPP per 1000 ICU kg^-1. Between 2,500 and 12,500 ICU kg^-1 diet, the response was equal to approximately 0.003% NPP per 1000 ICU. The addition of 537 units of phytase kg^-1 to a diet with 0.20% NPP caused a response equivalent to 0.05-0.07% NPP. Increasing the phytase to 1074 units per kg of diet gave no additional response. A discussion of the results suggests the importance of the solubility product and the common ion effect for explaining results in calcium and phosphorus nutrition.