Energy of pet food is evaluated as metabolizable energy (ME). To determine ME heat of combustion in food (gross energy, GE), feces and urine is analyzed. Losses by gases are negligible. GE of the food can be calculated by standard values for heat of combustion of nutrients. Given a typical pet food potential errors in the standard values for heat of combustion of nutrients can add up to a deviation of a maximum of about 8% between GE as analyzed and GE as calculated. In practice there are often deviations of up to 30% between analyzed and calculated GE, mostly due to pitfalls of bomb calorimetry. Five repetitions with an intra-analysis variation coefficient of < 0.4% are necessary to ensure quality. To predict digestibility a fixed value for the digestibility of each nutrient can be used. This works well in a group of rather homogenous foods. Examples are unmodified Atwater factors for homemade western diet type foods for humans and pets. If digestibility is variable within a group of foods it is either necessary to make subgroups of foods with similar digestibilities or to use equations which adjust digestibility such as the prediction of digestibility by fiber in dry matter. For this crude fiber is the analysis of choice because in pet foods it detects mostly unfermentable fiber, which has a stronger impact on energy digestibility than fermentable fiber. Even so subgrouping the foods may present an advantage. The depressive effect of fiber on digestibility is stronger for carbohydrates than for fat, thus subgrouping according to carbohydrate or fat content may increase accuracy of prediction. Subgrouping of foods could even be done by in vitro tests such as an HCl-pepsin test to identify reducing diets with low digestibility of protein. There is little data on renal energy excretion in pets. So far the prediction of renal losses is done by a factor linked to protein intake, which is lower for cats than for dogs in most equations. Presumably the species difference reflects a difference in renal excretion of high energy nitrogen compounds such as hippuric acid, which is derived from phenolic food compounds. Both the content of such compounds in the diet and the ability to degrade these compounds to hippuric acid may contribute to the difference.