A deterministic, dynamic pig growth model is described that predicts the effects of genotype and the thermal and nutritional environments on food intake, growth and body composition of growing pigs. From the daily potential for protein gain, as determined by pig genotype and current state, the potential gains of the other chemical components, including ‘desired’ lipid gain, are calculated. Unconstrained voluntary food intake is predicted from the current protein and lipid contents of the pig, and the composition of the food, as that which is needed to permit potential growth to be achieved. The model allows compensatory lipid gain. The composition of the food is described in terms of its digestible energy content (DEC), ideal digestible crude protein content (IDCPC) and bulkiness. Both energy and protein can be limiting resources and the bulk of the food may constrain intake. The animal’s capacity for bulk is a function of its size. The thermal environment is described by the ambient temperature, wind speed, floor type and humidity and sets the maximum (HLmax) and minimum (HLmin) values possible for heat loss. A comparison with heat production (HP) determines whether the environment is hot (HP > HLmax), cold (HP < HLmin) or thermoneutral (HLmin < HP < HLmax). A constraint on intake operates in hot environments, while in cold environments, there is an extra thermal demand. If conditions are thermoneutral no further action is taken. Daily gains of each of the chemical components are calculated by partitioning energy intake between protein and lipid gains according only to the energy to protein ratio of the food. The model builds on the work of others in the literature as it allows predictions on how changes in: (i) the kind of pig; (ii) the animal’s current state, which is particularly relevant in cases of compensatory growth; (iii) the dietary composition, and; (iv) the climatic environment, affect food intake and growth, whilst maintaining simplicity and flexibility.