How much should you feed an underweight or thin dog? Here are some general rules:
Underweight dogs: Feed your dog 1-1/2 times the "usual" amount of food and make an appointment to see your veterinarian about your dog's body condition. Consider switching to a food with higher protein and fat content.
Lean dogs: Many healthy dogs are a bit thin, especially active young male dogs. Consider increasing total daily food or caloric intake by 25 percent. Weigh your dog every week if possible to chart progress.
For more information, please read Nutrition in Dogs.