To keep your dog happy and healthy, feed him a high-quality dog food at least twice a day. To feed him properly, it's important to know how much food a dog of his size and activity level needs. This is the amount that will provide the calories he needs to get through the day's activities without creating weight issues.
Activity Level
Like people, different dogs have different activity levels, which means they have different daily calorie requirements. DogFoodAdvisor.com says the daily calories required for a 40-pound dog are as follows: A typical dog needs 969 calories; an active dog, 1101 calories; an overweight dog, 616 calories; a highly active dog, 1541 calories; a senior, neutered, inactive dog, 792; a light-duty working dog, 1145 to 1321 calories; a moderate duty working dog, 1321 to 1541 calories; and a heavy-duty working dog, 1541 to 2201 calories.
Serving Size
Once you know the caloric requirement for your 40-pound dog, you need to figure out how much food to give. Different dog food types and brands have different calorie contents, so there's no across-the-board formula for the amount of food your dog should get. Many dog food brands list "calories per cup" on the dog food packaging near the nutrition facts. Using your dog's estimated caloric needs, you can calculate the quantity of your favored dog food food your dog needs each day. For example, if your dog food label says it provides “355 kcal per cup,” and you have an active 40-pound dog, your dog needs just over 3 cups of food per day.
How Often?
The American Society for the Prevention of Cruelty to Animals (ASPCA) recommends you feed your dog twice daily. Once you know your dog’s daily caloric needs, you can divide that into two meals per day. Space mealtimes 8 to 12 hours apart.
Considerations
Weigh your dog every few weeks to make sure he’s neither gaining nor losing weight. If he is, adjust the amount you feed accordingly. Use a measuring cup to portion out his food -- don’t rely on a scoop without correct measurements on it. Consult your vet if you have questions about your dog’s weight or types of dog food to use for your specific breed.