I get this question almost daily. The answer is “it depends.”
We have a complicated formula to calculate the Resting Energy Requirements (RER) of dogs and cats in kcal/day. It isn’t a straight-line graph, but we have chart. Some points on this chart is the RER for a 10-pound critter is 218 kcals/day, 20# is 366 kcal/day, 30# is 497 kcal/day, 40# is 616 kcals/day, 50# is 729 kcals/day. My chart goes up to 200#.
The number we really want is the Daily Energy requirements (DER). This represents the average daily energy expenditure of animal, depending on life stage and activity (work, gestation, lactation, and growth). To get DER from RER, we multiple RER with a number from 0.8 to 4.0, depending on the species, growth vs maintenance, obese vs weight loss, light work vs heavy work.
For examine, for an average 20# house dog with normal weight, we multiple 366 kcal/day x 1.6=586 kcal, but it is already obese and need to lose weight at 20#, we multiple by 1.0 for just 366 kcal/day. If this was a fat 20# cat needing to lose weight, we multiple by 0.8 for only 293 kcal/day.
So how many cups of food is that? First, we need to know the calorie density of the food. I seldom see it on the bag, but it should be available on the manufacturer website. My young dogs eat a Science Diet adult food for tiny dogs, and the website lists the calorie density at 291 kcal/cup. So, my little guys should eat about 2 cups/day each. I do meal feedings so each should eat about a cup, twice a day. Right now, I feed them together, but if one starts getting chubby, and the other thin, then I will need to feed them separately. I checked the feeding guide on the website, and it suggested 1 .5 cups/day. Usually I find the feeding guides to be “generous” in their portions, but it never hurts to do the math.
Another part of the equation is that not all days have equal activity. If you have a high energy dog, that gets walked every day, that is great. Many have different activities depending on the day of the week and the weather. The weekend walker or dog park goer could have a little more on those high energy days but remember to cut back if we have a run of nasty weather with no playtime. Mine get extra chow on the high energy days after running around the pasture chasing cows or digging for hours after gophers (not sure if they are eating gophers, hard thing to calculate).
The last part of the equation is how any “extra” calories a day they get in treats or food from the table (or stolen from kids). A quick search on Chewey for popular dog treats listed Milk Bones Large original at 125 kcals/treat, Denta Sticks at 76 kcal/treat, and Pup-peroni at 28 kcals/treat. Just like fun sized Snickers bars, they are that many calories, but they can add up over time.
So, the short answer is “it depends.”