The answer, in one line
Your dog's daily calories divided by the energy density of your food gives the portion. grams per day = daily kcal ÷ kcal per kg × 1000
cups per day = grams per day ÷ grams in one cup. A 40 lb neutered adult needs about 985 kcal a day; on a 3,500 kcal/kg food that is 281 g, which at 110 g per cup is about two and a half cups. Change the food and every one of those numbers changes, even though the dog did not.
Three ways the bag states energy, and how to convert each
Pet food labels declare calorie content in one of three units, and no law makes them use the same one. All three convert to the same figure:
- kcal per kg — divide by 1,000 to get kcal per gram, or just use it directly in the formula above. Typical dry food: 3,000–4,000. Typical canned: 800–1,200.
- kcal per 100 g — multiply by 10 to get kcal per kg. This is the unit used on most non-US labels, sometimes alongside kJ.
- kcal per cup — the least useful of the three, because a cup is a volume. Divide daily kcal by this figure to get cups per day directly, but you have inherited the manufacturer's idea of what a cup weighs.
If your bag only gives kcal per cup and you want grams, weigh one level cup and work backwards: kcal per kg = kcal per cup × 1000 ÷ grams per cup. That single weighing is worth more than any other number on this page.
Why cups are the weakest unit you can feed by
A cup measures volume, your dog eats mass, and the conversion between them depends on kibble size, shape and how the cup was filled. Two people filling the same cup with the same kibble routinely differ by 20% or more, and the same "cup" of a different food can differ by 40%. Scooping is consistently generous, which is one reason so many dogs on measured-by-eye rations are overweight.
The fix takes one minute: put the empty cup on a kitchen scale, tare it, fill level, read the grams, write it on the bag. Do it once per food. From then on a gram scale does the rest, and you can change brands without silently changing the ration.
How long a bag lasts, and what it costs
This is the question every owner asks and no calculator answers: how long will this bag actually go, and what am I spending per day. Both fall straight out of the portion — days = bag weight in grams ÷ grams per day.
At 3,500 kcal/kg and the neutered-adult factor, a 12 kg bag feeds a 10 lb dog for about 121 days and a 100 lb dog for about 21 days. The cost curve is the same shape inverted: the same bag that costs a small-dog owner half a unit a day costs a large-dog owner nearly three. That gap is also why the cheapest food per bag is often not the cheapest per day — a calorie-dense food fed in smaller portions can beat a bulky one that has to be fed by the bucket.
Dry, canned, and mixed feeding
Calories add across foods, so mixed feeding is arithmetic, not a special case: work out the grams of each food from its own energy density, and make the two portions add up to the daily target. What does not work is averaging the percentages on the two labels — a canned food at 78% moisture and a kibble at 10% moisture cannot be compared as-fed. Convert both to a dry-matter basis first, or compare grams of protein per 100 kcal.
One practical note: canned food at roughly 1,000 kcal/kg means a large dog would need to eat a physically enormous volume of it to meet its energy needs. This is why most large dogs are fed mainly dry food, with wet food as a topper rather than the base.
Mistakes that cost more than the formula does
- Changing food without recalculating. Two foods both labelled "complete" can differ by 30% in energy density. Same cup, different calories, weight gain nobody can explain.
- Trusting the bag's feeding guide over the dog. Those guides cover the whole weight band and the active end of the range; they are a ceiling more often than a target.
- Forgetting treats. Treats come out of the portion, not on top of it. Ten percent of the day is two or three biscuits, not a handful.
- Buying by price per bag. Compare price per 1,000 kcal, not price per kilo. A denser food that costs more per bag can be cheaper per day.
Frequently asked questions
How many cups should I feed my dog per day?
There is no cup number that survives a change of food. Work it out from your own bag: divide the daily calorie target by the kcal per cup, or by the kcal per kg converted through the weight of your cup. A 40 lb dog on a 3,500 kcal/kg food lands near two and a half cups a day; the same dog on a 4,200 kcal/kg food lands near two.
How do I work out calories if my bag only gives kcal per cup?
Weigh one level cup in grams, then kcal per kg = kcal per cup × 1000 ÷ grams per cup. Once you have that, every other number on this page works. If you cannot weigh it, use 110 g as a rough middle for dry kibble and treat the result as an estimate.
How long does a 12 kg bag of dog food last?
About 121 days for a 10 lb dog, 72 for 20 lb, 43 for 40 lb, 31 for 60 lb, 25 for 80 lb and 21 for 100 lb — at 3,500 kcal/kg and the neutered-adult factor. Halve the bag size or double the dog and the number moves accordingly.
Is a more expensive food actually cheaper to feed?
Often, because you feed less of it. Compare cost per 1,000 kcal rather than cost per kilogram. A food at 4,000 kcal/kg that costs more per bag may deliver a lower daily cost than one at 3,000 kcal/kg that has to be fed in larger portions.
Does canned food have fewer calories than dry food?
Per kilogram, yes — roughly 800–1,200 kcal/kg against 3,000–4,000 for dry — because most of a can's weight is water. That does not make it lower in calories per feeding, since you have to feed far more of it to deliver the same energy.
Do I need to change the amount if I switch from dry to mixed feeding?
Yes. Work out each component from its own energy density and make the portions add up to the daily target. Feeding the same volume of kibble plus a can on top is a large daily surplus.
Sources
- RER = 70 × kg0.75 and the maintenance factors used here — MSD Veterinary Manual, caloric requirements of adult dogs.
- Calorie content labelling and its placement on pet food packaging — AAFCO guidance for consumers.
- Body condition scoring and feeding assessment — WSAVA Global Nutrition Guidelines.
Need the daily calorie figure first? Use the dog calorie calculator, or the feeding guide for meals, treats and the two-week correction rule. If your bag gives energy in kcals and you are not sure what it means, see how many kcals should my dog eat.