Here's a quick, simple overview of how to calculate the energy requirement (required kcal per day) of a healthy adult dog or cat.
I know it looks like a lot of text (not actually quick or easy!) but I wanted to really break it down and give a few examples, so it's as clear as possible. There is a quick summary at the end that you can skip to, if preferred.
So, why bother to do this? You may want to:
Advise clients how much to feed their pet for healthy weight maintenance
Check if a pet is being overfed or underfed
Only very simple maths is required - you can use any phone calculator (just turn the phone sideways).
Let's consider a case example, to review the steps:
Molly is a healthy 2 year old spayed female Weimaraner. She is only moderately active (one 30-40 minute walk every day and a run at the park) and her owner would like to buy a bag of 'Hill's Science Diet' Healthy Mobility Chicken Meal, Brown Rice & Barley dry diet from your clinic.
Molly's owner is worried about joint health and would like to keep Molly in good condition. She asks you how much of the diet she should feed everyday?
First of all, what information do you need to work this out?
Molly's current body weight
Molly's body condition score (BCS)
The kcal/gram of the diet the owner has selected
Whether Molly is fed any other treats or foods
Molly's current weight is 26.7 kg, and you assess her body condition score as 4/9 (if you need to, review body condition scoring here). You ask the owner about treats, and learn that Molly is fed one large Greenies dental chew every day.
To work out the kcal/gram of the selected diet, it is usually fine to just look on the packaging, or check the manufacturer's website. Most manufacturers should provide the kcal/kg of the diet, and Hill's does just that:
This obviously equates to 3.61 kcal per gram of this particular diet. But what about the Greenies chew? Her owner needs to account for the calories provided by that daily treat, or Molly might gain excessive weight. It's a common mistake that owners feed an appropriate amount of the selected diet, but then feed a whole lot of additional treats, without adjusting or reducing the amount of diet fed.
To find out about kcal in the chew, again, check the manufacturer's website:
So now you have all the information you need to get started with some calculations. To summarise:
Body weight: 26.7 kg
Energy density of chosen diet: 3.61 kcal/gram
Treats fed per day: One large Greenies chew = 147 kcal
Here are the steps:
1) Calculate ideal body weight, as needed
Molly has a BCS of 4/9, which is healthy but lean. It's probably ideal for most young, large-breed dogs. So let's say her current weight and condition is ideal.
If for some reason you assessed her as too lean and wanted her to have a BCS of 5/9, you would multiply her current weight by 1.1, because every BCS point over or under ideal weight equates to about 10%.
So for a dog with a weight of 10 kg and a BCS of 3/9, you would multiply 10 kg by 1.2 (this dog is 20% too lean) to provide an ideal weight of 12 kg.
For a dog weighing 10 kg with a BCS of 6/9, you would multiply the current weight by 0.9 (this dog is 10% too heavy), yielding an ideal weight of 9 kg.
It is important to use the pet's ideal weight for the following calculations, because you don't want overweight animals to gain further weight if you use their current weight, or underweight animals to fail to gain weight.
2) Calculate required kcal/day for weight maintenance
To do this, you need to use an equation. We often use those provided by the papers published by Bermingham et al. (2010 and 2014); here they are:
Adult pet dogs:
(Ideal weight in kg)^0.93 x 62.5 = required kcal/day
Adult pet cats:
(Ideal weight in kg)^1.115 x 46.8 = required kcal/day
Just to note, the ^ symbol means 'to the power of'.
There are other ways to calculate kcal/day, but these equations are easy to use and accurate for the majority of pet dogs and cats (low to moderate activity level).
So for Molly, here's the calculation:
(26.7 kg)^0.93 x 62.5 = 1326 kcal per day
This is Molly's maintenance energy requirement (MER), or essentially the number of kcal/day she needs to consume to maintain her ideal weight.
3) Allow for any treats fed by the owner
Now we come back to the Greenies chew. Remember that one large chew provides 147 kcal. So subtract this amount from Molly's kcal per day (as per above):
1326 - 147 = 1179 kcal per day
So Molly's diet needs to provide 1179 kcal, with the remainder of her requirement being provided by the daily Greenies chew.
As an aside, generally treats or additional foods should contribute a maximum of 10% daily kcal, because otherwise there's a risk of the overall diet becoming unbalanced or deficient. Here's an example of that - if a dog owner feeds their 25 kg dog half a cup of a balanced, AAFCO-formulated commercial dry diet per day, plus 3 pig's ears, a cooked chicken breast, a handful of pretzels, a blueberry muffin, and a rump steak, you can see that a lot of the calories are being provided by the treats or additional foods, and these treats aren't complete and balanced (which will cause deficiencies).
In Molly's case the Greenies chew provides 11% of her daily calories, which should be fine.
4) Calculate required grams of diet per day
To review, Molly needs 1179 kcal per day from the dry diet, and the selected Hill's diet provides 3.61 kcal/gram. So now, just divide the kcal/day by the kcal/gram, to determine the grams/day:
1179 / 3.61 = 326 grams of diet per day
Done! To meet her energy requirement, Molly's owner should feed 326 grams (325 grams is ok!) of the Hill's dry diet, plus one large Greenies dental chew per day
I often recommend that vet practices create a simple form or Word document, so that the veterinary staff can quickly and easily refer to the steps and equations, and calculate a patient's requirement.
You could include these headings (or similar):
1) Patient's current body weight
2) Patient's current BCS
3) Patient's ideal weight
4) Name of selected diet
5) Kcal per gram of selected diet
6) List any treats or additional foods fed by the owner
7) Total kcal provided by treats
8) Patient's calculated MER (required kcal per day)
9) MER less any kcal from additional treats
10) Required grams of diet per day
This is the information you need, and the steps you need to take, to work out how much to feed a healthy adult dog or cat. Ensure that the pet's owner can take home a printed copy to stick on their fridge, and that they understand they need to weigh the amount of food to be fed everyday, not guesstimate using a scoop or cup.
I hope that's been helpful; as always any questions very welcome: firstname.lastname@example.org
Read the full paper (dogs) by Bermingham et al. (2014) here: https://pubmed.ncbi.nlm.nih.gov/25313818/
Read the full paper (cats) by Bermingham et al. (2010) here: https://pubmed.ncbi.nlm.nih.gov/20100376/