When you browse the aisles of a British supermarket like Tesco or Sainsbury's, you will find countless bags of pet food with generic 'cups per day' charts. However, these guidelines are often based on broad averages that may lead to overfeeding. Calculating resting energy requirements (RER) is the most scientifically accurate way to determine your pet's baseline metabolic needs. By understanding the math behind the bowl, you can move away from guesswork and ensure your dog or cat maintains a healthy weight. In the United Kingdom, where pet obesity is a rising concern, taking control of these calculations is the first step toward a longer, healthier life for your companion.
The Limitations of Commercial Feeding Guidelines
Most pet owners in the UK rely on the feeding tables printed on the back of kibble bags. While these are a helpful starting point, they are designed as 'one size fits all' estimates. These charts often use weight ranges that are too broad, failing to account for whether your pet is a neutered indoor cat or a highly active working Border Collie. Manufacturers often suggest portions on the higher end of the spectrum to ensure no pet is underfed, which, unfortunately, contributes to the growing epidemic of pet obesity across the country.
Furthermore, these guidelines do not account for the caloric density of treats or the 'human food' scraps that often slip off the dinner table. When we rely solely on these generic labels, we ignore the unique metabolic rate of the individual animal. A dog living in a chilly flat in Edinburgh during January will have different thermoregulatory needs compared to the same breed living in a temperature-controlled home in London. By calculating resting energy requirements, you bypass these generalities in favour of biological data tailored to your specific pet's frame and physiological status.
What Exactly is Resting Energy Requirement (RER)?
Resting Energy Requirement, or RER, represents the amount of energy an animal expends while at rest in a thermoneutral environment. Essentially, it is the number of calories required to maintain vital bodily functions such as breathing, heart rate, and brain activity, without any additional energy spent on exercise or digestion. It is the feline or canine equivalent of the human Basal Metabolic Rate (BMR).
In veterinary medicine, RER is the foundation of all nutritional planning. Once we have the RER, we apply 'multipliers' to account for lifestyle factors—this result is known as the Maintenance Energy Requirement (MER). For example, a neutered adult dog generally requires 1.6 times their RER to maintain their weight, while a very active dog might require 2.0 or higher. Understanding RER allows you to see the 'fuel' your pet needs just to exist before you even consider their daily walk through the local park or a vigorous game of fetch.
The Step-by-Step RER Formula
To begin calculating resting energy requirements, you first need an accurate weight for your pet in kilograms. In the UK, most vets use the standard scientific formula: $70 \times (body weight in kg)^{0.75}$. For those without a scientific calculator, a simplified 'linear' formula for dogs between 2kg and 45kg is $(30 \times weight in kg) + 70$. While the exponential formula is more accurate across all sizes, the linear version is a reliable tool for most domestic pets.
Let us look at a practical example: a 10kg Cavalier King Charles Spaniel. Using the linear formula, you would multiply 10 by 30 (300) and add 70, resulting in an RER of 370 calories per day. This is the baseline. If this dog is a typical neutered adult, we multiply 370 by 1.6 to find the MER, which is 592 calories. If you were strictly following a bag guide that suggested a range for 'dogs up to 15kg', you might accidentally be feeding for a much larger animal. Having this specific number allows you to look at the 'kcal/kg' information on your pet food label and measure out the exact grammage required.
Adjusting for Life Stages and Activity
Once you have your RER, you must adjust it based on your pet's life stage and 'job'. A multiplier of 1.2 to 1.4 is often used for weight loss, while a puppy may require a multiplier of 3.0 or more because they are building bone and muscle. British breeds like the Working Cocker Spaniel or various Terriers may have much higher needs if they are spending hours running across fields versus a more sedentary life in a city flat.
It is also vital to consider 'neutering' as a factor. Neutering reduces the metabolic rate; research suggests that neutered animals require roughly 20-30% fewer calories than their 'intact' counterparts. If you do not adjust the RER multiplier after your pet is spayed or castrated, they will almost certainly gain weight. We also recommend accounting for the 'treat tax'. If you use high-value treats for training, those calories must be subtracted from the total MER calculated. Veterinary experts suggest that treats should never exceed 10% of the total daily caloric intake to ensure nutritional balance.
Troubleshooting: When the Math Meets Reality
Calculations provide a brilliant baseline, but they are not infallible. Biology is 'dynamic energy' and individual metabolism varies. You should monitor your pet's Body Condition Score (BCS) every two weeks. If you are feeding the calculated amount but your dog is still gaining weight, their individual metabolism may be slower than the average, or you may be overestimating their activity level. Conversely, if they are becoming too lean, you should increase the multiplier slightly.
Common signs that you need to adjust your approach include a loss of a visible waistline, inability to feel the ribs under a light layer of fat, or a noticeable drop in energy levels. If your pet seems constantly ravenous despite receiving their calculated MER, consult your vet to rule out underlying issues like thyroid dysfunction or parasites. Always remember that environmental factors—such as a particularly cold British winter—can increase energy expenditure as the body works harder to stay warm. Adjusting by 5-10% increments is usually the safest way to find the 'sweet spot' for your specific pet.
FAQ
Can I use the same RER formula for both dogs and cats?
Yes, the basic RER formula ($70 \times weight^{0.75}$) is standard for both cats and dogs. However, the multipliers used to reach the Maintenance Energy Requirement (MER) differ significantly between the two species due to their different metabolic pathways.
What should I do if my pet is already overweight?
If your pet is overweight, you should calculate their RER based on their 'ideal' target weight rather than their current weight. It is crucial to work with a vet to ensure weight loss is gradual, as losing weight too quickly can be dangerous, especially for cats.
How do I find out how many calories are in my pet's food?
In the UK, look for the 'Metabolisable Energy' (ME) value on the packaging, usually expressed as kcal/100g or kcal/kg. If it is not listed, you can contact the manufacturer or use an online calculator that estimates ME based on the protein, fat, and fibre percentages provided.
Conclusion
Calculating resting energy requirements is the most empowering tool a pet owner can have. It moves the conversation from 'how many scoops' to 'how many calories', allowing for a level of precision that generic feeding guides simply cannot match. By starting with the RER and adjusting for your pet's specific life stage, activity level, and the damp British weather, you are directly investing in their long-term health. However, these formulas are starting points. Always pair your calculations with regular weigh-ins and Body Condition Score assessments. If you notice sudden weight changes or have concerns about your pet's nutritional needs, consult your local Royal College of Veterinary Surgeons (RCVS) registered vet for a tailored nutritional plan.
References & Sources
This article was researched using the following sources:
