RER, or resting energy requirement, is the number of calories a dog or cat needs per day just to maintain basic body functions like breathing, digestion, and circulation. The formula is: RER = 70 × (body weight in kg)^0.75, and the result is in kilocalories per day. This baseline number then gets multiplied by a factor based on your pet’s life stage, activity level, and whether they’re spayed or neutered.
The Standard RER Formula
The widely used equation, refined by the physiologist Max Kleiber, is:
RER = 70 × (body weight in kg)0.75
The 0.75 exponent accounts for how metabolism scales with body size. A 40 kg dog doesn’t burn exactly twice the calories of a 20 kg dog. Smaller animals have higher metabolic rates relative to their size, and the 0.75 power captures that relationship across species. This is sometimes called “metabolic body weight.”
How to Do the Math Step by Step
The trickiest part is raising a number to the 0.75 power, but any smartphone can handle it. Here’s how:
- Step 1: Convert your pet’s weight to kilograms. Divide pounds by 2.2. A 10-pound cat is about 4.5 kg.
- Step 2: Raise that number to the 0.75 power. On most smartphones, turn the phone sideways to reveal the scientific calculator. Type the weight in kg, then use the exponent button (usually xy) and enter 0.75.
- Step 3: Multiply the result by 70.
For a 10-pound (4.5 kg) dog: 4.50.75 = roughly 3.09. Multiply by 70, and you get about 216 kcal/day as the resting energy requirement.
For a 30 kg (66-pound) dog: 300.75 = roughly 12.82. Multiply by 70, and the RER is about 897 kcal/day.
The Simpler Linear Formula
There’s a shortcut that avoids the exponent entirely:
RER = (30 × body weight in kg) + 70
This linear version is only accurate for animals between 2 kg and 45 kg (roughly 4.4 to 99 pounds), according to the Merck Veterinary Manual. Outside that range, it becomes unreliable. For very small pets or giant breed dogs, stick with the exponential formula.
Turning RER Into Daily Calories
RER is just a starting point. Your pet’s actual daily calorie needs, called the maintenance energy requirement (MER), depend on life stage and other factors. You calculate MER by multiplying the RER by a specific number.
Dogs
- Intact adult: 1.8 × RER
- Neutered adult: 1.6 × RER
- Obesity-prone: 1.4 × RER
- Puppy under 4 months: 3.0 × RER
- Puppy over 4 months: 2.0 × RER
Cats
- Intact adult: 1.4 × RER
- Neutered adult: 1.2 × RER
- Obesity-prone: 1.0 × RER
- Kitten: 2.5 × RER
So for that 10-pound neutered dog with an RER of 216 kcal/day, the daily calorie target would be 216 × 1.6 = about 346 kcal/day. These multipliers come from the AAHA and Merck Veterinary Manual guidelines, and they’re meant as starting points. You may need to adjust up or down based on how your pet’s weight responds over time.
Which Weight to Use for Overweight Pets
If your pet needs to lose weight, don’t plug their current weight into the formula. AAHA guidelines recommend using your pet’s estimated ideal weight instead, then feeding a percentage of that calculated amount. Your vet can help estimate what that ideal weight should be based on body condition scoring. Using current weight for an overweight pet just calculates the calories needed to maintain the excess.
RER in Human Exercise Science
If you landed here looking for something different, “RER” also stands for respiratory exchange ratio in exercise physiology. That’s a completely separate measurement: the ratio of carbon dioxide your body produces to the oxygen it consumes (VCO2 ÷ VO2). It’s measured during metabolic testing on a treadmill or bike and tells you whether your body is burning more fat or carbohydrate during exercise. An RER near 0.7 indicates primarily fat burning, while values approaching 1.0 or above indicate carbohydrate dominance. No weight-based formula is involved.