The Complete Guide to Calculating Pet Food Calories (RER & DER)

DigestiveQ&AMeongsiljang Veterinary Advisory Board

I’ll walk you through how to calculate your pet’s daily caloric needs using the RER and DER formulas, step by step. I’ve carefully organized the coefficients based on weight, activity level, and neuter status.

What is Pet Food Calorie Calculation?

A veterinarian showing a nutrition chart to a puppy and a kitten.

Calculating your pet’s food calories is a method to determine the daily energy (kcal) your dog or cat needs based on their weight and activity level. The process involves two key steps. First, calculate the RER (Resting Energy Requirement), then multiply it by a factor that accounts for activity level, neuter status, and age to derive the DER (Daily Energy Requirement). Guessing portion sizes is a leading cause of obesity and underweight conditions in pets. With just one calculation, you can accurately determine the right feeding amount by simply checking the "kcal/100g" value on the back of the food bag.

Step 1: RER (Resting Energy Requirement) Formula

RER, or Resting Energy Requirement, is the number of calories your pet needs for basic metabolism when they are completely at rest. There are two formulas to calculate it:

Exact formula: RER = 70 × (body weight in kg)^0.75

Simplified formula (for pets 2–45 kg): RER = (30 × body weight in kg) + 70

For example, for a 5 kg dog, using the simplified formula: (30 × 5) + 70 = 220 kcal. However, while both formulas give similar results for pets weighing between 3 and 25 kg, the simplified formula tends to overestimate RER for pets over 25 kg (for a 40 kg pet, it overestimates by about 14%). Therefore, it is recommended to use the exact formula (with the 0.75 exponent) for very small dogs under 2 kg or medium-to-large dogs over 25 kg to reduce calculation errors.

An illustration of a puppy sitting on a scale and a calculator

Quick RER Reference Table by Weight

Item	Simple formula	Accurate formula (^0.75)
2kg	130 kcal	118 kcal
5kg	220 kcal	234 kcal
10kg	370 kcal	394 kcal
20kg	670 kcal	662 kcal
30kg	970 kcal	897 kcal
40kg	1,270 kcal	1,113 kcal

Above 25 kg body weight, the simple formula increasingly tends to overestimate RER (about 8% difference at 30 kg, about 14% at 40 kg). For medium and large dogs, using the accurate formula is recommended.

Step 2: Multiply by DER (Daily Energy Requirement) Coefficient

DER is calculated by multiplying RER by a coefficient based on the pet’s lifestyle. The coefficients below are clinical estimation guidelines, and individual requirements may vary. Multiply the appropriate coefficient for your pet’s condition by their RER to determine their total daily caloric needs. - Neutered adult dogs: RER × 1.6 - Intact adult dogs: RER × 1.8 - Weight management/loss: RER × 1.0 - Highly active adult dogs: RER × 2.0–3.0 - Adult cats (neutered): RER × 1.2 - Adult cats (intact): RER × 1.4 - Kittens and puppies (under 4 months): RER × 3.0 For a 5 kg neutered adult dog, the target is 220 × 1.6 = 352 kcal/day.

An illustration contrasting an active dog with a resting cat

Note: The Calculated Value is a 'Starting Point'

DER is an estimate based on average values. Even with the same body weight and coefficient, individual requirements can vary by up to ±20%. After feeding the calculated calories for 2–4 weeks, be sure to reevaluate your pet’s weight change and body condition score (BCS). Adjust accordingly: reduce intake by 5–10% if gaining weight, or increase by 5–10% if losing weight. Pets that are pregnant, nursing, or ill have different coefficient requirements, so please consult your veterinarian.

Step 3: Convert to Feeding Amount Using the Food Bag

Now that you have the DER, it’s time to convert it into the actual feeding amount in grams. Look for the Metabolizable Energy (ME) kcal/kg or kcal/100g value on the back of the food bag. Feeding amount (g) = (DER ÷ food kcal/kg) × 1,000 Example: If the DER is 352 kcal and the food provides 3,800 kcal/kg, then (352 ÷ 3,800) × 1,000 = approximately 93 g/day. Divide this amount into 2–3 meals per day. If you give treats, reduce the main meal portion by the same number of calories as the treats to keep the daily total accurate (treats should make up no more than 10% of daily calories).

Illustration of the nutrition facts label on a bag of pet food and a measuring scoop

Life Stage Coefficient Checklist

The multiplier varies significantly depending on age and condition, so please choose the one that best fits your pet’s situation. - Puppy (under 4 months): RER × 3.0 — Rapid growth phase - Puppy (4 months to 1 year): RER × 2.0 — Late growth phase - Senior dog (7+ years, indoor): RER × 1.2–1.4 — Reflects reduced activity - Pregnant dog (late stage): RER × 3.0 (approx.) — Energy for fetal growth (consult your veterinarian for the precise multiplier) - Lactating dog: RER × 4.0–6.0 — Nursing dogs require roughly three times their maintenance energy, proportional to the number of puppies - Kitten (up to 1 year): RER × 2.5 - Overweight cat (weight loss): RER × 0.8

Don't Use the 'Dog Formula' for Cats

Cats require more protein than dogs and metabolize carbohydrates differently. While the Resting Energy Requirement (RER) formula is the same, the Daily Energy Requirement (DER) coefficient is significantly lower for cats (around 1.2 for neutered adult cats). Using the dog coefficient (1.6–1.8) can quickly lead to obesity. Additionally, cats are at high risk of developing fatty liver disease (hepatic lipidosis) if their food intake drops suddenly or if they experience prolonged loss of appetite. Therefore, even during weight loss, drastic dietary restriction is strictly prohibited.

Management Points After Calculation

Calorie calculations aren’t a one-time task. Regular reassessment is key to accuracy. - Weigh your pet weekly: Use the same scale at the same time each week. - Check Body Condition Score (BCS) monthly: A score of 4–5/9, where ribs are lightly palpable, is ideal. - Adjust for seasonal changes: Reduce calories by 5% in winter if activity decreases, and increase by 5% in summer if walks increase. - Include treats and supplements: Even one dental chew can contain 20–40 kcal. - Use feeding apps or notes: Tracking daily totals helps reduce cumulative errors.

The owner is weighing their dog and recording the weight.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Shouldn't I just follow the "recommended feeding amount" listed on the food bag?

The recommended serving size on the bag is based on an intact adult dog with average activity levels, so it often results in overfeeding for most pets. For neutered indoor dogs, feeding about 70–80% of the bag’s recommendation is usually more appropriate. We recommend calculating your dog’s Resting Energy Requirement (RER) and Daily Energy Requirement (DER) directly.

Which one should I use, the quick formula or the precise formula?

For dogs weighing approximately 3 to 25 kg, both formulas yield similar results, so the simplified formula (30 × weight + 70) is sufficient in this range. However, for medium- and large-breed dogs over 25 kg, the simplified formula increasingly overestimates the Resting Energy Requirement (RER), making the precise formula using the 0.75 exponent more accurate. The precise formula also has less error for cats and extra-small dogs under 2 kg.

How do I calculate the right amount when mixing wet and dry food?

You can check the kcal/100g for each type of food separately and add them up. For example: 60g of dry food (about 220 kcal) + 50g of wet food (about 50 kcal) = 270 kcal in total. The most accurate way is to measure the food with a scale so that you don’t exceed the daily DER target.

How much should I reduce the calories for my senior dog?

After age 7, when activity levels decrease, reduce the DER coefficient from 1.6 to 1.2–1.4. However, it is advisable to maintain the same protein ratio to support muscle preservation. If your pet has arthritis or heart disease, consult your veterinarian about combining therapeutic diets with treatment.

I’m trying to lose weight—how quickly is it safe to do so?

For dogs, a safe weight loss rate is 1–2% of their body weight per week, while for cats it’s 0.5–1% per week. Rapid weight loss can lead to muscle wasting and hepatic lipidosis in cats. If the target weight loss is substantial, we recommend planning for a gradual 3–6 month timeline.

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References

[1] Fascetti AJ, Delaney SJ. Applied Veterinary Clinical Nutrition, 2nd Ed. Wiley-Blackwell, Chapter 8: Commercial and Home-Prepared Diets

[2] National Research Council (NRC). Nutrient Requirements of Dogs and Cats, 2006

[3] Ettinger SJ, Feldman EC. Clinical Medicine of the Dog and Cat, 4th Ed. Section V: Elements of Therapy