How to Calculate RER and Daily Calories for Dogs and Cats

How to Calculate RER and Daily Calories for Dogs and Cats 🐶🐱🍽️

By Dr Duncan Houston

🔎 Quick Answer

Resting Energy Requirement (RER) is calculated as:
👉 RER = 70 × (body weight in kg ^ 0.75)

Quick version:
👉 RER = 30 × body weight (kg) + 70

To convert energy:
👉 1 kcal = 4.184 kJ
👉 1 kJ = 0.239 kcal

Feeding your pet properly is one of the most important things you can get right.

Too much → obesity
Too little → poor health

And the confusion usually starts with calories vs kilojoules.

Let’s make it simple.

🧠 What Is RER?

RER is the number of calories your pet needs just to:

• stay alive

• run organs

• maintain basic function

👉 It is your baseline energy requirement

📐 How to Calculate RER

🔬 Accurate formula:

RER = 70 × (body weight in kg ^ 0.75)

⚡ Simple formula:

RER = 30 × body weight (kg) + 70

👉 Works well for most pets

📊 Example RER Calculations

🐶 10 kg dog:

RER = 30 × 10 + 70 = 370 kcal/day

👉 In kilojoules:
370 × 4.184 = ~1548 kJ/day

🐱 4 kg cat:

RER = 30 × 4 + 70 = 190 kcal/day

👉 In kilojoules:
190 × 4.184 = ~795 kJ/day

🔁 Calories vs Kilojoules (Important)

Pet foods may use either.

Conversion rules:

👉 kcal → kJ
Multiply by 4.184

👉 kJ → kcal
Divide by 4.184

Example:

Food label says:
👉 1500 kJ per cup

Convert to kcal:
1500 ÷ 4.184 = ~358 kcal

🍽️ What Is MER?

MER = what you actually feed

👉 MER = RER × multiplier

📈 MER Multipliers

🐶 Dogs

🐱 Cats

📊 Example Full Calculation

🐶 10 kg neutered dog

RER = 370 kcal
MER = 370 × 1.6 = ~590 kcal/day

👉 In kJ:
590 × 4.184 = ~2468 kJ/day

🐱 4 kg neutered cat

RER = 190 kcal
MER = 190 × 1.2 = ~230 kcal/day

👉 In kJ:
230 × 4.184 = ~962 kJ/day

⚖️ Weight Loss Calculations

Always use target weight, not current weight.

Example:

Dog = 30 kg but should be 25 kg

Use:
👉 25 kg in RER

Then:
👉 Multiply by ~1.0

🍗 Converting Calories to Food

Example:

Dog needs:
👉 590 kcal/day

Food = 350 kcal per cup

👉 590 ÷ 350 = ~1.7 cups/day

If using kJ:

Food = 1500 kJ per cup
Dog needs 2468 kJ

👉 2468 ÷ 1500 = ~1.6 cups/day

⚠️ Common Mistakes

❌ Mixing kcal and kJ incorrectly
❌ Feeding based on overweight body weight
❌ Not adjusting after desexing
❌ Treat overload
❌ Not reassessing regularly

🧪 Special Situations

🐾 Sick pets

May need adjusted calories

🐕 Working dogs

👉 Up to 2–5× normal

🐱 Seniors

Lower calories but often need better protein

🩺 When to Recalculate

• weight change

• diet change

• illness

• activity change

👉 Always adjust based on body condition

💬 Final Thoughts

RER gives you a starting point
MER gives you your feeding target

But the real key is:
👉 adjust based on your pet

Because the perfect calculation means nothing if your pet is:

• gaining weight

• losing weight

• or not thriving

❓ FAQ

Is kcal the same as Calorie on food labels?

Yes. 1 kcal = 1 Calorie (capital C)

Should I use kcal or kJ?

Either is fine, just be consistent and convert correctly.

How often should I check my pet’s weight?

Every 2–4 weeks when adjusting diet.

What is the biggest feeding mistake?

Overfeeding, especially with treats.

If you want help calculating your pet’s exact calorie needs or converting between kcal and kJ for specific foods, the ASK A VET™ app can help you build a tailored feeding plan that actually works in real life.