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

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🔎 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

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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.

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🧠 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

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📐 How to Calculate RER

🔬 Accurate formula:

RER = 70 × (body weight in kg ^ 0.75)

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⚡ Simple formula:

RER = 30 × body weight (kg) + 70

👉 Works well for most pets

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📊 Example RER Calculations

🐶 10 kg dog:

RER = 30 × 10 + 70 = 370 kcal/day

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

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🐱 4 kg cat:

RER = 30 × 4 + 70 = 190 kcal/day

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

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🔁 Calories vs Kilojoules (Important)

Pet foods may use either.

Conversion rules:

👉 kcal → kJ
Multiply by 4.184

👉 kJ → kcal
Divide by 4.184

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Example:

Food label says:
👉 1500 kJ per cup

Convert to kcal:
1500 ÷ 4.184 = ~358 kcal

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🍽️ What Is MER?

MER = what you actually feed

👉 MER = RER × multiplier

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📈 MER Multipliers

🐶 Dogs

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🐱 Cats

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📊 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

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🐱 4 kg neutered cat

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

👉 In kJ:
230 × 4.184 = ~962 kJ/day

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⚖️ 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

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🍗 Converting Calories to Food

Example:

Dog needs:
👉 590 kcal/day

Food = 350 kcal per cup

👉 590 ÷ 350 = ~1.7 cups/day

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If using kJ:

Food = 1500 kJ per cup
Dog needs 2468 kJ

👉 2468 ÷ 1500 = ~1.6 cups/day

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⚠️ Common Mistakes

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

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🧪 Special Situations

🐾 Sick pets

May need adjusted calories

🐕 Working dogs

👉 Up to 2–5× normal

🐱 Seniors

Lower calories but often need better protein

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🩺 When to Recalculate

• weight change

• diet change

• illness

• activity change

👉 Always adjust based on body condition

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💬 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

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❓ FAQ

Is kcal the same as Calorie on food labels?

Yes. 1 kcal = 1 Calorie (capital C)

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Should I use kcal or kJ?

Either is fine, just be consistent and convert correctly.

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How often should I check my pet’s weight?

Every 2–4 weeks when adjusting diet.

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What is the biggest feeding mistake?

Overfeeding, especially with treats.

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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.