What is the difference between RER and MER?

RER (Resting Energy Requirement) is the baseline calories needed for basic organ function at complete rest. MER (Maintenance Energy Requirement) is RER multiplied by a life-stage and activity factor — it is the number a healthy, normally active pet actually needs day to day. RER alone is used clinically when lifestyle factors are removed from the equation (e.g. a hospitalised patient).

Is RER = 70 × weight^0.75 accurate for all pets?

The formula is the standard accepted by veterinary nutrition authorities for dogs and cats and gives a reliable metabolic baseline for healthy pets of known weight. It becomes less accurate at very low body weights (under about 2 kg) and in pets with conditions that alter metabolic rate (hyperthyroidism, severe cachexia, etc.). For clinical use in those cases, direct calorimetry or vet-guided targets are more appropriate.

Why does the calculator only ask for weight — no age or breed?

RER is deliberately weight-only. Age, breed, spay/neuter status, and activity level all affect the MER factor applied on top of RER, but RER itself is purely a function of metabolic body size (weight^0.75). Adding those inputs would turn it into a MER calculation, which is what the MER Calculator and main Calorie Calculator are for.

My vet mentioned "caloric restriction to 80% of RER" — what does that mean?

80% of RER is a commonly used starting point for weight-loss feeding in cats (and sometimes dogs) under veterinary supervision. It means feeding 80% of the raw RER number — not 80% of the full MER. Always confirm the exact target with your vet, since the right restriction level depends on the individual patient.

Can I use the RER calculator for exotic pets or pocket pets?

The 70 × weight^0.75 formula is validated for dogs and cats. Some exotic animal veterinarians use a similar allometric formula with different coefficients for other species. For rabbits, birds, ferrets, or reptiles, consult a specialist exotic vet rather than applying this formula directly.

How the RER Calculator Works | PetCalculator

Resting Energy Requirement (RER) is the baseline calorie number vets use for hospitalized and critically ill pets — and the foundation of every other energy calculation. Here is the formula and when RER alone (not MER) is the number that matters.