Cat RER Calculator

RER (Resting Energy Requirement) is the energy a cat needs at rest. It is estimated as 70 × body weight in kg^0.75. This cat RER calculator estimates your cat’s Resting Energy Requirement from body weight alone, using the veterinary equation RER = 70 × body weight in kg^0.75. Enter the weight in kilograms or pounds to see the RER and the full working. RER is a baseline estimate of the energy needed for essential body functions at rest, not your cat’s final daily calorie target. For a daily calorie estimate based on life stage and profile, use the Cat Calorie Calculator.

Last updated: 26 September 2026 · Educational tool, not veterinary advice

Your cat’s RER

Enter your cat’s weight and press Calculate RER to see the Resting Energy Requirement in kcal/day.

How to Use the Cat RER Calculator

Enter your cat’s weight.
Select kg or lb.
Press Calculate RER.
Review the RER estimate and the formula working.
Need daily calories? Use the Cat Calorie Calculator for the next step.

What Is RER for Cats?

RER stands for Resting Energy Requirement. The Pet Nutrition Alliance describes it as a function of metabolic body size used to estimate the daily energy needed to sustain essential bodily functions while the cat is at rest, and states that the RER equation is the same for cats and dogs. Today’s Veterinary Nurse describes RER as the energy required for essential physiological functions such as digestion, respiration, and cardiac and brain activity.

RER is useful because it gives a consistent, weight-based starting point that veterinary energy calculations build on. On its own it doesn’t account for growth, activity or the individual cat, so it isn’t the same as your cat’s daily calorie needs.

Cat RER Formula

RER (kcal/day) = 70 × (body weight in kg)^0.75

Weight must be in kilograms, so pounds are divided by 2.2046226218 first. The calculator uses full precision and rounds only the displayed result.

The 0.75 power means energy rises more slowly than weight. An 8 kg cat weighs twice as much as a 4 kg cat, but its RER is only about 1.7 times higher (333 vs 198 kcal/day). Smaller cats use more energy per kilogram than larger ones.

Illustrative example

Worked Example: RER for a 4 kg Cat

Weight: 4 kg (about 8.8 lb)
4^0.75 ≈ 2.828
RER = 70 × 2.828 ≈ 198 kcal/day

198 kcal/day is the RER estimate. It is not automatically this cat’s final daily calorie recommendation.

Cat RER by Weight: Example Table

Illustrative Cat RER Estimates Using 70 × kg^0.75
Weight (kg)	Approx. weight (lb)	RER (kcal/day)
1	2.2	70
2	4.4	118
3	6.6	160
4	8.8	198
5	11.0	234
6	13.2	268
7	15.4	301
8	17.6	333
10	22.0	394

These are RER estimates, not universal feeding or daily calorie recommendations.

RER vs Daily Calorie Needs for Cats

RER is a resting baseline calculated from weight alone.
A daily calorie estimate usually starts with RER and multiplies it by one feline factor that fits the cat’s life stage and profile.

For example, a 4 kg cat’s RER is about 198 kcal/day. A separate daily-energy calculation for that cat can give a different result once an appropriate profile factor is applied. That step is what the Cat Calorie Calculator does.

Is RER the Same as MER?

No. The Pet Nutrition Alliance and the Merck Veterinary Manual use MER (maintenance energy requirement) for total daily energy, estimated by multiplying RER by an appropriate factor. Today’s Veterinary Nurse calls a similar figure DER (daily energy requirement): RER modified by a factor for life stage and activity. The names vary between sources, but the takeaway doesn’t: RER is the baseline, and the daily estimate is a separate step.

Does Spaying or Neutering Change a Cat’s RER?

No. The RER equation uses body weight only, so a spayed or neutered cat and an intact cat of the same weight share the same RER. Reproductive status matters later, when a daily energy estimate is made. That’s why the Cat Calorie Calculator asks about it and this calculator doesn’t.

Does Activity Change a Cat’s RER?

No, the formula stays the same. A playful cat and a sleepy cat of the same weight share the same RER, and indoor or outdoor living doesn’t change it either. Lifestyle and activity can affect practical daily energy needs, which are estimated in a separate step.

Do Kittens Use a Different RER Formula?

No. The sources used here apply the same RER equation regardless of life stage. Growth is accounted for afterwards, when a daily energy estimate is made: Merck lists 2.5 × RER for kittens and the Pet Nutrition Alliance gives a growth range of 2–3 × RER. This calculator doesn’t apply either; it shows the baseline only.

For a kitten growth estimate and food portions, use the Kitten Feeding Calculator.

Do Senior Cats Use a Different RER Formula?

No. There’s no separate senior RER equation or senior multiplier in the sources used here. Older cats vary widely in body condition, muscle condition, activity and health, which can all affect their practical calorie needs. Their RER is still calculated from weight in the same way.

Can RER Be Used for Cat Weight Loss?

RER isn’t a weight-loss target or a feeding prescription. Safe feline weight loss needs an individual plan, an appropriate method and regular monitoring. Cats shouldn’t be put on severe or abrupt food restriction: the Merck Veterinary Manual explains that feline hepatic lipidosis (fatty liver disease) is associated with a period of poor appetite lasting from a few days to several weeks, especially in obese cats. If your cat needs to lose weight, ask your vet for a plan.

Cat Weight Loss Calculator

Why Your Cat’s Actual Calorie Needs May Differ From RER

RER is a calculated baseline. A cat’s real daily needs also depend on life stage, reproductive status, body condition, activity and lifestyle, health, and individual variation. For a fuller explanation, read How Many Calories Does My Cat Need?

If you already have a daily calorie estimate and want to turn it into grams, cups, cans or pouches of your food, use the Cat Food Calculator.

Cat RER: Key Takeaways

RER means Resting Energy Requirement.
RER is calculated from body weight only; pounds are converted to kilograms first.
The formula is RER = 70 × kg^0.75, the same equation used for dogs.
RER is not automatically your cat’s final daily calories.
Life stage and profile factors belong to daily calorie estimation.
Use the Cat Calorie Calculator for the next step.

Cat RER Calculator FAQ

What is RER for cats?

RER is the Resting Energy Requirement: an estimate, based on body weight, of the daily energy needed to sustain essential body functions at rest. It is a baseline, not a daily feeding amount.

How do I calculate my cat’s RER?

Convert weight to kilograms, raise it to the power 0.75, then multiply by 70. RER (kcal/day) = 70 × kg^0.75.

What is the RER of a 4 kg cat?

About 198 kcal/day: 70 × 4^0.75 ≈ 70 × 2.828 ≈ 198.

Is RER the same as daily calorie needs?

No. Daily calorie needs are usually estimated by multiplying RER by a feline factor for life stage and profile. The Cat Calorie Calculator does this.

Is RER the same as MER?

No. MER (maintenance energy requirement) is estimated by multiplying RER by an appropriate factor. Some sources call this DER (daily energy requirement).

Does spaying or neutering change RER?

No. RER depends on weight only. Neuter status is considered later, when estimating daily energy.

Do kittens use a different RER formula?

No. The same equation is used; growth is accounted for afterwards with a growth factor in the daily energy estimate.

Can I use RER to decide how much food to feed my cat?

Not on its own. First estimate daily calories with the Cat Calorie Calculator, then convert calories into food with the Cat Food Calculator.

Sources & Methodology

RER = 70 × kg^0.75, calculated at full precision and rounded to the nearest kcal for display. No energy factor is applied. The 0.1–20 kg input range is only a typing-error safeguard, not a biological limit for cats.

Pet Nutrition Alliance: Calculating Calories Based on Pet Needs. RER definition and equation (the same for cats and dogs); MER as RER × an appropriate factor; feline growth range.
Today’s Veterinary Nurse: Energy Calculations (2022). RER description; the equation for dogs and cats of any size; DER as RER × a life-stage/activity factor.
Merck Veterinary Manual: Daily Maintenance Energy Requirements for Dogs and Cats. Cat MER expressed as multiples of RER, including kittens at 2.5 × RER; calculations are a starting point.
Merck Veterinary Manual (Cat Owners): Disorders of the Liver and Gallbladder in Cats. Hepatic lipidosis and its association with poor appetite.

Educational purpose: PawCalcHub provides general estimates for education. They are not veterinary advice, diagnosis or treatment. This page has not been reviewed by a veterinarian.