Convert to kg
4 kg = 4.00 kg
The RER formula requires body weight in kilograms.
Vet-Standard Formula · NRC/AAFCO Approved · Full Calculation Shown
Cat RER Calculator: Resting Energy Requirement for Cats
RER is the foundation of feline nutrition math. Calculate your cat's resting energy requirement with the gold-standard 70 x W^0.75 formula, compare it with the linear shortcut, and see every step behind the number.
RER = 70 x W(kg)^0.75Individual cats can vary by +/-50%MER, weight loss, and recovery targets start with RER
Live RER Result
Luna's resting baseline
Recommended RER
198
kcal/day
Current MER
317
kcal/day
Typical RER Range
158-238
kcal/day
Neutered adult cat: RER x 1.6 = 317 kcal/day.
Calculator
Calculate your cat's RER instantly
Use current body weight for general RER. Use ideal weight when planning weight loss so the calorie target does not reward excess body fat.
Transparent Math
How we calculated this RER
Calculate metabolic body weight
4.00^0.75 = 2.8284
The 0.75 exponent reflects Kleiber's Law: metabolism scales with metabolic body weight, not straight body weight.
Multiply by 70
70 x 2.8284 = 198
70 is the empirical mammalian constant used in veterinary RER calculations.
Apply MER multiplier
198 x 1.6 = 317
Neutered adult cat. MER adds life stage, activity, reproduction, or recovery needs beyond rest.
MER Scenarios
All MER scenarios for Luna
Based on RER = 198 kcal/day. These multipliers turn a resting baseline into daily needs for growth, adults, seniors, weight change, and recovery.
| Life stage / scenario | Multiplier | MER | Typical Range | Note |
|---|---|---|---|---|
| Kitten < 4 months | 3x | 594 kcal | 475-713 kcal | - |
| Kitten 4 months-1 year | 2x | 396 kcal | 317-475 kcal | - |
| Pregnant cat | 2x | 396 kcal | 317-475 kcal | - |
| Nursing cat | 2.5x | 495 kcal | 396-594 kcal | - |
| Intact adult cat | 1.8x | 356 kcal | 285-428 kcal | - |
| Neutered adult cat ← current | 1.6x | 317 kcal | 253-380 kcal | Most common adult baseline |
| Senior cat (7-11y) | 1.4x | 277 kcal | 222-333 kcal | - |
| Geriatric cat (11y+) | 1.1x | 218 kcal | 174-261 kcal | - |
| Weight gain | 1.7x | 337 kcal | 269-404 kcal | - |
| Weight loss | 1x | 198 kcal | 158-238 kcal | Use ideal weight for RER |
| Post-surgery / hospital | 1x | 198 kcal | 158-238 kcal | RER only, no multiplier |
RER Education
RER vs MER vs DER: what changes?
RER
Resting baseline
Resting Energy Requirement is the calories needed for breathing, circulation, body temperature, organ work, and cellular repair at rest. It excludes activity, digestion, growth, pregnancy, and lactation.
MER
Daily target
Maintenance Energy Requirement multiplies RER by a life-stage factor. A neutered adult commonly uses 1.6x, while kittens and nursing cats need much higher multipliers.
DER
Practical usage
Daily Energy Requirement is often used like MER in feeding guides. It is the real-world daily target after adjusting for life stage, activity, health, and weight goals.
Formula Detail
Why W^0.75 beats straight body weight
The 0.75 exponent comes from Kleiber's Law, the observation that metabolic rate scales with metabolic body weight instead of raw mass. A larger cat does not need exactly twice the calories of a cat half its size, because heat loss, organ demand, and surface area do not scale linearly.
The linear shortcut, 30W + 70, is useful for mental math in small animals, but the exponential formula is the preferred transparent baseline when building MER, recovery, or weight-loss calculations.
Exponential formula
RER = 70 x Wkg^0.75
Most accurate for a broad range of cat sizes because it uses metabolic body weight.
Linear approximation
RER ≈ 30 x Wkg + 70
Easy to calculate quickly. For a 4 kg cat, it returns 190 kcal/day versus 198 kcal/day from the exponential formula.
Post-surgery use
Initial feeding = RER x 1.0
Hospitalized or post-surgery cats often start at RER only because activity is minimal and overfeeding can be counterproductive.
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Resting energy baseline
RER is the first-principles number behind every feeding target
Resting Energy Requirement is the calories a cat needs before activity, growth, pregnancy, lactation, recovery, or weight goals are added. The calculator makes that baseline visible by showing both the gold-standard exponential formula and the simpler linear approximation.
Result guide
How to read the RER result
The recommended RER is the exponential result from 70 x Wkg^0.75. The linear result is a useful shortcut, but RER should still be treated as a baseline, not the final daily feeding target for most healthy cats.
- Use the exponential result when accuracy matters or when comparing MER scenarios.
- Use the linear estimate only as a quick approximation for ordinary small-animal weights.
- Use ideal weight for weight-loss RER rather than current overweight body weight.
- Use the MER table to understand how life stage turns RER into a practical daily target.
RER is not a diet plan by itself
RER excludes activity, digestion, growth, reproduction, and most daily movement. Adult maintenance, kitten growth, lactation, and recovery all require different interpretation.
The 0.75 exponent matters
Metabolic demand does not scale in a straight line with body weight. Wkg^0.75 prevents larger cats from being overestimated and smaller cats from being underestimated.
Ranges are part of the result
The +/-50% range is intentionally visible because thyroid status, muscle mass, health, environment, and individual metabolism can move a cat away from the calculated baseline.
RER decisions
Use the baseline to choose the next calculator or care step
RER is most useful when it explains which downstream decision you need next. A resting baseline can become a MER target, a food portion plan, a weight-loss discussion, or a recovery feeding question.
Maintenance planning
A healthy adult usually needs MER, not bare RER. Neuter status and life stage should decide whether the daily target is closer to 1.6x, 1.8x, or another multiplier.
Open the calorie calculator to turn RER into a full MER feeding target.
Weight-loss planning
Weight-loss RER should be based on ideal weight. If body condition is high, the plan should also account for hepatic lipidosis risk and slow weekly loss.
Use BCS or ideal-weight tools before reducing portions.
Recovery or hospitalization
Post-surgery cats are often started around RER x 1.0, but appetite, nausea, pain, and diagnosis can change the feeding plan.
Use the RER number as discussion context for veterinary recovery instructions.
Weekly after changes
Check weight weekly when calories or feeding amounts change.
Every life-stage shift
Recalculate when a kitten matures, a cat is neutered, pregnancy starts, lactation ends, or senior status changes.
Immediately for illness
Escalate appetite loss, rapid weight change, vomiting, weakness, or post-surgery feeding problems to a veterinarian.
How to use RER without overreacting
1
Start with current purpose
Decide whether you are learning the resting baseline, building a maintenance target, planning weight loss, or supporting recovery.
2
Apply the correct multiplier
Use the MER table for life stage and goal context instead of applying one adult multiplier to every cat.
3
Validate with trend
RER and MER are estimates. Weekly weight checks and body condition changes are the feedback loop that tells you whether the number fits the individual cat.
FAQ
Frequently Asked Questions About Cat RER
What is the RER formula for cats?
The standard RER formula for cats is RER = 70 x body weight in kg^0.75. For example, a 4 kg cat has RER = 70 x 4^0.75 = 198 kcal/day. The simpler linear approximation is RER approximately 30 x weight in kg + 70.
What is the difference between RER and MER in cats?
RER is the calories needed at rest for basic life functions. MER is RER multiplied by a life-stage factor for activity, digestion, growth, reproduction, recovery, or weight goals.
How do I calculate my cat's RER?
Weigh your cat in kilograms, raise that weight to the 0.75 power, then multiply by 70. A 5 kg cat has 5^0.75 = 3.34, so RER is about 234 kcal/day.
Why is RER calculated using weight to the power of 0.75?
The 0.75 exponent reflects metabolic body weight, sometimes explained through Kleiber's Law. Metabolism does not scale in a straight line with body weight, so W^0.75 gives a better baseline than W^1.0.
What RER multiplier should I use for my cat?
Common MER multipliers include 1.6x for a neutered adult, 1.8x for an intact adult, 1.0x for weight loss based on ideal weight, 1.7x for weight gain, 3.0x for kittens under 4 months, and 2.5x for nursing cats.
How is RER used for post-surgery cats?
Veterinarians often use RER x 1.0 as the initial feeding target for hospitalized or post-surgery cats because activity is low and feeding usually ramps up gradually during recovery.