Based on: WSAVA Global Nutrition Guidelines. MER multipliers from Thatcher et al. and the NRC Nutrient Requirements of Dogs and Cats (2006).
Understanding RER and MER
Feline caloric calculations start with RER — Resting Energy Requirement, the number of kilocalories a cat needs to maintain basic physiological functions at complete rest. The WSAVA-recommended formula is:
This metabolic scaling formula, derived from Kleiber's Law, accounts for the fact that metabolic rate scales with surface area rather than body mass linearly. A cat weighing 4 kg has an RER of approximately 166 kcal/day.
RER is then multiplied by a MER factor — Maintenance Energy Requirement — that adjusts for real-world conditions. The WSAVA publishes life factors for different physiological states:
- Neutered adult (indoor): 1.2 × RER
- Intact adult: 1.4 × RER
- Active indoor: 1.4 × RER
- Very active / outdoor access: 1.6 × RER
- Kitten (under 1 year): 2.0–2.5 × RER (varies with age)
- Mature/senior (7+ years): 1.1–1.2 × RER (metabolism slows)
- Weight loss target: 0.8 × RER calculated at ideal body weight
- Weight gain target: 1.6–1.8 × RER
These ranges exist because individual variation is real — genetics, muscle mass, thyroid function, and gut microbiome all affect actual caloric needs. The calculator produces a target range, not a single number, for this reason. Monitor body condition score (BCS) over 2–4 weeks and adjust intake accordingly.
This calculator gives estimates for healthy adult cats. Cats with diabetes, kidney disease, hyperthyroidism, or other conditions have specific caloric requirements that should be determined with your vet. Weight loss protocols for significantly overweight cats should always be supervised to avoid hepatic lipidosis.
Why so many cats are overfed
Feline obesity is now the most common nutritional disorder in domestic cats — affecting an estimated 25–40% of the pet cat population in North America and Europe. There are a few structural reasons this happens. First, most cat food packaging recommendations are intentionally generous — they're calculated for an average intact adult at maintenance, which is systematically higher than a neutered indoor cat at rest. Second, neutering reduces metabolic rate by 20–30% in cats, and few owners receive explicit guidance to reduce food portions after the procedure. Third, ad libitum (always-available) dry food feeding is common but incompatible with normal feline food-seeking behaviour — cats evolved as solitary hunters who eat multiple small meals per day, and unrestricted access disrupts this regulatory mechanism.
The practical implications are significant. An average 4.5 kg indoor neutered cat has an RER of approximately 183 kcal/day and an MER of about 220 kcal/day. If the food packaging recommends 275 kcal/day (a typical instruction for an intact or more active cat), the gap of 55 kcal/day compounds across months. Over a year, that's approximately 20,000 excess calories — enough to add roughly 0.5–1 kg of body fat.
Related Lab Notes
The Science of Cat Obesity: Why 40% of Cats Are Overweight
Dry vs. Wet Cat Food: What the Research Actually Says
Why Cats Cannot Survive on Dog Food
Frequently asked questions
RER (Resting Energy Requirement) is the calories a cat needs just to maintain basic bodily functions at rest. Formula: 70 × (body weight in kg)^0.75. All feline nutritional calculations start here.
A neutered indoor adult at 4.5 kg (10 lb) typically needs approximately 200–250 kcal/day. This varies with activity, age, and individual metabolism.
Check the caloric density on your cat food packaging (kcal/kg or kcal/can). Divide your daily kcal target by the food's kcal per gram. Example: 220 kcal ÷ 3.5 kcal/g = 63g per day.
Yes. Neutering reduces metabolic rate by approximately 20–30%. This is a primary driver of feline obesity when food portions are not adjusted post-surgery.
Only under veterinary guidance. Cats can develop hepatic lipidosis (fatty liver disease) from overly aggressive food restriction. Weight loss should be gradual (max 0.5–1% body weight per week) and ideally supervised.