Pet Food Portion Calculator (RER/MER)

Quick answer

Science-based daily food portions for dogs and cats: RER = 70 × kg^0.75, MER factor 0.8-2.0 by life stage, BCS-9 adjustment — exact grams and cups per day.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How the Pet Food Portion Calculator Works

RER = 70 × weight(kg)^0.75; MER = RER × factor (0.8 weight loss … 2.0 growth); grams = MER ÷ kcal per gram

Every portion starts from resting energy: RER = 70 × body weight in kg raised to the power 0.75, the energy a pet burns at rest. Daily needs (MER) multiply RER by a life-stage factor — about 0.8 for a slimming cat, 1.0 for a weight-loss dog, 1.2 for a neutered cat, 1.6 for a typical neutered adult dog, up to 2.0 for growth and hard-working dogs. A body-condition check tunes it: each BCS point above the ideal 5/9 means roughly 10-15% over ideal weight, so the target weight, not the current weight, drives the math. Worked example: a 10 kg dog at ideal weight has RER = 70 × 10^0.75 ≈ 394 kcal; at factor 1.6 that is 630 kcal/day, which is 157 g of a 4 kcal/g food, split over two meals.

1
Step 1
Enter species, current weight and body condition score (1-9)
2
Step 2
Choose the life stage: weight loss, neutered adult, intact, senior, growth or working
3
Step 3
Type your food's calorie density — kcal/kg or kcal per cup from the label
4
Step 4
Read the daily portion in grams and cups, and the split per meal

Use Cases

Setting exact daily grams after a vet flags a BCS of 7/9 or higher

Portioning a new food with a different calorie density than the old one

Building a weight-loss plan that keeps meals satisfying but in deficit

Splitting daily calories correctly across multi-pet households

Tips

1
Portion to the target weight, not the current weight, when BCS is high
2
Weigh food on a 0.1 g kitchen scale — cups can be off by 20%
3
Count treats inside the daily calories, never on top
4
Puppies under 4 months need their portion split across 3-4 meals

Common Mistakes

Feeding by cup scoops that vary with every fill
Using the current weight of an overweight pet for RER
Forgetting to re-portion after neutering drops the factor
Mixing foods without converting calorie density first

FAQs

What is RER and why 70 × kg^0.75?

Resting energy requirement is the energy a pet burns at rest in a day. The 70 × kg^0.75 formula is the allometric equation veterinary nutritionists use because energy needs scale with body surface (the 0.75 power), not linearly with weight — doubling weight does not double RER.

Which MER factor fits my pet?

Common factors: 0.8 for feline weight loss, 1.0 canine weight loss, 1.2 neutered cat, 1.4 intact cat, 1.6 neutered adult dog, 1.8 intact dog, 2.0 for puppies 4-12 months and working dogs. When in doubt start at the neutered value and adjust by weigh-ins.

How does body condition score change the portion?

Each BCS point above 5/9 on the nine-point scale equals roughly 10-15% above ideal weight. The calculator portions to the target weight from your BCS entry, so an 8/9 dog is fed for a smaller ideal weight — that is what makes the deficit safe.

Why does the pack's feeding guide disagree with my result?

Bag charts are averages for a typical pet and often round generously. Label guides assume factor ~1.6-2.0 and no treats. If your weigh-ins show gain or loss, trust the scale trend over the chart.

How often should I re-run the portion calculation?

Re-check every 2-4 weeks during weight-loss plans and after any weight change of half a kilogram or more, a food switch, or a life-stage change such as neutering or leaving growth.