How the calculator works
The calculator uses the widely cited veterinary formula for Resting Energy Requirement (RER): RER = 70 × (body weight in kg)0.75. That is multiplied by a life-stage factor to estimate Maintenance Energy Requirement (MER), the calories your pet needs per day. From your food’s calorie density it works out grams (and cups) per day, how many days a bag lasts, and the cost per day, month (365.25 ÷ 12 days) and year.
Life-stage factors used
| Life stage | Dog | Cat |
|---|---|---|
| Adult, spayed/neutered | 1.6 | 1.2 |
| Adult, intact | 1.8 | 1.4 |
| Inactive / prone to weight gain | 1.2 | 1.0 |
| Weight loss (at target weight) | 1.0 | 0.8 |
| Active / light work | 2.0 | — |
| Senior | 1.4 | 1.1 |
| Puppy under 4 months / kitten | 3.0 | 2.5 |
| Puppy, 4 months to adult | 2.0 | — |
Approximate daily calories by weight
| Weight | RER | Neutered adult dog (×1.6) | Neutered adult cat (×1.2) |
|---|---|---|---|
| 5 lb (2.3 kg) | 129 | 207 | 155 |
| 10 lb (4.5 kg) | 218 | 348 | 261 |
| 15 lb (6.8 kg) | 295 | 472 | 354 |
| 20 lb (9.1 kg) | 366 | 585 | 439 |
| 30 lb (13.6 kg) | 496 | 794 | — |
| 40 lb (18.1 kg) | 615 | 985 | — |
| 50 lb (22.7 kg) | 727 | 1,164 | — |
| 60 lb (27.2 kg) | 834 | 1,335 | — |
| 70 lb (31.8 kg) | 936 | 1,498 | — |
| 80 lb (36.3 kg) | 1,035 | 1,656 | — |
| 100 lb (45.4 kg) | 1,223 | 1,958 | — |
Why compare cost per day, not price per bag
Two bags at the same price can cost very different amounts to feed. A food with more calories per kg means smaller portions, so the bag lasts longer. Use the “Compare a second food” option to put two foods side by side using your own prices and label figures – the calculator shows which is cheaper per day and the yearly difference.
Where to find kcal per kg and kcal per cup
In the US, dog and cat food labels carry a “Calorie content (calculated)” statement, for example “3,600 kcal/kg, 380 kcal/cup”. If you only have kcal per cup, weigh one level cup of the food in grams so the calculator can convert cups to bag weight.
Portion control and automatic feeders
Overfeeding is the easiest way to blow a food budget – and the most common cause of weight gain in pets. Measuring meals with a kitchen scale, rather than an eyeballed scoop, keeps portions consistent with the number above. An automatic feeder goes further: it dispenses set portions on a schedule, which helps with weight-loss plans, multiple small meals, and feeding while you’re out. A smart water fountain encourages drinking, which matters most for cats on dry food.
See all our picks: smart feeder reviews and smart water fountain reviews.
Automatic feeder
PETLIBRO Granary (camera model)
Schedules portioned meals from an app and includes a 1080p camera so you can see that meals are actually eaten. A good fit for portion-controlled diets and busy schedules.
Smart water fountains
PETLIBRO Dockstream
Stainless-steel tray fountain with app monitoring of water levels – a widely used pick for cats and small dogs.
Catit PIXI Smart Fountain
App-connected smart water fountain designed for cats, from the Catit PIXI range.
PETKIT Eversweet Ultra
Premium filterless cat fountain with a built-in AI camera for monitoring drinking habits.
FAQ
How many calories does my dog or cat need?
A common starting estimate is RER = 70 × kg0.75, multiplied by a life-stage factor (1.6 for a neutered adult dog, 1.2 for a neutered adult cat). It is approximate; your vet can tailor it.
How do I work out how long a bag lasts?
Divide the bag weight by the amount fed per day. Daily amount in grams = daily calories ÷ kcal per kg × 1000.
Is a cheaper bag always cheaper to feed?
No. Calorie-dense food means smaller portions, so a pricier bag can cost less per day. Compare cost per day.