Feeding calculator

How it works

First we work out the energy your pet needs at rest (RER = 70 × body weight in kg to the power of 0.75). Then we multiply it by a factor for age and lifestyle.

Dividing the daily calories by the energy in the food gives grams. If the packet has no calories, we estimate them from the analytical constituents with the NRC (2006) method that pet food makers use. The estimate can be off by around 10%, so the maker's own figure is better when you can find it.

Before you change anything

• These amounts are a starting point. Individual animals can need up to about 30–50% more or less.
• Weigh your pet and check its body condition every 2–4 weeks, and change the amount in small steps of about 10%.
• If the packet's feeding guide and the calculator disagree, go by your pet's body shape, not either number.
• Large and giant breed puppies need a large-breed puppy food and controlled growth. Ask your vet rather than relying on adult maths.
• Weight loss should be slow: about 1–2% of body weight a week. Never starve a cat — sudden fasting can cause serious liver disease.
• Pregnant, nursing, ill or medicated animals, or those with kidney, liver, heart, gut disease or diabetes: ask your vet for the amount.