Do neutered pets need less food?

A 30 kg dog drops from about 1,615 kcal a day to about 1,436 once neutered, by the Merck Veterinary Manual's factors. That's 11% less. PDSA's neutering guide says "around 30% less calories", which would mean about 1,131 kcal. So the answer is yes, and the size of the cut depends on who you read.

Cats show the same split. For a 4 kg cat, Merck's factors give about 277 kcal intact and about 238 neutered, or 14% less. PDSA's cat guide says approximately 25% less, about 208 kcal.

These are sums from published factors. I've read the sources named here and haven't measured any animal, so the opinions below are mine alone.

Why the two figures differ

Merck's numbers are 70 x kg^0.75, multiplied by 1.8 for an intact dog and 1.6 for a neutered one, or 1.4 and 1.2 for cats. It calls any such sum a starting point, and says one animal may need as much as 30% more or less than another of the same weight. PDSA gives a rule of thumb for owners and doesn't show its working. It pairs that with practical advice: smaller portions, or a food made for neutered pets.

I wouldn't read them as a real conflict. Merck's factor comes from a table of adult animals in general. PDSA's advice comes with an instruction to watch the dog and talk to your vet or vet nurse if it gains weight. Between them, they bracket where to start.

Appetite counts too

It's easy to assume neutering just slows the body down. For cats, the research I read points at the bowl as well.

Jennifer Larsen of UC Davis reviewed the evidence in 2017. She calls increased food intake the most likely factor in weight gain after neutering. One study she cites found neutered cats gained 30.2% against 11.8% for intact males, and 40% against 16.1% for females. Another found male cats eating significantly more within 3 days, with weight significantly higher by 7 weeks. FEDIAF, the European pet food trade body, says much the same: neutering can affect a cat's ability to feel full.

So a cat may ask for more just as it needs less. Larsen also says that when cats are fed freely after neutering, weight gain is likely. That's the case for measuring meals. International Cat Care says neutered cats are more prone to weight gain, and Cats Protection says they won't need to eat as much.

What that means in the bowl

Take Hill's Science Plan Sterilised Cat, with 3,570 kcal/kg stated by Pets at Home.

Hill's own table on that page says 60 g for a 4 kg adult, about 214 kcal. That lands near PDSA's number.

Over a year, 78 g a day is 28.3 kg and 67 g a day is 24.3 kg. At £27.89 for 3 kg (£9.30/kg), the regular price read on 2 November 2026, that's about £263 against about £226. A nice side effect, if your cat will put up with it.

Do you need a special food?

PDSA says you can either cut portions or buy a food for neutered pets, and Cats Protection says the same for cats. The difference may be smaller than the label suggests. Hill's Sterilised has 3,570 kcal/kg and Whiskas 1+ Adult Chicken has 3,760 kcal/kg, about 5% apart, both as stated by Pets at Home. Per calorie, Hill's costs £2.60 per 1,000 kcal and Whiskas £0.84 per 1,000 kcal. Whether Hill's is worth that is a separate question, covered in the Hill's Sterilised review.

There's one catch with cutting a normal food by a lot. Merck warns that feeding an adult maintenance food in restricted amounts also cuts every nutrient in it, and that a food with less energy may be the better answer when a pet needs 10% to 30% less than the label says. If the cut you're making looks that large, ask your vet.

What I'd do

Weigh the pet before the operation, then every couple of weeks after it. Start near Merck's lower factor, and measure every meal, treats included. If the weight climbs, talk to your vet or vet nurse, as PDSA advises, before moving towards its figure. How much to feed your dog has the weighing routine, and the pet cost calculator uses Merck's neutered factors by default, so you can see the yearly food bill for your pet.

Weight gain or loss that worries you goes to the vet, who can rule out other causes.