How to switch your pet to a new food

Going from Royal Canin Maxi Adult to Pedigree Adult Complete, a 30 kg neutered dog needs about 407 g a day of the new food where it had about 364 g of the old. Swap gram for gram and you've cut its calories by about a tenth. None of the switching guides I read mention that.

The sums come from stated calories, 3,941 kcal/kg and 3,530 kcal/kg on zooplus.co.uk, checked 2 November 2026, and the Merck Veterinary Manual's formula, which gives that dog 1,436 kcal a day (70 x 30^0.75 x 1.6). I've read the sources below and haven't run a feeding trial, so the plan is my reading of their advice.

How long it should take

The published advice doesn't agree.

Two charities sit at the ends: Dogs Trust is quickest and PDSA slowest. I'd plan for 10 days, and take PDSA's 14 if your pet has reacted badly before. Nothing is lost by going slowly except a little patience.

The plan

Purina UK's steps are the clearest I read, so I've used them.

1. Work out the full daily portion of each food by calories. Divide your pet's daily calories by the kcal/kg on each bag, or use the numbers on each pack if you trust them. Reading the calories on a label shows where to look.
2. Days 1 and 2: three-quarters of the old portion, plus a quarter of the new portion.
3. Days 3 and 4: half of each.
4. Days 5 to 7: a quarter of the old, plus three-quarters of the new.
5. Days 8 to 10: only the new food.
6. Weigh your pet at the end and adjust the portion. Merck says any given animal may need as much as 30% more or less than another of the same weight.

For that 30 kg dog, the grams come out like this.

Note that the daily total climbs while the calories stay level. That's the point.

Don't buy the full bag of the new food if a smaller one exists. Keep enough of the old food to last the whole change, plus a few days in case you have to slow down. Then compare the running cost with the pet cost calculator, or browse the dry dog food list.

Where the advice differs by animal

Puppies. PDSA and Battersea both say to stick with the breeder's food while the puppy settles, then change slowly. If you're choosing that next bag, puppy food by adult size has the sums.

A more cautious start. PDSA's steps for dogs begin with three days of a teaspoon of the new food on a separate plate, beside the regular meal. Mixing starts from day 4. I'd use that for a dog that has reacted badly before.

Cats. Cats Protection says to add a little of the new food to the usual one, offer more only if the cat eats it, and carry on over a week or more. Its warning is plain. A cat that doesn't eat the new food or becomes unwell goes back on the old food, and you speak to your vet.

When to stop and ring the vet

Purina UK suggests a few more days if a dog isn't adjusting, or a call to your vet. Cats Protection says to call if a cat's eating or drinking suddenly changes. I'd ring sooner rather than later.

Foods prescribed by a vet are outside this plan. PDSA also says sudden changes are out unless your vet advises one, so follow the vet's own instructions if that's your situation.