How we work out the numbers

We do not just use the middle of the breed's weight range, because a big-framed Labrador can be perfectly lean at a weight that would be heavy for a small one. Instead, two answers are turned into a 9-point body condition score (4 to 5 is ideal): whether you can feel the ribs, which is how vets check, and the body shape picture. Each point above 5 is treated as about 10% excess weight, and the estimate is kept inside a plausible range for the breed. Where the two answers disagree, or the dog's weight is above the breed's own range, the target is shown as a wider range instead of a falsely precise number: this follows published research showing even vets often disagree on body condition by a point, and that owners of overweight dogs commonly rate them as ideal. Breed ranges (206 breeds and sizes) are the adult healthy ranges from the American Kennel Club and Kennel Club breed standards. Greyhounds and other sighthounds are healthy with a deep tuck and faintly visible ribs, so the "very tucked" shape counts as normal for them.

Resting energy is 70 × weight^0.75 kcal a day. For weight loss the plan feeds the resting energy of the healthy weight, the standard veterinary starting point. To keep weight steady it multiplies resting energy by 1.2 to 2.0 depending on activity, and by a little more for dogs that are not desexed (about 1.6 for a typical desexed adult and 1.8 for an entire one). Seniors get 10% less. Puppies get twice their resting energy as a growth allowance and are never put on a weight-loss plan. Grams per meal use the calories from the food label if the owner enters them, otherwise a typical value for that kind of food (for example about 3.3 kcal per gram for dry kibble), and the plan says which. Treats are capped at 10% of daily calories.

A safe rate of loss is 1 to 2% of body weight a week, so the time to reach the target is shown as a range, with a checkpoint at week 4. The single figure used inside the plan assumes about 1.2% a week.

The starting numbers are estimates, so the plan is meant to be corrected by the dog's own result. When you log a weight, we compare the pace with the safe 1 to 2% a week: on track keeps the portions, slower trims them by 5 or 10%, faster adds 5% back, much too fast (over 3% a week) or unexplained weight loss says to see your vet, and reaching the goal switches to a keep-weight budget. We never advise going below 80% of the resting energy at the healthy weight. After a couple of weigh-ins, the "time to reach target" figure also switches from the general guideline to your own dog's measured pace. We send a few reminder emails to re-weigh, with a link to stop them.

The years are shown as a range. For 12 breeds we use a study of 50,787 pet dogs, which found overweight dogs had a shorter median life span than normal-weight dogs of the same breed: from about 5 months for large breeds to 2.5 years for toy breeds. For other breeds we estimate from the size class the study covered, which is why the result says "up to". A dog that is only mildly over gets a share of the figure. This is an association from observational data, not a promise that losing weight will add that time. Separately, in a 14 year study of Labrador retrievers, dogs fed 25% less than their littermates lived a median 1.8 years longer.