Puppy Weight Calculator

How the estimate works

In 2017 researchers from the WALTHAM Petcare Science Institute, the University of Liverpool and Banfield Pet Hospital published growth standards for dogs, built from the vet-visit weights of about 50,000 dogs and grouped by the adult size of the breed. The median curve says what share of its adult weight a typical puppy of each size has reached at each age: a typical male in the 15 to 30 kg band is at about 41% of it at 16 weeks and 65% at 24.

The calculator divides your puppy’s weight by that share. Pick the breed and it uses the band the breed’s typical adult weight falls in; leave it on “cross or not sure” and it uses the band whose weight range agrees with its own answer. Between the ages on the chart it interpolates, and it reads males and females on their own curves, as the study does.

For small breeds the curves keep creeping up in the second year, which is mostly condition rather than growth, so the estimate errs a little heavy. That is the safe direction for buying a crate or a harness.

How far to trust it

Every puppy is an estimate until it is grown. On the study’s own data, even the best method, following a purebred puppy’s line across several weighings, leaves about eight in ten dogs within roughly a quarter of the estimate at 12 weeks and within about 15% at a year. That is the range the calculator prints, and a single weighing does a little worse, so treat it as the least uncertainty there is. The researchers’ follow-up work found that most healthy puppies drift across at least one line of the growth chart as they grow, and that two thirds of the dogs that were obese by three had climbed across two or more lines while growing. Cross-breeds vary more than purebreds.

So weigh the puppy every few weeks, at the same time of day, and watch the estimate settle: WALTHAM’s own guide to its charts suggests three weighings before reading a puppy’s line, and caution with cross-breeds. Before 12 weeks the curves are rougher, and for giant breeds they start at 12 weeks, so the calculator waits for that. A puppy that looks too thin or too round is a question for the vet, not for a calculator.

Why the rules of thumb miss

• Double the weight at 16 weeks. Close for toy and small breeds, 14 to 18% low for medium to extra-large dogs, and 15 to 25% low for giants, which are well short of half grown at four months.
• Weight divided by age in weeks, times 52. Assumes a puppy grows in a straight line for a year. It does not, and the formula comes out 10 to 66% high.
• Two and a half times the weight at 14 weeks. 4 to 10% high for toy and small dogs and 6 to 15% low for the bigger ones.

Buying for the grown dog

The reason to know the adult weight is to buy things once. A crate should be the adult size from the start, with a divider panel moved back as the puppy grows, because a crate with spare floor becomes a toilet at one end: the crate size calculator has the adult size for 46 breeds. A bed is the opposite: puppies chew, so buy a cheap washable one now and the good one, sized with the bed size calculator, once the chewing stage is over. Harnesses are sized by the chest, not the weight, so an adjustable one re-measured every month with the harness size guide beats guessing the adult size.