How big will my puppy get?

Why not just use the usual formula?

Most puppy weight calculators divide the weight by the age in weeks and multiply by 52. That assumes a puppy keeps growing at the same rate all year, which no puppy does. Growth is fastest early and slows down, so the formula overestimates, badly: when we tested it against real dogs, it was out by around half at 12 to 26 weeks.

This calculator uses growth charts instead. They come from a study by the University of Liverpool and the WALTHAM Centre for Pet Nutrition, which used the clinic records of around 50,000 healthy dogs weighed repeatedly from puppyhood to two years. The researchers published their raw data, and we fitted our own curves to it. Tested against dogs the curves had not seen, the estimate was typically within about 11% from 12 weeks, 8% from six months and 6% from a year.

What the centile means

A puppy on the 25th centile weighs more than 25 in 100 puppies of the same size and sex at that age. It is not a score, and a puppy on the 10th is not doing worse than one on the 60th. Healthy dogs sit everywhere on the chart.

What matters is staying roughly on the same line. A puppy that drops across two or more of the centile lines between weigh-ins is worth showing to your vet, even if it seems well. One that climbs steadily across the lines may be eating more than it needs.

Puppies at the extremes do tend to drift back towards the middle as they grow, so the projection allows for that instead of assuming a very heavy puppy will be a very heavy adult.

Getting the best estimate

• Pick the size group carefully. It is the biggest source of error. For a crossbreed, go by the expected adult weight rather than either parent’s breed.
• Weigh on the same scales each time, ideally at the vet or a pet shop. Holding a puppy on bathroom scales and subtracting your own weight is good enough to spot a trend but not for a single reading.
• Check again every few weeks. The range narrows as your puppy gets older, because there is less growing left to guess at.

What it cannot do

The dogs in the study were purebreds seen at US clinics, so the curves describe typical growth rather than any one breed. Giant breeds over 40kg were left out of the study because they grow too differently from each other, and puppies under 12 weeks grow too unevenly to project from. Neither is covered here.

It is a guide to growth, not a health check. Body condition tells you more than any chart: you should be able to feel ribs easily under a thin layer of fat. If you are unsure, ask your vet at the next vaccination or check-up. Our first-year timeline covers when those fall.