How the calculator works
Puppies don't grow in a straight line. They put on weight fastest in the first few months, then slow down as they near adult size, and bigger breeds keep growing for longer. So the useful question is: what share of its adult weight has a puppy of this size reached at this age? Divide today's weight by that share and you have an estimate of the adult weight.
The shares come from a peer-reviewed study: Hawthorne, Booles, Nugent, Wilkinson and Gettinby, "Body-weight changes during growth in puppies of different breeds", Journal of Nutrition 134 (2004), 2027S–2030S. Researchers at the WALTHAM Centre for Pet Nutrition weighed 173 puppies of 12 breeds every week, from toy to giant, until about 12 months old (18 months for large and giant breeds). For each breed they fitted an S-shaped growth curve,
weight = adult weight ÷ (1 + e^(−(age − x0) ÷ b)),
where x0 is the age in weeks at which the breed reached half its adult weight and 1/b its early growth rate. The calculator uses the published x0 and growth rate for each breed, sorts the breeds into the same size bands as the dog age calculator by their adult weight, and shows the range across the breeds in the size you choose:
| Size | Adult weight | Breeds in the study |
|---|---|---|
| Small | 20 lb or less (up to 9 kg) | Papillon, Cairn Terrier, Miniature Schnauzer |
| Medium | 21–50 lb (10–23 kg) | Cocker Spaniel, English Springer Spaniel, Beagle |
| Large | 51–90 lb (23–41 kg) | Labrador Retriever |
| Giant | over 90 lb (over 41 kg) | Newfoundland, Great Dane, Irish Wolfhound, St. Bernard, English Mastiff |
The study followed only one large breed, so a large puppy gets a single Labrador-based estimate rather than a range. The calculator works from 8 weeks, the age from which the study weighed every breed, to 78 weeks.
Puppy growth chart
Share of adult weight reached at each age, from each breed's fitted curve in the study. Adult weights are the study's fitted values, rounded. The last column is the age at which the breed reached 99% of its adult weight.
| Breed | Size | Adult weight | 8 weeks | 12 weeks | 16 weeks | 20 weeks | 26 weeks | 36 weeks | 99% grown |
|---|---|---|---|---|---|---|---|---|---|
| Papillon | Small | 5 lb (2.2 kg) | 39% | 53% | 68% | 79% | 90% | 98% | 10 months |
| Cairn Terrier | Small | 14 lb (6.3 kg) | 22% | 36% | 52% | 68% | 86% | 97% | 10 months |
| Miniature Schnauzer | Small | 15 lb (7 kg) | 26% | 41% | 58% | 73% | 88% | 98% | 9 months |
| Cocker Spaniel | Medium | 22 lb (10.2 kg) | 21% | 33% | 48% | 63% | 81% | 95% | 11 months |
| English Springer Spaniel | Medium | 33 lb (14.8 kg) | 24% | 40% | 58% | 74% | 89% | 98% | 9 months |
| Beagle | Medium | 38 lb (17.4 kg) | 24% | 38% | 55% | 71% | 87% | 97% | 10 months |
| Labrador Retriever | Large | 62 lb (28.1 kg) | 19% | 29% | 41% | 55% | 73% | 92% | 12 months |
| Newfoundland | Giant | 95 lb (43.3 kg) | 15% | 25% | 40% | 56% | 78% | 95% | 11 months |
| Great Dane | Giant | 113 lb (51.1 kg) | 20% | 31% | 43% | 56% | 74% | 92% | 12 months |
| Irish Wolfhound | Giant | 116 lb (52.4 kg) | 15% | 25% | 38% | 53% | 74% | 93% | 11 months |
| St. Bernard | Giant | 144 lb (65.2 kg) | 14% | 21% | 31% | 44% | 63% | 87% | 13 months |
| English Mastiff | Giant | 147 lb (66.9 kg) | 16% | 23% | 32% | 42% | 58% | 81% | 15 months |
Even within one size, breeds differ: at 16 weeks a Springer Spaniel was 58% grown and a Cocker Spaniel 48%. That spread is why the calculator gives a range.
Example: a 12-week-old medium puppy
A 12-week-old puppy weighs 12 lb and is expected to be a medium dog.
- Cocker Spaniel curve: 33.1% grown at 12 weeks, so 12 ÷ 0.331 = 36 lb.
- English Springer Spaniel curve: 39.6% grown, so 12 ÷ 0.396 = 30 lb.
- Beagle curve: 38.3% grown, so 12 ÷ 0.383 = 31 lb.
The estimated adult weight is about 30–36 lb, and the puppy is about a third of the way there. Puppies of these breeds reached 99% of adult weight at about 9 to 11 months.
Why not use a simple formula?
You will see quick rules online, such as "double the weight at 4 months" or "divide the weight by the age in weeks and multiply by 52". Each one is a single multiplier for every dog. The measured curves show why that can't fit everyone. Here is what you would need to multiply a 16-week (about 4-month) weight by to get the adult weight in the study:
| Size | Multiply the 16-week weight by |
|---|---|
| Small | 1.5–1.9 |
| Medium | 1.7–2.1 |
| Large (Labrador) | 2.4 |
| Giant | 2.3–3.2 |
| "Double it" rule | 2 |
| "÷ weeks × 52" rule | 3.25 |
Doubling is close for medium dogs but overshoots small ones and undershoots big ones. The ÷ weeks × 52 rule assumes a puppy grows at a steady rate until its first birthday, so it overshoots every size at 16 weeks, and small dogs by roughly double.
Limits of the estimate
- Few dogs per breed. The study had between 3 and 37 puppies per breed, all raised at one research center and fed to keep an optimum body composition.
- Breed averages. The curves describe the average puppy of each breed. Males and females can grow differently, and individual puppies vary.
- One large breed. Large puppies are estimated from Labradors alone. A large dog of another breed may mature a little earlier or later.
- Not a health check. A projected adult weight says nothing about whether your puppy is too thin or too heavy now.
Tips
- Weigh at the same time of day, before a meal, on the same scale. For a small puppy, weigh yourself holding it and subtract your own weight.
- Re-check every few weeks. The older the puppy, the closer the estimate, because more of its growth is behind it.
- Feed for the puppy you have. Follow the puppy food's guide and your veterinarian's advice for its current weight and body condition, not a hoped-for adult size. The study's authors note that in large breeds, overfeeding causes bone and joint problems as well as obesity.
- Large and giant breeds grow for longer. Ask your vet when to switch from puppy to adult food, since St. Bernards and English Mastiffs in the study were still growing after their first birthday.
Frequently asked questions
How big will my puppy get?
Divide its weight now by the share of adult weight puppies of its size have reached at the same age. In a study of 12 breeds, medium-sized puppies had reached about a third of their adult weight at 12 weeks, so a 12 lb medium puppy at 12 weeks points to an adult of roughly 30 to 36 lb. Treat it as a range, not a promise.
At what age is a puppy half its adult weight?
In the WALTHAM study by Hawthorne and colleagues, Papillons reached half their adult weight at about 11 weeks, the small and medium breeds at 14 to 17 weeks, Labrador Retrievers at about 19 weeks and the giant breeds at 18 to 23 weeks.
When do puppies stop growing?
The same study found that toy, small and medium breeds reached 99% of adult weight at about 9 to 11 months, Labradors at about 12 months and giant breeds at 11 to 15 months, with English Mastiffs the slowest.
Does doubling a puppy's weight at 4 months give its adult weight?
Only for some sizes. At 16 weeks the study's small breeds had reached 52 to 68% of adult weight, so doubling overshoots them, while the Labradors were at 41% and the slowest giant breeds at about 31%, so doubling undershoots. The multipliers on this page come from the measured curves for each size.
Does the calculator work for mixed breeds?
Yes, as long as you pick a sensible adult size. Go by the parents' size if you know it, or try two sizes and see where the ranges overlap. Mixed-breed puppies were not part of the study, so expect more spread.
How do I know if my puppy is a healthy weight?
Ask your veterinarian. Vets judge a puppy by body condition, how its ribs, waist and belly look and feel, and can plot its weight on growth charts such as the WALTHAM puppy growth charts, which were built from records of healthy dogs at US veterinary hospitals. A projected adult weight cannot tell you whether a puppy is too thin or too heavy today.
Last updated October 20, 2026.