How the prediction works
Puppies grow on a predictable curve. By any given age, a puppy has reached a known share of the weight it will carry as an adult — and that share depends heavily on how big the dog will end up. A small terrier is half-grown at three months. A great dane at the same age is not even a third of the way there.
So the calculation is simply:
A 12 kg large-breed puppy at 12 weeks is around 38% grown, which puts it on course for roughly 31 kg. The same 12 kg in a small-breed puppy at the same age would mean something very different — which is why picking the right size category matters more than any other input.
Puppy growth chart: % of adult weight by age
These are the percentages the calculator uses. Find your puppy's age down the left, then read across to their expected adult size:
| Age | Small | Medium | Large | Giant |
|---|---|---|---|---|
| 8 weeks | 30% | 26% | 22% | 18% |
| 3 months | 54% | 48% | 41% | 33% |
| 4 months | 68% | 61% | 53% | 45% |
| 6 months | 85% | 78% | 72% | 62% |
| 9 months | 97% | 92% | 88% | 78% |
| 12 months | 100% | 99% | 96% | 88% |
Small = under 9 kg / 20 lbs as an adult · Medium = 9–22 kg / 20–50 lbs · Large = 22–41 kg / 50–90 lbs · Giant = over 41 kg / 90 lbs. Individual puppies vary by roughly 10–15% either way.
Quick rules of thumb
If you want to check the calculator in your head, these shortcuts follow the same curve:
- Small breeds: weight at 6 weeks × 4
- Medium breeds: weight at 12 weeks × 2.2
- Large breeds: weight at 16 weeks × 2
- Giant breeds: weight at 6 months × 1.6
Shortcuts are blunter than the calculator because they ignore everything between those ages. Genetics, feeding and health still decide the final number.
When to weigh, and what to do with the number
Weigh your puppy weekly until four months, then monthly. Steady gains along the curve are what you want to see — a week with no gain at all, or sudden loss, is worth a call to your vet rather than a recalculation.
The predicted adult weight is genuinely useful for planning:
- •Food. Puppy feeding charts are based on expected adult weight, not today's weight.
- •Crate size. Buy for the adult dog and use a divider while they grow.
- •Joint care. Large and giant breeds do better kept lean through growth — extra weight on developing joints is a real risk.
- •Vet doses and insurance bands are often priced by adult weight.
Treat it as an estimate. This is a growth-curve calculation, not a breed database or a medical assessment. If your puppy is far off the curve in either direction, or is not gaining at all, your vet is the right next step.
Now work out the food
Once you know the adult weight, you can set portions properly and check your puppy is on track month by month.
Frequently Asked Questions
How do I predict my puppy's adult weight?
Take your puppy's current weight and divide it by the share of adult weight they have already reached for their age and breed size. A 12-week-old large-breed puppy is usually around 38% of its adult weight, so a 12 kg puppy works out at roughly 31 kg fully grown. The calculator above does that maths for you.
When does a puppy stop growing?
Small breeds are usually finished by 9–12 months, medium breeds by 12–15 months, large breeds by 15–18 months, and giant breeds can keep filling out until 18–24 months. Height is reached before weight — most dogs keep adding muscle and chest width for months after they stop getting taller.
How accurate is this puppy weight calculator?
Expect to land within about 10–15% of the true adult weight, which is what the range under the result shows. It is at its most accurate between 4 and 6 months, when the fastest growth is behind the puppy but the frame is still filling out. Very young puppies, mixed breeds and dogs neutered early are harder to call.
What if my puppy is a mixed breed?
Go by the parents if you know them — a puppy usually lands somewhere between the two adult weights. If you don't, use the paws and frame as a clue: oversized paws and loose skin suggest more growth to come. When you are torn between two size categories, pick the larger one and check again at six months.
Does neutering affect adult size?
It can. Neutering before the growth plates close tends to delay their closing, so some dogs end up slightly taller and leaner than they otherwise would have been. The effect on weight is small compared with genetics and feeding, but it is another reason to treat any prediction as an estimate and talk timing through with your vet.
Is my puppy overweight or just growing?
Body shape answers this better than the scale. You should be able to feel the ribs easily under a thin layer of fat, see a waist when you look down from above, and see the belly tuck up behind the ribs from the side. A growing puppy that feels padded over the ribs is being overfed, and extra weight during growth is hard on developing joints, especially in large breeds.