The six size classes
We use the size categories from the 2017 PLOS ONE growth-chart study by Salt and colleagues, because they came from clustering real growth curves, not from marketing labels. Breeds with similar growth patterns ended up in the same class. The study defined five classes up to 40 kg (88 lb). We add a sixth for dogs above that.
| Class | Name | Adult weight |
|---|---|---|
| I | Toy | under 6.5 kg (14 lb) |
| II | Small | 6.5 to 9 kg (14 to 20 lb) |
| III | Small to medium | 9 to 15 kg (20 to 33 lb) |
| IV | Medium | 15 to 30 kg (33 to 66 lb) |
| V | Large | 30 to 40 kg (66 to 88 lb) |
| VI | Giant | over 40 kg (88 lb) |
The classes matter because they change the timetable. A toy breed is about 90% of the way to adult weight by 9 months. A giant is still filling out at 18.
From your puppy's weight
This is the more reliable route. Pick 'From my puppy', enter age and weight, and the calculator finds the size class where your puppy's numbers fit best. It's the same method as the puppy weight calculator, shown as a class, with the likely adult weight underneath.
From the parents
Averaging the mother's and father's weights is the old standby. It's useful when the puppy is very young, and it is less useful when the parents differ a lot in size, or when one parent is a breed the other isn't. Expect to be wrong by a class sometimes. Treat it as a hint until you have a few weigh-ins.
We don't use paw size. We haven't found good evidence that it predicts adult size better than weight does, and puppies that are heavy for their age tend to have big paws anyway.
Why size class changes how you care for a puppy
- Large and giant puppies are fed to grow steadily, not fast. Growing too fast in these breeds can strain joints, so vets often recommend a food made for large-breed puppies and careful portions.
- Small and toy puppies are more prone to low blood sugar when young, so frequent meals in the first months matter.
- Spay or neuter timing, joint supplements and exercise limits are all discussed differently by size. That is a conversation to have with your vet.
How to use it, step by step
- Choose a method. Start from your puppy's weight, or from the parents' weights if you can't weigh the puppy yet.
- Enter the numbers. Age and weight for the puppy, or each parent's weight.
- Read the size class. The calculator shows the class and the adult weight band it covers.
- Re-check as the puppy grows. A weigh-in at 12 to 16 weeks is far better than parents' weights.
Questions people ask
How do I know what size my mixed-breed puppy will be?
Weigh the puppy and use the calculator. Weight at 12 to 16 weeks says much more about adult size than any guess based on looks. If you know a parent's weight, enter that too.
What counts as a medium dog?
In the study we draw on, medium dogs end up between 15 and 30 kg (33 to 66 lb). Small to medium is 9 to 15 kg, and large is 30 to 40 kg.
Do big paws mean a big dog?
Not reliably. We don't use paw size, because we haven't found data showing it adds anything beyond the puppy's weight.
When is a puppy fully grown?
Toy and small breeds typically finish by 9 to 12 months. Medium dogs reach most of their weight by 12 months and finish by 18. Large and giant dogs keep filling out until 18 to 24 months.
Last reviewed October 31, 2026. General information, not veterinary advice. Growth curves are PupCurve's analysis of data from Salt et al. 2017 (CC BY 4.0). How it works.