How big does a Mastiff get?
A full-grown Mastiff is a giant dog, weighing about 120–230 lb as an adult.
- Adult weight
- 120–230 lb
- Also in kilograms
- 54.4–104.3 kg
- Size class
- Giant
As a giant breed, a Mastiff keeps growing for longer than a small dog — larger breeds reach their adult weight months later (Hawthorne et al., 2004). Expect steady growth well past the first year.
Common questions
How much does a full-grown Mastiff weigh?
A Mastiff weighs 120–230 lb (54.4–104.3 kg) as an adult. That range is the weight the American Kennel Club publishes in the breed's official standard, re-checked here every week — it is a typical weight across many adult dogs, not a target for one dog.
Is a Mastiff a giant dog?
Yes — a Mastiff counts as a giant breed, going by the middle of its adult weight range (54.4–104.3 kg). The cut-offs between small, medium, large and giant are a common veterinary convention rather than one official standard, so a breed near a boundary can be described either way elsewhere.
When does a Mastiff stop growing?
Later than a small dog. Hawthorne et al. (2004) measured that larger breeds keep gaining weight for longer, so a Mastiff is likely to fill out well past its first birthday. We don't publish a month-by-month figure, because no source gives one breed by breed — and an invented number would be worse than none.
Why is there a range instead of one weight?
Because the source publishes a range. Within any breed, weight varies with sex, genetics, build and health, and a mixed-breed dog usually falls between its parents' breeds. Narrowing 120–230 lb down to a single number would look more precise while being less true.
Breeds of a similar size
Weight varies within the Mastiff by sex, genetics and health, and mixed-breed dogs fall between their parents. This is a typical range, not a guarantee.
Verified on . Based on Breed weights — official AKC breed standards (American Kennel Club, breed pages, accessed 2026-07-29) and Body-Weight Changes during Growth in Puppies of Different Breeds (Hawthorne et al., Journal of Nutrition, 2004, vol. 134, p. 2027S). See the method.