# How big does a Mastiff get?

A full-grown Mastiff is a giant dog weighing about 120–230 lb (54.4–104.3 kg) as an adult. Larger breeds take longer to reach adult weight (Hawthorne et al., 2004).

| Field | Value |
|---|---|
| Adult weight | 120–230 lb |
| In kilograms | 54.4–104.3 kg |
| Size class | Giant |

## 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

- English Mastiff: https://petmeasure.com/dog/breed/english-mastiff
- Great Dane: https://petmeasure.com/dog/breed/great-dane
- Newfoundland dog: https://petmeasure.com/dog/breed/newfoundland-dog
- Neapolitan Mastiff: https://petmeasure.com/dog/breed/neapolitan-mastiff

## Sources

- Wikidata — dog breed mass (property P2067) — Wikimedia Foundation, query run 2026-07-22, CC0 license (https://query.wikidata.org/)
- Wikidata — cat breed mass (property P2067) — Wikimedia Foundation, query run 2026-07-28, CC0 license (https://query.wikidata.org/)
- Breed weights — official AKC breed standards — American Kennel Club, breed pages, accessed 2026-07-29 (https://www.akc.org/dog-breeds/)
- Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome — Wang et al., Cell Systems, 2020, vol. 11, no. 2, p. 176–185.e6 (DNA methylation in 104 Labradors) (https://doi.org/10.1016/j.cels.2020.06.006)
- Body-Weight Changes during Growth in Puppies of Different Breeds — Hawthorne et al., Journal of Nutrition, 2004, vol. 134, p. 2027S (https://doi.org/10.1093/jn/134.8.2027S)
- Management of Reproduction — gestation period in the bitch — Merck Veterinary Manual, “Approximate Gestation Periods” table, accessed 2026-07-23 (https://www.merckvetmanual.com/multimedia/table/approximate-gestation-periods)
- Dystocia in Small Animals — Merck Veterinary Manual, “Diagnosis of Dystocia in Small Animals” section, accessed 2026-07-28 (https://www.merckvetmanual.com/reproductive-system/reproductive-diseases-of-the-female-small-animal/dystocia-in-small-animals)
- Factors affecting gestation duration in the bitch — Eilts et al., Theriogenology, 2005, vol. 64, no. 2, p. 242–251 (https://pubmed.ncbi.nlm.nih.gov/15955350/)
- Nutritional Requirements of Small Animals — energy requirements of the dog and cat — Merck Veterinary Manual, “Energy” section, accessed 2026-07-23 (https://www.merckvetmanual.com/management-and-nutrition/nutrition-small-animals/nutritional-requirements-of-small-animals)
- Chocolate Toxicosis in Animals — Merck Veterinary Manual, “Methylxanthine Content of Various Types of Chocolate” table and “Etiology” section, accessed 2026-07-28 (https://www.merckvetmanual.com/toxicology/food-hazards/chocolate-toxicosis-in-animals)
- ASPCA Poison Control — ASPCA, 24-hour emergency line, accessed 2026-07-28 (https://www.aspca.org/pet-care/aspca-poison-control)
- How old is your cat? — life stage and human equivalent age — International Cat Care (Cat Friendly Clinic), accessed 2026-07-28 (https://icatcare.org/resources/cat-friendly-clinic/how-old-is-your-cat-handout.pdf)
- The Special Needs of the Senior Cat — cat age in human years — Cornell Feline Health Center, Cornell University, accessed 2026-07-28 (golden-test source, not used to build the calculation) (https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/special-needs-senior-cat)
- Calorie Needs for an Average Healthy Adult Cat in Ideal Body Condition — WSAVA Global Nutrition Committee, updated July 2020 (golden-test source, not used to build the calculation) (https://wsava.org/wp-content/uploads/2020/07/Calorie-Needs-for-Healthy-Adult-Cats-updated-July-2020.pdf)
- Assessment of body weight for age determination in kittens — DiGangi et al., Journal of Feline Medicine and Surgery, 2019 (tables 1 and 2, weight-for-age, male and female) (https://doi.org/10.1177/1098612X19844846)
- Kitten Age & Weight Chart — ASPCA Pro, 2023 (golden-test source, not used to build the calculation) (https://www.aspcapro.org/sites/default/files/2023-08/aspca_kittenchart_2023.pdf)

Full method: https://petmeasure.com/sources

_Verified 2026-07-28. Estimates only — this does not replace veterinary advice._