Mastiff Weight, Size & Lifespan
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
- Life expectancy
- 6–10 years
Mastiff Size
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.
Mastiff Lifespan
How long do Mastiffs live?
The American Kennel Club publishes a life expectancy of 6–10 years for the Mastiff, on its breed page. That is a figure for the breed, not a prediction for one dog — an individual Mastiff can fall on either side of it.
A second source measured it instead of publishing it. McMillan et al. (2024) followed 1,840 dogs of this breed in the United Kingdom and recorded 1,419 deaths, giving a median age at death of 9 years (95% CI 8.9–9.2 years).
The two roughly agree here, which is not always the case: on 34 of the breeds on this site the measured median falls outside the range the AKC publishes, in both directions. They are still not the same measurement — a life expectancy for the breed on one side, the age half the studied dogs had died by on the other.
Mastiff Growth Chart
What a Mastiff puppy weighs week by week, from the growth curve FEDIAF publishes (Nutritional Guidelines 2025, Table VII-8a) for a dog whose expected adult weight is 79.4 kg. It is an average, not a target, and it is only valid from 8 weeks to one year — FEDIAF notes that growth patterns show a certain individual variability. Read a row as the middle of a spread.
| Age | Weight | % of adult weight |
|---|---|---|
| 8 weeks | 24 lb (10.9 kg) | 14% |
| 10 weeks | 38 lb (17.4 kg) | 22% |
| 12 weeks | 50 lb (22.7 kg) | 29% |
| 14 weeks | 60 lb (27.2 kg) | 34% |
| 16 weeks | 68 lb (31.0 kg) | 39% |
| 18 weeks | 76 lb (34.5 kg) | 43% |
| 20 weeks | 83 lb (37.5 kg) | 47% |
| 24 weeks | 94 lb (42.8 kg) | 54% |
| 28 weeks | 104 lb (47.3 kg) | 60% |
| 32 weeks | 113 lb (51.2 kg) | 64% |
| 36 weeks | 120 lb (54.6 kg) | 69% |
| 40 weeks | 127 lb (57.7 kg) | 73% |
| 44 weeks | 133 lb (60.4 kg) | 76% |
| 48 weeks | 139 lb (63.0 kg) | 79% |
| 52 weeks | 144 lb (65.3 kg) | 82% |
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.
How long do Mastiffs live?
The American Kennel Club publishes a life expectancy of 6–10 years for the breed, re-checked here every week. That is a figure for the breed, not a prediction for one dog: an individual Mastiff can fall on either side of it, and how a dog is fed, exercised and cared for moves the odds. A second source measured it rather than publishing it: McMillan et al. (2024) followed 1,840 dogs of this breed in the United Kingdom, recorded 1,419 deaths, and report a median age at death of 9 years (95% CI 8.9–9.2 years).
Breeds of a similar size
Every dog calculator
- Dog Age Calculator — Your dog’s age in human years, the science-based way
- Puppy Weight Calculator — How big will your puppy get, by breed
- Dog Pregnancy Calculator — Your dog’s due date from her mating date
- Dog Calorie Calculator — How many calories your dog needs a day
- Dog Chocolate Toxicity Calculator — How much chocolate is dangerous for your dog, by weight
- Puppy Feeding Calculator — How much to feed a puppy, by weight and age
- Dog Benadryl Dosage Calculator — How much Benadryl you can give a dog, by weight
- Puppy Vaccination Schedule — Puppy shots by age, from the AAHA’s published guidelines
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), Breed life expectancy — AKC breed pages (American Kennel Club, breed pages, accessed 2026-08-10), Longevity of companion dog breeds: those at risk from early death — Supplementary Table S3 (McMillan et al., Scientific Reports (2024), 14:531, 584,734 UK dogs — Kaplan-Meier median survival for 155 breeds), Body-Weight Changes during Growth in Puppies of Different Breeds (Hawthorne et al., Journal of Nutrition, 2004, vol. 134, p. 2027S) and Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs (FEDIAF (European Pet Food Industry Federation), 2025 edition (growth curves Table VII-8a, growth energy VII-8b and VII-10)). See the method.