When Do Mastiffs Stop Growing? 
Growth Rates And Stages From Puppy To Adult

Ever wonder when do Mastiffs stop growing, especially now that you've noticed that your Mastiff puppy is gaining height and weight every day? Are you starting to wonder when he will eventually reach full size? You are not the only one! Many owners admire their puppies’ changing features in awe.

So, when do Mastiffs stop growing and reach full size? While most of the bones, muscles, and joint development happens during their first two years, Mastiffs reach their full size at around three years of age. Until then, not only their physical features keep changing, but they also have a puppy-like personality. Once fully grown, they can be as big as:

  • 90kg (198 lbs.) and 80cm (31 inches) tall for males
  • 70kg (154 lbs.) and 70cm (27 inches) tall for females

Now that you have realistic expectations of how big and how quickly your baby Mastiff is going to grow let’s have a look at what to expect from the first years of sharing your life with a Mastiff puppy! Read More Below...

Mastiff ramps are a great way to save your dog's joints and your back. You can find them on Amazon by clicking here now.

Chihuahua riding on the back of a Great Dane


Mastiffs Growth Rates And Stages

Mastiffs are incredible creatures that won’t look the same for two days in a row during their first two years of life. In the space of three months, they can even double in size! While Mastiff puppies reach a size similar to the one of an adult mastiff within its first 18 to 20 months, he will keep growing slower for the next year. Here is what to expect from a growing Mastiff!

4 to 11 weeks of age

A four-week-old mastiff puppy weighs around 6kg (13 lbs.). During this phase, the puppy should be with his mother and littermates and encouraged to socialize with other humans and pets. Once he has reached 11 weeks of age, your Mastiff will weigh around 18kg (39 lbs.) and is almost ready to leave his first home and join you in your household! 

11 weeks to 3 months

These months are a critical period of the life of Mastiffs. Once in your home, they will learn how to live in your company, and you should be ready to start their training as soon as they are used to the new environment. During this time, Mastiffs are very active and excitable, but their bones and joints can be fragile. You can expect them to grow from 9kg (19 lbs.) to 15kg (33 lbs.)

3 to 6 months

The following three months are a delicate period for the physical and psychological development of your puppy. During these three months, Mastiff puppies almost doubled in size, reaching a weight of 45kg (99 lbs.). If you have not started to train them yet, their size might make managing them much more challenging!


Mastiffs drool over their monthly Super Chewer box from BarkBox which contains Mastiff proof toys as well as treats they love and beefy chews with FREE delivery…Click here now!

6 to 12 months 

During these six months, Mastiff puppies reach a weight of 79kg (174 lbs.), which is close to what they are going to weigh as adults. In this period, they double in size again, and they can be extremely excited, playful, and active. However, this time is also when their joints and bones are developing and can be fragile. Devising an efficient training plan that is safe for them and can yield the most significant results is essential. 

12 to 18 months

Your Mastiff is becoming an adult! During this phase, they will only grow to weigh around 85kg (187 lbs.) for males and 60kg (132 lbs.) for females. While can seem fully grown Mastiffs, puppies will be puppies, and they will still need supervision! Until 18 months of age, demanding physical exercise, excessive jumping, and stair climbing is not recommended to preserve the health of their limbs and joints.

18 months to 3 years

This period marks the last primary physical development phase your Mastiff puppy will go through. At the end of the first two years, a Mastiff will have reached his adult size. Yet, bones and joints will still be settling until three years of age. Don’t expect your Mastiff to give up his puppy-like excitement any time before then!

Want to learn more about what to expect in the first months of puppy growth?
This article Mastiff Puppy Weight Guide will reveal just how fast they do grow in their first year.

Watch Just How Fast Mastiffs Grow...


When Is A Mastiff Full Grown?

Mastiff puppies reach physical maturity around two to three years of age. The fully developed bones, joints, and muscles mean that your dog is about the size he will be for the rest of his life. At this time, males can be as tall as 80cm (31 inches) and as heavy as 90kg (198 lbs.). Yet, it is not uncommon to have a lean dog that weighs over 85kg (187 lbs.). Females, on the other hand, can be as massive as 80kg (176 lbs.) but only 70cm (27 inches) tall.

As your puppy reaches adulthood, any underlying health problem or condition can start to show. Be aware that often such conditions originate during the first years of the life of your puppy but only become visible as he enters adulthood. Understanding what the telltale signs of dysplasia and other bone conditions are can guarantee a much healthier life for your dog.

When Do Mastiffs Reach Psychological Maturity?

Psychological or emotional maturity can vary from breed to breed and from dog to dog. While we can consider a Mastiff an adult at around 2 ½ years of age, he might still act like a puppy for a few months. 

In dog years, 4 to 7 months of age is the period before adolescence because your dog will reach sexual maturity. However, Mastiff puppies only reach emotional maturity almost a year after that. During this time, they will try to understand what their place in the “pack” is and can even show dominant behaviors. Training is strictly necessary during this phase to ensure you are bringing up a well-behaved dog.

Harnesses are a much safer way to control your Mastiff and protect their necks than a leash. Find harnesses on Amazon by clicking here now.

When Do Mastiffs Reach Sexual Maturity?

Mastiff puppies enter sexual maturity at around 6 to 9 months of age, and this is one of the first milestones reached during their development. However, sexual maturity comes with several different behavioral changes, and it is an extremely delicate phase of the growth of your puppy.

Female Mastiffs: will go into heat twice a month and develop a vaginal discharge that can last anything between 7 to 10 days. Females can get pregnant even during their first heat so, if you have not spayed her, you should supervise her on walks or while socializing. 

Male Mastiffs: while they won’t go into heat like females, male dogs are at their most fertile while 12 to 15 months of age. At this time, males can be prone to mounting and roaming, which makes it essential to supervise them.

What You Should Look For While Your Mastiffs Growing

Mastiffs are powerful and massive creatures that have loads of love to give. Yet, they can be incredibly vulnerable during their first three years of life. 

Mastiffs, similarly to other large and giant dog breeds, are predisposed to many physical and medical conditions that can lower their lifespan and pose a threat to their lives. Here are the diseases you should look out for a while your Mastiff puppy is developing:

  • Hip and elbow dysplasia
  • Bloat or Gastric Torsion (GDV)
  • Cardiomyopathy
  • Overheating and low tolerance to heat
  • Wobblers syndrome
  • Arthritis
  • Training-related issues such as aggressiveness

Conclusion...

Mastiffs are complex creatures who rely mostly on us to take care of their physical and psychological needs. While puppies are little bundles of hyperactivity and love, there are essential factors to consider while bringing up a giant breed dog such as Mastiff.  when do Mastiffs stop growing

They tend to grow exceptionally quickly and occasionally develop bones, joints, digestive, and cardiovascular problems that can shorten their lives. Even if the rate your puppy is growing up is amusing you, it won’t be this size for much longer. Training and attention are essential during this time.

Learn Even More About The Mastiff Breed Here...


  1. Mastiff Guide Home
  2. Mastiff Dog Information
  3. When Do Mastiffs Stop Growing