French mastiffs, otherwise known as Dogue de Bordeaux, are unquestionably unique in appearance. They are known for their larger heads which wear a near-constant serious expression on their profoundly wrinkled faces. But what kind of size are these dogs when fully grown? Here is what you should know.

So, how big do french mastiffs get? French Mastiffs typically reach between 23-27″ in height and weigh between 120-140 pounds (54-63 kg), on average. They generally reach their final size by 19 months of age and there is little variation in size between the males and females of the breed.

With those dimensions, the French mastiff is classified as a large breed.

And they are a very powerful, athletic, and muscular dog too.

Their build keeps them low to the ground too, and added to their strong legs enables them to accelerate at incredible speeds for short periods.

They can be intimidating dogs for sure.

But aside from how they look, underneath is an even-tempered, loyal, affectionate, and sensitive dog.

Let us now take a closer look at the Dogue de Bordeaux and its rate of growth. This way, you can see exactly how a cute young pup will grow into a larger, more formidable adult dog.

We will then take a closer look at some size considerations for owners, so if you are particularly interested in the breed – I suggest you keep reading!

At What Age Is A French Mastiff Full Grown?

The average French mastiff will reach their full adult size around 19 months of age, and this is commonly seen in both the males and the females of the breed.

Let us now take a closer look at their growth trajectory; from the early stages of ownership through to their adult maturity.

Growth Of A Male French Mastiff

The weight of a male French Mastiff at 3 months should be between 40 and 48 pounds.
At 6 months, they will likely weigh between 73 and 85 lbs (pounds).
At 1 year, a male French Mastiff should weigh between 110 and 130 lbs (pounds).

Age	Weight – Low Average (lbs)	Weight – High Average (lbs)
3 Months	40	48
4 Months	51	61
5 Months	62	73
6 Months	73	85
7 Months	82	93
8 Months	91	102
9 Months	96	111
10 Months	101	119
11 Months	106	124
12 Months	110	130
13 Months	111	132
14 Months	113	133
15 Months	115	135
16 Months	117	137
17 Months	118	138
18 Months	119	139
19 Months	120	140

Growth Of A Female French Mastiff

The weight of a female French Mastiff at 3 months should be between 40 and 48 lbs (pounds).
At 6 months, they will likely weigh between 73 and 85 lbs (pounds).
At 1 year, a female French Mastiff should weigh between 110 and 130 lbs (pounds).

Age	Weight – Low Average (lbs)	Weight – High Average (lbs)
3 Months	40	48
4 Months	51	61
5 Months	62	73
6 Months	73	85
7 Months	82	93
8 Months	91	102
9 Months	96	111
10 Months	101	119
11 Months	106	124
12 Months	110	130
13 Months	111	132
14 Months	113	133
15 Months	115	135
16 Months	117	137
17 Months	118	138
18 Months	119	139
19 Months	120	140

As you can see, there is minimal difference between the males and females of this breed.

You’ll also notice upon closer viewing of the data, that these dogs grow very quickly during the first 12 months.

They then quite drastically slow down in speed of growth, doing so gradually for the next 7 months, where they tend to finish up.

The first 6 months is the most rapid period of growth in particular.

So, the French Mastiff will acquire the majority of their size while they are still young. They’ll fill out as time goes on.

Spatial Requirements Of French Mastiffs

Despite being a large and sturdy breed, the French Mastiff does not need an endless amount of space like some other breeds.

Sure, they need enough to be comfortable and move about freely, but they are generally very laid back and easy going.

They are not the most energetic of breeds or active of dogs, so having your own outdoor space is not so much of a necessity.

This is, of course, assuming that you are able to take them out for walks fairly regularly. More so if they are kept inside for the majority of the day.

While some will argue that this breed is suitable for apartment living; but it does depend on the apartment and the dimensions of the home.

They are a large dog after all.

This means that traveling in the car may be particularly challenging, especially when they do reach their final adult size.

What is imperative however, is that these dogs are not left at home alone for too long.

This breed is known for attaching very strongly to their owners and can suffer with separation anxiety fast.

Slowly introducing this dog to spending time alone, along with spending sufficient time with them regularly throughout the day is a must.

Exercise And Dietary Needs Of French Mastiffs

When owning any breed of dog, its essential to be aware of their own exercise and dietary needs.

Besides, this can ensure they reach their full-size potential and keep a healthy weight.

Let us now take a closer look at what this looks like for the French Mastiff: