What size harness does a Bullmastiff need?
The breed standard puts a Bullmastiff at 45–59kg (99–130lb). Weight alone settles the size at 2 of the 5 brands. 2 brands size by girth only and are left blank rather than guessed.
What that weight means at each brand
| Brand | Size | What it means |
|---|---|---|
| Julius-K9 | 3 | Every Bullmastiff in the breed's weight range falls in Julius-K9 3. |
| Ruffwear | measure | Ruffwear sizes by chest girth only and publishes no weight guidance, so a Bullmastiff's weight cannot be converted to one of their sizes. Measure the chest and the tool above will answer it exactly. |
| PetSafe | no size made | A Bullmastiff is heavier than the largest PetSafe size, which stops at 43.1kg. This brand does not make a harness for this breed. |
| Kurgo | X-Large | Every Bullmastiff in the breed's weight range falls in Kurgo X-Large. |
| Blue-9 | measure | Blue-9 sizes by chest girth only and publishes no weight guidance, so a Bullmastiff's weight cannot be converted to one of their sizes. Measure the chest and the tool above will answer it exactly. |
Blank entries are deliberate. Ruffwear and Blue-9 publish no weight guidance to convert, and printing a size anyway would be a guess dressed as an answer.
PetSafe publishes weight guidance, but its range does not reach a Bullmastiff — there is no size to recommend rather than one we have withheld.
Now measure, and get the exact size
A breed weight range covers both sexes and every build, which is why it spans more than one size almost everywhere. One measurement replaces all of it.
Measure right around the deepest part of the ribcage, just behind the front legs.
6 size ranges overlap here. The marker shows where your dog falls inside each one — nearer the middle means more adjustment room either way.
How to measure a Bullmastiff
Take a soft tape right around the deepest part of the ribcage, just behind the front legs. Keep it snug against the coat rather than pulled tight — you should be able to slide two fingers under it. That single number, the chest girth, is what every brand on this page sizes by. Weight is only ever a tie-breaker.
Why the answer is a range, not a number
Every page that hands you one size for a Bullmastiff is hiding the same problem: the breed standard covers a 31% spread in weight between the lightest and heaviest adults, and girth varies more than weight does. Two Bullmastiffs on the same scale reading can be a size apart. The honest answer is the shortlist above plus the tape.
Other giant breeds
- What size harness does a Rottweiler need?
- What size harness does a Great Dane need?
- What size harness does a Bernese Mountain Dog need?
- What size harness does a Mastiff need?
- What size harness does a Newfoundland need?
- What size harness does a Akita need?
- What size harness does a Saint Bernard need?
- What size harness does a Great Pyrenees need?
Weight range from the breed standard, read 2026-08-25. Sizes come from each manufacturer’s own published chart — nothing on this page is estimated or averaged between brands.