Which dog is the biggest? It depends whether you mean tallest, heaviest, or simply one of the breeds commonly described as giant. The Irish Wolfhound wins one version of the question; the Mastiff dominates another.
Six breeds that redefine “large dog”
Irish Wolfhound
AKC describes the Irish Wolfhound as the world’s tallest dog breed. Adult males commonly stand around 34–35 inches at the shoulder and can weigh roughly 140–180 pounds.
Mastiff
AKC’s breed weight chart lists Mastiffs at roughly 120–230 pounds, giving the breed one of the largest standard weight ranges.
Great Dane
Typically around 110–175 pounds and famously tall, with adult males reaching about 32 inches at the shoulder.
Saint Bernard
AKC lists a typical range of roughly 120–180 pounds.
Leonberger
A giant working breed commonly listed around 90–170 pounds.
Newfoundland
Usually around 100–150 pounds, originally developed for demanding water work with Canadian fishermen.
Size changes the care equation
Food costs, vehicle space, joint health, training, veterinary handling and simple household logistics all scale up. A gentle temperament does not make a 70-kg dog physically easy to manage.