American Kennel Club (AKC)
General Appearance: The Mastiff is a large, massive, symmetrical dog with a well-knit frame. The impression is one of grandeur and dignity. Dogs are more massive throughout than bitches. Size: Dogs, minimum 30 inches at shoulder; Bitches, minimum 27½ inches at shoulder. Fault: Dogs under 30 inches; bitches under 27½ inches.
Head: In general outline giving a massive appearance when viewed from any angle. Breadth greatly to be desired. Skull broad between the ears, forehead flat, but wrinkled when excited. Brows slightly raised; muscles of the temples and cheeks well developed. Arch across the skull from ear to ear a flattened curve with a furrow up the center of the forehead. Eyes set wide apart, medium in size, never too prominent. Expression alert but kindly. Color of eyes brown, the darker the better, and showing no haw.
Ears small in proportion to the skull, V-shaped, rounded at the tips. Leather moderately thin, set widely apart at the highest points on the sides of the skull continuing the outline across the summit. They should lie close to the cheeks when in repose. Ears dark in color, the blacker the better, conforming to the color of the muzzle.
Muzzle short, broad under the eyes and running nearly equal in width to the end of the nose. Truncated, showing great depth from the point of the nose to the underjaw. Underjaw broad to the end and slightly rounded. Muzzle dark in color, the blacker the better. Muzzle should be half the length of the skull. Lack of foreface with nostrils set on top of muzzle is a reversion to the Bulldog and is very undesirable. A dark muzzle is preferable.
Body: Neck slightly arched, moderately long, very muscular, and measuring in circumference about one or two inches less than the skull before the ears. Chest wide, deep, rounded between the forelegs, well let down between them. Ribs extremely well rounded. False ribs deep and well set back. Shoulder and arm slightly sloping, giving stability and power. Forearms straight, strong and set wide apart, heavy boned. Elbows parallel to body. Feet large, round, and compact with well arched toes. Pasterns strong and bent only slightly.
Back muscularly powerful and straight. Loins wide and muscular. Flanks properly drawn up. Hindquarters broad, wide and muscular. Second thighs well developed, hocks bent, wide apart, and quite squarely set when standing or walking. Feet large, round, and compact with well arched toes.
Tail set on moderately high and reaching to the hocks or a little below them. Wide at the root, tapering to the end, hanging straight in repose, forming a slight curve, but never over the back when the dog is in action.
Coat: Outer coat moderately coarse. Undercoat, dense, short, and close lying. Coat should not be so long as to produce 'fringe' on the belly, tail, or hind legs. Color: Apricot, silver fawn, or dark fawn-brindle. Fawn-brindle should have fawn as a background color which should be throughout completely covered with very dark stripes. In any case, muzzle, ears, and nose should be dark in color, the blacker the better, with similar color tone around the eye orbits and extending upward between them.
Fédération Cynologique Internationale (FCI)
General Appearance: Head in general outline giving a square appearance when viewed from any angle. Breadth greatly to be desired. Body massive, broad, deep, long, powerfully built, on legs wide apart and squarely set. Muscles sharply defined. Size should be combined with soundness. Height and substance important if both points are proportionally combined.
Important Proportions: Length of body from point of shoulder to point of buttock greater than height at withers. Height of body from withers to elbow equal to height from elbow to ground. Circumference of skull in front of ears greater than height of dog at withers.
Behaviour and Temperament: A combination of grandeur and good nature, courage and docility. Dignified rather than gay. Head: Skull broad between ears, forehead flat, with wrinkles when attention is excited. Brows slightly raised. Muscles of temples and cheeks well developed. Arch across skull a flattened curve, with a furrow up the center of the forehead from between the eyes to halfway up the sagittal suture.
Stop: There should be a distinct stop between the eyes with some depth of furrow. Nose: Broad with widely spreading nostrils when viewed from front, flat, not pointed or turned up in profile. Muzzle: Short, broad under the eyes, and keeping nearly parallel in width to end of nose. Truncated, showing great depth from point of nose to underjaw. Lips: Diverging at obtuse angles with the septum and sufficiently pendulous to show a square profile.
Eyes: Small, wide apart, divided by at least the space of two eyes. The stop between the eyes well marked but not over abrupt. Colour hazel brown, the darker the better, showing no haw. Free from obvious eye problems. Ears: Small, thin to the touch, wide apart, set on at the highest points of the sides of the skull, so as to continue the outline across the summit, and lying flat and close to the cheeks when in repose.
United Kennel Club (UKC)
General Appearance: The Mastiff is a large, massive, powerful, symmetrical, well-knit dog. The head is massive and the body is rectangular rather than square. The Mastiff is courageous but not aggressive, and combines a grandeur and dignity with a kindly nature. The breed has an ancient lineage, and over the centuries has been consistently bred for size, substance, and the typical Mastiff head and expression.
Characteristics: The Mastiff is innately good-natured, calm, easygoing, and surprisingly gentle for such a large dog. They are self-confident and watchful, and are naturally protective of home and family. They make excellent family companions. The breed is characteristically docile, but is capable of guarding. Typically slower to mature, both physically and mentally.
Head and Skull: The head gives a rectangular appearance when viewed from any angle. The breadth of the head is greatly desired and should be in proportion to the length of the entire dog. The skull is broad and flat between the ears. The forehead shows marked wrinkles which are particularly distinctive when the dog is alert or excited. The brows are slightly raised, and the muscles of the temporal region and cheeks are well developed.
Size and Weight: Dogs minimum 30 inches at the withers. Bitches minimum 27.5 inches at the withers. Dogs typically weigh 160-230 pounds. Bitches typically weigh 120-170 pounds. Great size is desirable provided that the proportions and type are maintained. Equal consideration is to be given to height and substance.
Other Organizations
The Kennel Club (UK): Recognizes the breed as the "Mastiff" with similar standards emphasizing the massive head, broad skull, and gentle temperament. Slight variations in wording but consistent emphasis on size, proportion, and character.
Canadian Kennel Club: Follows standards very similar to the AKC, with the breed recognized as "Mastiff." Emphasis on the combination of size with soundness and the characteristic gentle but watchful temperament.
Australian National Kennel Council: Similar standards to FCI, recognizing the breed's need for both impressive size and maintained proportion. Special attention to the breed's suitability as a family companion despite their imposing appearance.
Various smaller kennel clubs worldwide generally follow either the AKC, FCI, or Kennel Club (UK) standards with minor regional adaptations. All recognized standards emphasize the breed's distinctive massive head, powerful body, gentle temperament, and the importance of maintaining breed type while selecting for size and substance.