The English mastiff -- often simply referred to as the mastiff -- is a giant breed that may date back more than 5,000 years. Predicting how big your English mastiff puppy will be when he's fully grown is an inexact science based on breed averages and the pup's parents' weights.
Average English Mastiff Size
Adult males usually weigh around 160 pounds and stand at least 30 inches tall at the shoulder. Adult females are slightly smaller, weighing an average of 150 pounds and measuring at least 27 inches tall. Early nutrition will affect the dog's size later: A high-protein big puppy diet generally will suit a mastiff, but consult a vet to ensure your mastiff puppy reaches his full potential.
Just Like Mom and Dad
You can also take a look at the size of your pup's parents -- if his mom and dad were both particularly tall, it's likely that he might be, too. Two smaller mastiffs will likely produce smallish offspring, but not always. All bets are off when small mastiffs mate with large ones. For the record, an English mastiff named Zorba de la Susa -- the biggest dog in history -- grew to weigh 343 pounds and measured 8 feet 3 inches from nose to tail.