Dogs age more rapidly than humans, with an average lifespan of about 14 years, depending on the dog's size, health and lifestyle. While many people calculate a dog's age in human years by multiplying their actual age in years by seven, this isn't correct because a dog ages more rapidly in his early years than his later ones. Most breeds age at the same pace for the first five years of life, but smaller ones age more slowly than larger ones after this age and can live almost twice as long in some cases.
Estimate your dog's age if you don't know exactly when your dog was born. A dog's age usually can be determined by the state of his teeth. A puppy gets all of his baby teeth by 8 weeks of age and permanent ones by 7 months, according to WebMD. A dog 1 to 2 years old has little tartar buildup, and a dog up to 5 years old has some wear and yellowing of the teeth. Dogs over 5 years old may have serious tartar buildup and dental disease. Senior dogs, over 10, may also show signs of graying fur, cataracts and joint disease.
Determine if your dog's size is considered small, medium or large. Small dogs weigh less than 20 pounds, medium dogs range between 21 and 50 pounds and large dogs are over 50 pounds in size. While most dogs age at the same rate until they reach 5 years old, after this age, larger breeds age more rapidly than smaller ones, affecting the calculation of the dog's age in human years.
Calculate the approximate human age of your puppy by multiplying the age in months by 2 for puppies up to 5 months old. Puppies age very rapidly during their first year of life, especially during these first 5 months, a rate of growth which decreases as the dog approaches his first year of life.
Multiply the age in months by 1.5 for puppies from 6 months old to 8 months old to account for the decreased rate of aging.
Multiply the age in months by 1.25 for puppies from 9 months old to 1 year old. A 1-year-old dog's age in human years is approximately 15 years old, and the dog ages 3/4 of a human year for each month after that until the dog reaches 2 years old, or approximately 24 years old in human years. This figure is the same for all sizes and breeds.
Subtract 2 from the dog's age in years and multiply the remainder by 4 for dogs over 2 years old and under 5 years old. Add 24 to this number to determine the age in human years. This figure is the same for all sizes and breeds up to 5 years old.
For example, to determine the human age of a 4-year-old dog, you would subtract 2 from 4, which would leave you with 2 as the remainder. Multiply this number by 4, which would give you 8. Add 24 to 8 to get 32, which is a 4-year-old dog's age in human years.
Subtract 5 from the dog's age in years for dogs over 5 years old. For small breeds of dogs, multiply the remainder by 4 and add it to 36. For medium-sized breeds, multiply the remainder by 5 and add it to 36.
For large breeds, multiply the remainder by 6 and add it to 36. Note that large breeds of dogs over 15 years old age about 27 human years between 15 and 16 years old in dog years due to their larger bone structure, which can lead to joint and other health issues.