The classic formula cited for calculating a dog's age in human years is to multiply his age by seven. However, this formula proves overly simplistic when you consider that many smaller dogs live into their late teens, making a 18-year-old dog 126 in human years. According to Purina, the first year of a dog's life equals 15 human years, but the age progression that follows differs by breed and size.
Add 15 years for the first human year of your dog's life regardless of the breed or size. For example, a 1-year-old dog is 15 dog-years-old.
Add nine years for the second human year of your dog's life, regardless of size or breed. For example, a 2-year old dog in human years would be 24 dog-years-old.
Add four dog years for each human year through age five. This calculation applies for all sizes and breeds. For example, a 5-year-old dog in human years would be 36 dog-years-old.
Add nine years for a large-breed dog after the fifth human year. For example, a large-breed dog who was six in human years would be 45 in dog years. Add six years for a medium-sized dog and four years for a small-breed dog. This calculation applies only to the sixth year of a dog's life in human years.
Add six years for each additional human year for extra-large breeds such as Great Danes and English Mastiffs. Add five years for each additional human year for large dogs between 22.7 and 31.8 Kilogram and medium-sized dogs, such as Springer Spaniels and Staffordshire Terriers, weighing between 30 and 50 pounds. Add four years per human year for small dogs, such as Cocker Spaniels or Basset Hounds, weighing between 15 and 30 pounds and for toy breeds, weighing less than 4.54 Kilogram.
Note that dogs rescued from abusive situations can appear older than they are due to malnutrition or illness.
This formula provides an approximation. Factors such as genetic quality, diet and medical care all affect a dog's lifespan.