While it is true that dogs age at a different rate than humans, the “one human year equals seven dog years” rule of thumb is not close to accurate. Dogs can reproduce at one year of age and some dogs live 20 years. If the 1-to-7 ratio were correct, then humans could reproduce at age seven and live to age 140. Further compounding the 1-to-7 ratio is that different dog breeds age at different rates and also have different life expectancy.
If you are looking for a formula, the “15, 9, 4” formula is a good estimator. It was created taking into account certain life milestones: when they reach puberty, when they reach adulthood, and the maximum years they live. The key is that dogs don’t age in parallel with humans; instead, they age much more quickly at first, then a little less quickly later on. Here’s how the 15,9,4 formula works: In your dog’s first year it will become nearly fully grown – similar to a human adolescent. So, at the end of year one, your dog is 15 in dog years. At age two, your dog is fully adult. Add 9 more years. Now he’s 24. Then, each year thereafter add four years. So, when your dog is five, he’s 36 in dog years. At age seven he’s 44, and so on. This formula more accurately expresses a dog’s quicker physical and mental maturity in the early years and then slows the comparative aging in the later years as compared to the 7 for 1 straight line formula.
Here’s a chart that takes the 15,9,4 formula and further refines it based on the size of breed: