How old is your dog in human years? New method better than 'multiply by 7'. How old is your tail-wagging bundle of joy in human years? According to the well-known ''rule of paw,'' one dog year is the equivalent of 7 years. Now scientists say it's wrong. Dogs are much older than we think, and researchers devised a more accurate formula to calculate a dog's age based on the chemical changes in the DNA as organisms grow old. Source 2n.