How To Calculate Dog Years

Things To Consider When Calculating Dog Years

The 7 Year Rule

Most people are aware of the “7 Year” rule, that is: each year in a dog’s life is equivalent to seven human years. Over time we have learned that the difference between dog years and human years is not linear and that a simple 7 to 1 ratio does not fully explain the relationship. Let’s go over the reasons why this isn’t very accurate...

Sexual Maturity

Dogs and humans reach an age where adults can give birth at different ages. The fact of the matter is that a dog can give birth, indicated by their first menstrual cycle, at about 9 months. Dogs (and cats) have generally reached full growth and sexual maturity around one year old. One modern school of thought is that the first two years of a dogs life is the human equivalent of 18-25 years. This number is based on both age of sexual maturity and life expectancy (which we will get to next).

Breed Size

The size of the breed plays a large role in life expectancy and therefore is a factor in determining their age relative to humans. Generally speaking smaller dogs live longer and age slower. Large breeds tend to live shorter lives and show signs of aging sooner (grey hairs, lethargy, joint pain, etc.) According to the UC Davis Book of Dogs, small-breed dogs (such as small terriers) become geriatric at about 11 years; medium-breed dogs (such as larger spaniels) at 10 years; large-breed dogs (such as German Shepherd Dogs) at 8 years; and giant-breed dogs (such as Great Danes) at 7 years.

What About the World’s Oldest Dog?

Using the 7 Year formula makes it extremely difficult to explain that the world’s oldest living dog. Max, a terrier born in 1983 made it to twenty-nine years, 282 days, which adds up to 208 in human years! There’s never been a case in which a human has reached the two century mark. So you might want to re-think your whole approach if you feel the need to believe that one dog year is equivalent to seven human years.

Are Dogs Living Longer Now?

Modern day dogs have a much higher quality of life on average than their canine ancestors. Medical and nutritional advances are allowing dogs to live longer all the time. Humans are also living much longer than in the past for many of the same reasons. Both dogs and humans in more developed countries will have longer life expectancies. For example, the average human life expectancy worldwide is only 66 years, but in developed countries that number jumps to 80 years. While both species are living to more advanced ages, they aren’t doing so at the exact same rate. This further discredits the 7 to 1 ratio previously used since humans and dogs lifespans, as stated before, do not have a linear relationship..

The Modern Approach

Modern thinking has found a more accurate approach to calculate a dog’s age in human years. That being said, no one formula is fully agreed on by the scientific community. The following charts and graphs are an attempt to generally explain the relationship between the rate at which a dog ages compared to a human.

This chart also applies to Dr. Randall's video about Senior Dog Food. Notice that dogs of heavier weights become "senior" as soon as Age 5, and you should consider providing them with senior dog food, along with as their daily dose of Flexpet!

Dog Years Chart

Dog years chart

How Old Is My Dog in "Human Years"?

Pet's Age Pet's Size (in pounds)
0-20 lbs 21-50 lbs 51-90 lbs > 90 lbs
1 15 15 15 15
2 24 24 24 24
3 28 28 28 28
4 32 32 32 32
5 36 37 40 42
6 40 42 45 49
7 44 47 50 56
8 48 51 55 64
9 52 56 61 71
10 56 60 66 78
11 60 65 72 86
12 64 69 77 93
13 68 74 82 101
14 72 78 88 108
15 76 83 93 115
16 80 87 99 123
17 84 92 104  
18 88 96 109  
19 92 101 115  
20 96 105 120  

Dr David Randall author photoWritten by Dr David Randall, DVM
His father established the clinic in 1977 and the the younger Dr. Randall always knew that he would follow closely in his father’s footsteps. Dr. Randall graduated from the University of Florida, College of Veterinary Medicine in 1985 and began his career at Big Cypress Animal Clinic.

Topics: Pets, Dr David Randall, Ask the Expert, Videos