Converting between cat years and human years isn't as simple as using a factor of 7 (as some people think with dogs, and sometimes cats). The main reason is that cats mature quickly in the first couple of years of life.

To convert cat age to an equivalent human age, an accepted method is to add 15 years for the first year of life. Then add 10 years for the second year of life. After that, add 4 years for every cat year. This means that by year two, a cat has matured to about the same as a 25 year old human.

Not everyone agrees on the same conversion formula. For a slightly different calculation, the Cat Bible, a book by Tracie Hotchner, provides the following list:

• 1-month-old kitten = 6-month-old human baby 
• 3-month-old kitten = 4-year-old child 
• 6-month-old kitten = 10 human years old 
• 8-month-old kitten = 15-year-old human 
• 1-year-old cat  = adulthood, the equivalent of 18 human years 
• 2 human years = 24 cat years 
• 4 human years = 35 cat years 
• 6 human years = 42 cat years 
• 8 human years = 50 cat years 
• 10 human years = 60 cat years 
• 12 human years = 70 cat years 
• 14 human years = 80 cat years
• 16 human years = 84 cat years