There is no reliable scientific method for calculating exactly how old your cat is in human years. It's often suggested that one human year is the equivalent of seven 'cat years', but in reality a one year-old cat is far more mature than a seven year-old child.
Experts generally agree that the first two years of a cat's life are roughly equal to the first 25 of a human. Thereafter, each additional year equals approximately four 'cat years'.
You can use this model to calculate your cat's approximate age in human years on the comparison chart below.