Practically, there is no mathematical formula (well at least, not a simple one) because the age of a cat does not follow a linear curve compared with the human age. Like many animals, "childhood" and "adolescence" occur much faster than humans.
So it's rather from observations of the ages of cats that veterinarians have established a correlation table (age chart) to see an equivalence in human age:
|Cat real age||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20|
Throughout his life, the cat, just like human, sees physiology, health and physical change.
If humans had Jeanne Calment, the oldest cat in the world was a female cat named Creme Puff.
She died in 2005 in Texas (USA) at the age of 38 years (and 3 days), which would correspond to a human aged nearly 170 years !!!
The most incredible is that Jake Perry who raised Creme Puff, also had a cat of 34 years, Granpa Rexs Allen who died in 1998!