How much of your life have you spent together?
Two numbers most owners have never worked out: what fraction of your life your pet has been part of, and what fraction of theirs you have. They are rarely close to each other.
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When they overtake you
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Your pet gains human years far faster than you do. A cat adds about four a year against your one, a giant-breed dog eight — so whatever the gap is today, it is closing.
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How these numbers work
The first figure is simply the years you have had them divided by your age. The second is the years you have had them divided by their age — and if you have had them since they were born, that is 100%. You have been present for their entire life. They have been present for a slice of yours. That asymmetry is the whole point of the calculator, and it is the part people find harder to shake off than any age chart.
The crossover date uses the same ageing curves as the rest of the site: about 15 human years in the first year, nine in the second, then four a year for cats and 4–8 for dogs depending on breed size. Because they gain human years several times faster than you do, almost every owner is eventually outlived in "human years" by a pet who is chronologically much younger.