Formula based on dog size:
• Small: Year 1 = 12, Year 2 = 9, Subsequent = 4 human years
• Medium: Year 1 = 13, Year 2 = 9.5, Subsequent = 4.5 human years
• Large: Year 1 = 14, Year 2 = 10, Subsequent = 5.5 human years
• Giant: Year 1 = 15, Year 2 = 10, Subsequent = 6 human years

This formula is based on 2020 research that considers differences in dog size and breed. Large dogs tend to age faster than small dogs.

Research Reference: "Quantitative translation of dog-to-human aging"