Size-adjusted formula (not the old 7× myth).
The "multiply by 7" rule is inaccurate. Dogs mature quickly: year 1 ≈ 15 human years, year 2 ≈ 24. After that, each year adds 4-7 human years depending on size — larger dogs age faster. Small dogs (under 20 lbs): +4 human years/year after age 2, lifespan 12-16 years. Medium (20-50 lbs): +5/year, lifespan 10-14. Large (50-90 lbs): +6/year, lifespan 8-12. Giant (90+ lbs): +7/year, lifespan 6-10. A 2019 study using DNA methylation (epigenetic clock) found dogs age rapidly early: 1-year-old dog ≈ 30-year-old human at the molecular level. Senior dogs (last 25% of expected lifespan) need twice-yearly vet visits, joint supplements, and modified exercise.