His age in — what it is in human years by the chart everyone repeats (and what that chart is worth), his life stage by USDA’s national equine study, his official age the way the Jockey Club counts it, and how long horses live, by Merck.
In human years
About 43 in human years
His life, stage by stage
Sources checked 2026-09-26.
Quick answer: by the chart most websites print, a horse’s first year is worth 6.5 human years, and he is 13 at two and 18 at three. Every year after that adds about 2.5, so a 13-year-old horse is about 43 and a 20-year-old about 60½. It is a rule of thumb: we could not find a study or a veterinary school that published it. The Merck Veterinary Manual says horses live 25 to 30 years on average, and miniatures can live 40 or more. USDA’s national equine study counts a horse over 20 as geriatric.
How to use it
Type his age in years, or switch to his foaling date if you know it. Half years and months work: 12.5, 12 years 6 months, or 8 months for a foal. The answer gives his age in human years by the common chart. It also gives his life stage as USDA describes it, and where he stands against the average lifespan. With a foaling date, you also get his official age, the way the Jockey Club counts it. Say whether he is a male, a gelding or a female, and it gives him his proper name too: a colt or filly up to 4, then a horse or mare.
What the human-years chart is worth
Very little, beyond a smile. It appears on page after page with nobody’s name on it, and we searched for where it came from without finding a source. Its fast start at least matches how horses grow: USDA’s study says most reach sexual maturity at about 3 to 4 and are fully grown by 4 to 5. At the far end, it runs past any human. A 40-year-old miniature, which Merck says is possible, would be 110½. So the calculator shows the chart, and says what it is.
His life stage, by USDA’s study
USDA APHIS’s National Animal Health Monitoring System describes a horse’s life in stages. A foal stays with his dam and is weaned at 4 to 6 months. As a yearling he learns to lead, to be groomed and to have his feet done. In many disciplines, a competitive career starts at 2. He reaches sexual maturity at about 3 to 4 and is fully mature by 4 to 5. From about 5 to 20 is the adult stage, when most horses reach their best athletic potential. Over 20, he is generally counted as geriatric. Many geriatric horses still work, though perhaps without the stamina or soundness they had. The calculator shows the whole list and marks where he is.
Why every horse has the same birthday
Racing needs to know which horses are the same age, so registries count age by the calendar year. The Jockey Club’s rule book says that, for the purpose of determining age, the date of birth for all Thoroughbreds is deemed to be January 1 of the year of foaling. A foal born in May 2020 was officially 1 on January 1, 2021, and he is officially 6 all through 2026. Give the calculator his foaling date and it shows both ages: his real one, and the one on his papers. There is more in why horses all have the same birthday.
How long horses live
Merck puts the average at 25 to 30 years and says the real span depends on the horse’s health, care and size. Miniatures can live 40 years or more. Horses are living longer. In USDA’s studies, the share of U.S. horses aged 20 or more rose from 5.6% in 1998 to 7.6% in 2005 and 11.4% in 2015. In 2015, about 0.8% of horses aged 5 to 20 died in a year, against 3.1% of those over 20, and after 20 old age was the most common cause. The oldest horse on record is Old Billy, an English draft horse who died in 1822 at 62.