In which units is the height of a horse measured?

Horses are measured in 'Hands'. Each hand is 4 inches

You write it like this: 12.1hh. One inch higher would be 12.2hh. another inch higher is 12.3hh. But, one more inch taller, and it goes to the next highest number: 13hh

If they are under 14.2hh then they are a pony, if they are over, they are a horse.