How many feet are on the ground when horses are running?

Horses are measured in hands not feet, but you can always break down the hands measurement into feet and inches. Keep in mind one hand is 4 inches.

Horses range on average from 14.3 to 17 hands tall, but can get taller than this (ponies are 14.2 hands and under, as it is a height designation.)

So this means a 14.3 hand horse is 4'11" and a 17 hand horse is 5'8" at the withers.

An interesting side note. The worlds smallest horse, a dwarf miniature horse is Thumbelina at just 17 inches or 1'5" (4.1 hands), while the tallest horse on record is Big Jake a Belgian draft horse that stands 20.2 3/4 hands which is 82.75 inches or 6'10 3/4" at the withers.