Gypsy, the Equine Encyclopedia, has something to say about how to measure a horse’s height. Like all shetlands, she is always watching, listening and learning.
Horses, ponies and all equines are measured in hands. One hand equals 4 inches. Horses are measured from one front hoof to the top of the withers (where the neck ties into the back). Ideally, before being measured, the horse should stand on a flat surface with all four feet squarely under him.
If a horse is between hands, then his measurement is in hands and inches (examples are 14 hands, 2 inches, 12.3, 15.2).