Measuring the height of a horse is really easy, once you know a couple of tips.

Tip 1: Make sure your horse is standing on hard, level ground. It may sound obvious, but a horse standing on soft soil, isn't going to measure accurately. Your horse should have it's front feet as evenly placed as possible.

Tip 2: Use a horse measuring stick or a rigid plastic or steel measuring tape for accuracy.
You don't want your tape bending midway. Measure from the ground up, in a straight line to the highest point of the horse's withers.

Tip 3: Not sure where to stop measuring? An old rancher's trick is to place your horse's favorite treat on the ground in front of him. Watch his shoulders as he bends down to eat the treat. The highest part of the shoulder that sticks up is the measuring point. Measure in a straight line from the ground to this point.

How many hands?
The height of horses, all equines actually, is expressed in "hands" and not inches and feet. If you've measured in inches, you will have to convert from inches to hands. A hand equals four inches and is written as the letters HH or hh after the actual numbers. For instance, 14HH would mean the horse measures 14 hands or 56 inches tall. If your horse should measure, 58 inches, that would actually convert to 14.5 when divided by 4, with the .5 actually meaning another half a hand or 2 inches. Horsemen would express this measurement as 14.2, meaning 14 and one-half hands high. It would be written as 14.2.

This can get a little tricky for the novice. Just remember that There is no such thing as 10 hands 4 inches, as 4 inches would be an additional hand. However, even fractions of an inch can be listed, such as 10.3 ¾ hh. (Ten hands, 3 ¾" inches, or Ten-three and three-quarters: just ¼" shy of a full 11 hands.) The first method of measuring anything, before rulers, was to measure by the average man's hand or foot. Measuring horses was easier to do with hands than feet. Usually a "hand" was defined the width of a person's hand using the fingers and the thumb or as the height of a clenched fist, Eventually, it was standardized to mean four inches.