Summarize this article:

Horses can be measured with a measuring stick or measuring tape. There are specific measuring sticks that measure in a unit called “hands" but you can also use a standard measuring tape. However, a horse measuring stick is the easiest way to measure a horse quickly and accurately. Horse measuring sticks can be found at equestrian supply stores (tack shops), farm supply, and from various online retailers.  Since horse heights are typically talked about in terms of hand units, if you use a standard tape measure, you will need to convert your measurement into hands. The hand is the most common form of horse measurement in the United States, Canada, and England. However, throughout other locations around the world, the metric system is used to record the horse's height. Make sure that the horse’s hooves are all on equal footing and its body looks tall and well-balanced. It is particularly important to keep the horse’s body level, as uneven ground can impact your measurements. For example, you might position the horse near a wash rack with ties, if you have one in your barn. As another option, the flat, concrete area you typically use to groom or tack your horse will also work great. You want the hooves to be right next to each other, hips width apart. Having one of the feet in front of the other can throw off your measurement. Having the feet next to each other lines up both legs all the way up to the top of the horse’s body. This line is where you will measure your horse.

Summary:
Get a measuring implement before positioning your horse. Put the horse on a firm and level area of ground. Move the horse’s feet if they are not positioned squarely.