Traditionally horses are measured in hands. One hand will equal 4 inches (the average width of the human hand). The horse's height is termed as x “hands high” or “h.h”.
What tools to use?
You can use your hands to measure your horse's height by placing one on top of the other. However, this isn't very accurate and practical. Ideally, you should use a tape or a stick. A measuring stick will work best, because it won't move that easily and will stay taught (unlike the tape).
Make sure your horse is standing on flat solid ground. Get your horse to stand squarely to ensure you get an accurate measurement.
How to measure?
With your horse standing still place the stick/tape upright on the ground, keeping it as perpendicular to the ground as possible. Usually, a measuring stick will come with and arm, but if it doesn't you can just make use of a short straight stick.
Place this stick/arm on top of the withers and hole it perpendicular to the stick/tape. Take note of where the stick/arm touches the stick/tape and mark it if you want.
Calculating your horse's height
If you used a measuring stick/tape that measures in inches or centimeters you have to convert your measurement into hands. If you measured in inches, all you have to do is divide the amount by 4. The total will give you your horse's height in hands.
If your horse measured 58 inches, you divide it by 4 and get 14.5. So, your horse is 14 and a half hands high. Correctly written your horse would be 14.2 h.h because half a hand is 2 inches (remember one hand equals 4 inches).
If you measured your horse in centimeters you will have to convert your measurement into inches first and then divide it by 4.
My pony measures 129,5 centimeters. I divide this by 2.5 (1 inch equals 2.5 centimeters) and get 50.98 inches. Divide this by 4 inches and I get 12.77.
So I know my pony is 12 hands but I have to calculate how many inches 0.7 is (I usually just work with the first decimal). Now what I do is I times 7 by 0.4 and get 2.8 inches. Thus my pony measures 12 hands and 2.8 inches. So I can say that my pony is somewhere between 12.2 and 12.3 h.h.
Here is a LINK to a table that gives you all the heights in hands and their values in centimeters :)