For the most accurate measurement, make sure your horse is on firm, level ground with their feet relatively even. Take a tape measure and stand on the end and bring it up to the highest point of the withers. Record the inches.
After you have found how many inches your horse is, you will need to convert the results from inches to "hands." Horse height is correctly referred to by a unit of measurement known as a "hand." One hand is equal to four inches. Take the number that you recorded and divide it by 4. For example, if your horse measured 58 inches and you divide that by 4, you get 14.5. The .5 means your horse was a half of a hand which translates to 2 inches. This means that your horse is 14.2 hands tall (14 hands plus 2 inches).
There are measuring devices on the market for horses that have hand and inches measurements marked on them.
Some devices are rigid poles with short crossbars toward the top that can be raised or lowered to rest on a horse's withers. These are very handy and give accurate measurements easily.
WHERE DID THE MEASURING TERM “HANDS” COME FROM?
The origin of measuring a horse this way is very old, but easy to understand.
In the past, people did not have the common measuring devices like tape measures to measure a horse. They used their hands. This would mean holding your hand out with your thumb pointed up and the distance between the edge of your palm at the bottom to the tip of your thumb at the top was about 4 inches. This obviously would vary depending on your hand size so somewhere along the way, the measuring unit of a hand was standardized to mean four inches.