Horses and ponies are measured from the ground to the top of the withers and are traditionally measured in "hands". One hand is equal to 4 inches. Originally horses were measured by the width of a person's hand which was approximately 4 inches, and by placing one hand on the ground, the other above it and then moving the first hand over the second, etc to measure the horse.
The term used for height measurement of a horse is "hands high" or "hh". Often the height is just over a number of hands eg 16 hands and 2 inches and the height is therefore referred to as 16.2 hh. With Europeanisation horses are also now being measured in centimetres, particularly small ponies. See conversion table for horse's height in hh and cm.
To measure a horse or pony it is best to ensure they are stood squarely on solid ground as this will give the most accurate height measurement.
There are tape measures marked out in hands that can be used but it is difficult to ensure these are held straight and then judge the height accurately as the tape has to be held some distance away from the withers of the horse.
A measuring stick designed for measuring a horse's height is the best way to measure a horse. A measuring stick consists of an upright wooden "ruler" marked out in hands and/or centimetres with a sliding wooden arm containing a spirit level at right angles to the ruler. The stick is then stood upright alongside the horse and the horizontal arm lowered until it sits on the withers and adjusted until the spirit level in the arm indicates the arm is level. At this point the height measurement is taken from the ruler to give the height of the horse.