A horse measuring tape (or weight tape) is a soft, flexible tape used to find a horse's weight, height (in hands), and girth/length, providing crucial health data; you measure the girth right behind the elbow and the length from the point of shoulder to the point of the buttock, while height is from the ground to the withers, with the horse standing square, all to monitor growth, condition, and medication dosages accurately. 
How to Measure a Horse
 
Stand the horse square:
Ensure the horse is on level ground with all four feet planted evenly.
Measure Girth:
Wrap the tape around the horse's chest, just behind the front legs and elbow, keeping it snug but not tight.
Measure Body Length:
Start at the point of the shoulder and run the tape straight back to the point of the buttock (the bony part just before the tail).
Measure Height:
Place the tape at the ground and go up to the highest point of the withers (the ridge between the shoulder blades).
Read the Tape: 
Use the markings for height (in hands and inches) or the weight estimations printed on the tape (often in pounds or kilograms). 
Why Measure?
Monitor Weight: Track weight changes for proper feeding and condition.
Calculate Weight (Formula): Use the Body Weight Formula: (Girth² × Body Length) / 330 = Weight (lbs) for adult horses (adjust divisor for yearlings/weanlings).
Check Health: Ensure accurate medication dosing (dewormers, etc.) and monitor growth in young horses.