Figuring what size girth your horse takes is a simple process. All you need is a tape measure and a calculator (or a brain that can do simple math). Asking someone to assist you makes it a bit easier and eliminates trying to wrangle around the horse.
Here's how to do it:
Have your horse stand on a level surface. Pass the tape measure around his body where the girth will rest---between his elbows and withers. (See photograph below). Make the two ends meet and pull it snugly. Take a look at the number. This is your horse's heart girth.
This simple formula will calculate your horse's girth size. Take your measurement in inches, divide it by two, and subtract 3. (Girth measurement/2) - 3.
Round up and go with the girth size that is nearest to your measurement. Keep in mind that girths usually only come in even numbers.
The resulting number will only be an estimate, but it is a good starting point for ordering the correct size.