This is the method that I use and it works perfectly. I have done all my horses this way and never had on get sores from it being the wrong size.

Here is the instructions from Cowboy Way, they have a picture showing you exactly where to put the measuring tape, I was unable to cut and paste the picture. The link is below.

http://www.cowboyway.com/HowTo/MeasureGirth.htm


Measure Your Horse's Girth
And How To Calculate Western Cinch Size

It is easy to measure your horse's heart girth. This measurement can be helpful for several things including getting an estimate on the size of the cinch your horse will need, and for calculating an estimate of a horse's weight.

Measuring Heart Girth
To measure a horse's heart girth, measure from the base of the withers down to a couple of inches behind the horse's front legs, under the belly, then up the opposite side to where you started. Notice this will cause your tape measure to run at an angle as shown by the yellow line in the photograph at right.

The measurement you arrive at is your horse's heart girth.



Calculating Western Cinch Size
There are several different ways to measure your horse for a Western cinch. The one we have found to be the most reliable is to measure the horse's heart girth (in inches), divide that measurement by 2, then subtract 3 inches.

The horse in the photo above measured 77 inches for his heart girth. So, to calculate his cinch size we're going to take his hearth girth, divide that measurement by 2, then subtract 3 inches:

77 inches divided by 2 = 38.5 inches

38.5 inches minus 3 inches = 35.5 inches

That's darn close! The horse in the photograph wears a 36 inch cinch. A different horse we measured the same day had a 75 inch heart girth. Seventy-five divided by 2 is 37.5, minus 3 inches gives us an answer of 34.5. That's also very close - this mare wears a 34 inch cinch.

This Is Just An Estimate...
Calculating your horse's cinch size using the method above is not always completely accurate. There are enough differences in saddles and horse body types to make any method of measuring for a cinch little more than an educated guess. Measuring an old cinch (if you have one) that fit the horse and saddle well would be a better way of finding out a good size for a new cinch, but that's not always possible. Of the methods we have tried, however, the one we described above has been the most accurate.


Hope this helps