I ran across this on Facebook! I've not tried it yet but thought it interesting.
You can estimate the ideal body weight of your horse using an equation recently developed by University of Minnesota researchers. Ideal body weight (lbs) = (body length (in) x 15.56) + (height (in) x 23.47) – 1,344 (for Arabians), 1,269 (stock horses) or 1,333 (ponies). The image demonstrates how to properly measure body length (D) and height (A). Researchers are currently developing similar equations for warmbloods and draft horses.
I couldn't get the photo to imbed but here is a link to see where the measurements should be made: