How do I tell if my horse is overweight/underweight (or gaining/losing weight)?

The best way to determine the ideal weight of a horse is to evaluate the Body Condition Score (BCS).  Weight tapes are good for measuring trends or estimating weight for a drug dosage, but not for telling how much a horse should weigh.  We grade BCS from 1-9, 1 being emaciated, 9 being extremely fat.  Ideally, pleasure or show horses should fall around 5 – 6.5 ("racing fit" horses are generally in the 4-5 range).  This means the ribs will not be visible, but will be felt easily by pressing through a thin padding of fat.  There should be a bit of fat around the tailhead which is a little spongy but not soft or bulging.  The withers should appear rounded; a small amount of fat deposited on either side is acceptable.  The back should be flat (no ridge), but a slight crease down the middle is acceptable:

 

 A very thin horse will have visible ribs and other prominent bony projections, such as the spinous processes (causing a ridge along the back), hip bones, withers, shoulders and neck bones:

 

 A very fat horse will have an obvious crease down the back, the ribs will be difficult or impossible to feel with patchy fat overlying them.  Fat deposits may be present along withers, behind the shoulder, at the tailhead, and at the inner thighs:

 

Once a determination of your horse’s BCS is made, you may either document his estimated weight with a weight tape, or simply record the BCS number and any notes from the examination.  Thereafter, check the BCS monthly, and repeat the weight tape if performed.  This allows you to track trends, and tell whether or not your horse is gaining or losing.

If you absolutely need an actual weight on your horse (usually for medication purposes), and you cannot find someone with a horse or cattle scale, the next best option is to drive the truck and trailer through a truck scale, then load the horse, weigh again, and subtract.