The best way to get a completely accurate measurement of a horse’s weight is on a livestock scale. Some veterinarian clinics have livestock scales and some may have portable scales. Auction barns often have scales. It may be possible for you to get your horse weighed on a scale if you take him to one of these places.
You might also be able to weigh your horse on a truck scale, but you will have to load your horse onto your trailer to take him there and weigh the whole rig, with and without the horse loaded on. The horse’s weight will be the difference of the two weights. If you weigh your horse on a different day than you weigh your empty trailer, things like how much gear you’ve got stowed and how much fuel is in the tank will affect the total weight. Truck scales are not as finely calibrated as livestock scales, so the weight may be out as much as 20 or 40 lbs, depending on how the scales are set up. Getting a weight this way is tricky and time consuming.
If you can’t get your horse on a livestock scale, you can approximate its weight by measuring its body. An approximate weight can be measured with a special weight tape. Weight tapes are available at tack shops and feed stores. They are inexpensive (which is good because mine seems to go missing frequently) and easy to use. They aren’t completely accurate, because they only measure around the girth of the horse, and don’t take into consideration other things like fat/muscle ratio, height, body type and condition.
Have your horse tied safely and standing quietly. With a purchased weight tape , measure around the horse so the tape goes around the girth area, and up over the horse, just behind withers. Take the measurement where the end of the tape meets the scale on the tape. If you are tracking your horse’s weight, you’ll need to be careful that you place the tape in the same place each time, and hold it with the same tension. If you have a pony or foal, a regular tape may not be accurate. Weight tapes for specific body types can be purchased.
You don’t need a special weight tape to get an approximate weight however. You can use a regular tape measure, or a piece of twine which you measure. The weight chart on the OMAFRA site can then be used to determine an approximate weight.
Another way to find an approximate weight is to measure around the horse as I've described, and take an additional measurement from the point of the horse’s shoulder to the ‘edge’ of the haunch. With this measurement you can then do a bit of math: girth x girth x body length ÷ 300 = Horse’s weight. So if your horse is 76 inches around and 38 inches long you would calculate 76 x 76 x 38 ÷ 300 = 782 lbs. this method is the traditional formula for estimating weight, but other formulas do exist. A Better "Weigh" for Horses: Equine Weight Estimation looks at the accuracy of common formulas, and presents a slightly different way of measuring and calculating weight.
This calculation will not be as accurate for ponies and foals. Here is a a formula to calculate foal weight. To estimate a miniature horse’s weight, this Calculating body weight in minis formula can be used.
It’s important to remember that all of these methods, except the livestock scale will be estimations. These estimations should be sufficient for calculating feed, medications and supplements. If however, you need a very accurate weight, consult with your veterinarian for the best approach.