Most horse people have a weight tape hiding in their tack room somewhere, but if for some reason it is hiding in a place you can't find it, then there is another way to get a rough estimate of your horse's weight.

Begin by measuring the heart girth of the horse.  This is the area just behind the withers all around the girth area.  Then you will measure the length of the horse by measuring from the middle of the chest to the tail.  You will then use this equation to determine the horse's weight.

Heart Girth x Heart Girth x Length / 330 = Horse's Weight

This amount may not be exactly what your horse weighs but it is typically close enough for giving your horse medications such as Bute and dewormer.  The only true way of getting your horse's weight is to take him to a veterinarian with a scale.  Because this is not always available, this equation as well as a weight tape will give you an estimate that is close to your horse's actual weight.  In fact, there are actually few veterinarians who will weigh a horse and will merely take an educated guess, especially if they are out in the field and not in the clinic.