You may have access to a scale. Is there a vet school hospital or surgical hospital near you? I weighed my big boy at the vet school and was shocked to find out that he weighed 1600lbs. On the weight tapes, he is much less than that. It is really nice to have an accurate weight.
Most vet hospitals have scales, as another poster said. Some feed companies in the more "horsey" areas do as well.
An alternative option is to load the horse up and drive over the scales used to weigh semi trucks. Then go home, unload horse, and go back to weigh your empty rig. Loaded weight - empty weight = approximate horse weight.
The heart girth/length formula posted above is probably the closest estimate you will get sans scale. "Weight tapes" tend to underestimate weight, in my experience. For example, a few years ago Montana (15 hand Arab cross), at a BCS of 5, taped at 850 lbs. On the vet school's scale that same week, he weighed 938 lbs. That's almost a 100 lb. difference! I don't put much stock in what the weight tapes say now, except as an objective gauge of whether the horse is gaining or losing weight.
1. trailer horse to vet hospital/practice with a scale.
2. take empty truck/trailer to a semi scale - they are all over the place. Make sure you bed it and put in hay before you weigh it. Go home, add horse, go back. Subtract one from the other and you have the weight of your horse.
Weight tapes only work for a certain type of horse. Anything outside that and it will be wildly off.