Figuring Horse's Weight

Measure the heart girth just behind the elbow, taking the reading right
after the horse exhales. Measure body length from the point of the
shoulder to the point of the buttocks in a straight line.

Avoid using a cloth measuring tape because it may stretch. A metal
carpenter's tape is accurate (but noisy - can spook the horse!). A
plastic coated tape works best. If one is not available, use cord
or string that has no stretch and mark the spot with a pen; then
measure the cord with a carpenter's tape or yardstick.

Take the two measurements and multiply the heart girth in inches
by itself, then multiply that by the body length in inches. Divide
the total by 330 for the approximate weight in pounds.

For example, if the horse measures 75 inches around the heart girth
and body length is 64 inches: 75 x 75 x 64 / 330 = 1,090.9 lbs.

((heart girth x heart girth) x body length) / 330 = body weight

I think this is the most accurate way to measure weight without a scale.