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Drape a standard soft measuring tape across your horse's back, as in the step above. Hold the tape at the "zero" end.  The tape can be measured in feet and inches, or it can be measured in centimeters. The marking will affect the calculation that you do later in the process of determining your horse's weight. This is best done out of the wind. Wind can cause the tape measure to flap and twist, making it harder to manage. A flapping tape can also spook a horse, resulting in a difficult measuring session. The tape should be a few inches behind the horse's front legs. Raise the tape to meet the tape held in your other hand, and read the measurement where the "zero" end of the tape meets the other side. Write the measurement down on a paper.  You may notice that the tape runs at an angle from slightly behind the horse's front legs to the base of the withers. This is how it should be. To make sure you have the most accurate measurement, repeat the measuring process. You will need a second person to help you hold the measuring tape. Position the other person on the right side of the horse. This person should hold the "zero" end of the tape measure at the point of the horse's shoulder.  Don't stand too close to the horse's hind quarters, or he's likely to be spooked. Make sure the second person is attentive and quiet. Pull the tape taut once you reach the point of the horse's buttock or crease.  Make sure the tape isn't twisted as you pull it, or this may affect the accuracy of your measurement. The second person might hold your notebook and pen for marking your measurements, leaving your hands free for holding the tape. You now have your horse's heart girth and length written on a paper. For example, your horse's heart girth might be 78 inches, and his length 65 inches. If you've measured in centimeters, heart girth might be 200 cm, length 165 cm. You'll need these to calculate your horse's weight.  If your measurements are in centimeters, write down the number 11,990. Your weight will be in kilograms. If your measurements are in inches, write down the number 330. Your weight will be in pounds. For example, if heart girth were 78 and length were 65 inches, multiply 78 x 78 x 65.  You should arrive at the figure 395,460. When you have a total from the multiplication, divide this number by the number shown above.  If you're measuring in centimeters, divide your total by 11,990. The result will be your horse's weight, in kilograms. Find the weight of a horse with heart girth 200 cm, length 165 cm by using this equation: 200 x 200 x 165 = 6,600,000. Divided by 11,990, the final number would be 550. That is your horse's weight. If you're measuring in inches, divide your total by 330. The result will be your horse's weight, in pounds. So, using the example above, 78 x 78 x 65 = 395,460. Divided by 330, the final number would be 1,198. That's your horse's weight.

Summary:
Measure your horse's heart girth. Pull the tape gently beneath the horse's belly. Measure your horse's length. Walk beside the horse holding the measuring tape. Notice if your measurement is in inches (English) or centimeters (metric). Multiply the measured heart girth x heart girth x length.