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An old fashioned way to measure your horse's weight is as follows:
Take a tape measure and measure the length from the head to the top of the tail in inches (X), then measure round the girth (Y).
Multiply these measurements together (X x Y), then divide the result by 11. This will give the weight in pounds. If you divide the weight in pounds by 2.2 this will give the weight in kilos.
For an accurate assessment use a weighbridge.