Horse Weight from Girth

How it is calculated

The estimate uses the girth-and-length formula: weight (lb) = girth² × length ÷ 300. Heart girth is the tape measurement around the barrel just behind the elbows and withers; body length runs from the point of the shoulder to the point of the buttock. Both go in as inches, because the constant 300 is built for inches and pounds, and each must be between 30 and 100 inches or no result is shown. The kilogram figure is the pound result times 0.453592, and both are rounded to whole numbers. Girth is squared, so it influences the answer more than length does. It is a field estimate for tracking change and rough dosing, not a replacement for a scale.

Common mistakes

• Typing centimetres. The formula expects inches: 188 cm entered as 188 is above the 100 inch limit and gives no result, and it must be entered as about 74 in. Values like 90 cm fall inside the limit and give a wrong, far too small weight.
• Measuring girth too far forward over the shoulders, or too loosely. Girth is squared, so a 2 inch error moves the result by roughly 68 lb at this size.
• Measuring length from the wrong landmarks, such as to the tail or from the poll. It should be point of shoulder to point of buttock.
• Measuring right after a large meal or while the horse is holding its breath, then comparing with an earlier reading taken under different conditions.
• Dosing a drug from the estimate alone. Treat the figure as approximate and follow the vet's instruction on the safety margin.