Estimating Body Weight from Measurements

Horses November 05, 2014 Print Friendly and PDF

Jason Turner, New Mexico State University Extension Horse Specialist

The 1,000-pound estimate is good enough for many purposes. For example, for deworming, it's a safe ballpark figure because modern deworming drugs are designed with huge margins of safety, making overdosing a remote risk. Underdosing, on the other hand, can happen quite easily, and some researchers feel it might contribute to an incomplete worm kill and subsequent increased resistance by the remaining parasitic population in the gut. For other aspects of horse management, such as calculating the amount of feed a horse should eat each day, having an accurate idea of a horse's true weight is a definite asset.

Relying on "eyeballing" your horse's weight is not a good idea. One study of 77 farm managers and 62 veterinarians (averaging 17 and 21 years, respectively, in professional experience with horses), showed that visual examination failed to produce a reasonable weight estimate nearly 100 percent of the time. When asked to estimate the weight of five horses of varying sizes, these professionals guessed on the low side 87.5 percent of the time (by an average of 186 pounds) and on the high side 12.5 percent of the time (by an average of 92 pounds). Nearly 60 percent underestimated the weight of all five horses.

Measuring Body Weight 

The quick way to measure body weight is with a heart girth weight tape. It can be purchased at nearly any feed store or tack shop for a couple of dollars, and it may be provided free of charge. The measuring tape does the calculating for you. The tape is marked at intervals, so that when the tape is wrapped snugly around the horse's barrel (approximately over its heart girth), it will provide a rough estimate of the horse's body weight. Heart girth weight tapes only take a single measurement on a single portion of the horse's body, so their accuracy is somewhat questionable. They're completely useless for pregnant mares, foals, and miniature horses and nearly useless for very fit racehorses, but for a fast estimate, they are fine. You should expect a weight tape to be "off" by as much as 100 pounds in either direction.

The table below was taken from Lon Lewis’ "Feeding and Care of the Horse" and will help you to estimate the weight of a horse by its girth measurement when a weight tape is not available.

 

Using a Measurement Tape 

 

Girth Length (inches)
Girth Length (cm)
Weight (lbs.)
Weight (kg)
30 76 100 45.5
40 102 200 91
45.5 116 300 136.5
50.5 128 400 182
55 140 500 227
58.5 148 600 273
61.5 156 700 318
64.5 164 800 364
67.5 171 900 409
70.5 178 1000 455
73 185 1100 500
75.5 192 1200 454
77.5 197 1300 591

Using a Scale

The use of a portable weigh scale is the most accurate way to determine a horse’s weight. Portable scales are not an item commonly found on most horse farms. If scales are available, be consistent when taking weights, because feeding and watering will significantly affect the differences in weight between readings. Consistently weigh either before of after a feeding time, and weigh at the same time every day. Zero the scale after each weighing.

Weight Scale for Horses

 

 

Body Measurements to Determine Body Weight

Various body measurements are used singularly or together to estimate the body weight of horses. The weight tape normally uses the girth measurement. The girth measurement alone has a correlation of 0.87 (p<0.001), while girth and length have a correlation value of 0.90. This is the most accurate estimate as reported by Carroll and Huntington (1).

All measurements are in centimeters unless indicated.

Weight (kg) adult horses =
2(girth)2 x length
11880 cm3

Foal Weight 0 - 60 days

Foal Weight (kg) =
heart girth in inches - 25
.07

The average Thoroughbred or Standardbred foal will double its weight in the first 28 days.

Using Multiple Measurements to Estimate Body Weight

The following chart provides an alternative way of estimating a horse’s body weight. It was developed by C. L. Carroll and P. J. Huntington (1).

This method, which relates measurement around the girth to length, is slightly more accurate and can be used to estimate body weight for very fit racehorses. Girth measurement is taken immediately behind the elbow after the horse has breathed out. Length is taken from the point of the shoulder to the point of buttocks. Plot the two measurements on the chart and connect the two values with a straight line. Where the line crosses the center axis is the estimated weight. In the example below, a horse with a girth of 180 cm and length of 175 cm shows an estimated body weight of 475 kg.

Body Weight Measurement Chart

Chart for estimating body weight from girth and length measurements.

Measurements of girth and length are used to estimate body weight. None of these systems completely replaces the accuracy of a weigh scale when used over an extended period. However, they are often more accurate than a horseperson’s eye.

1. Carroll C.L., and Huntington P.J., (1988), Body Condition Scoring and Weight Estimation of Horses, Equine Veterinary Journal 20 (1), 41-45.

Body Weight Measurement Chart
 

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.