Weighing In

Accepted wisdom in the horse world tells us that an "average" light horse weighs about 1,000 pounds, or 450 kg. A draft horse, upwards of double that. But does that rather arbitrary figure really mean anything? After all, what is an "average" horse? To a dressage competitor, it might be a 17-hand warmblood, while to a cutting horse enthusiast, it might be a 14-hand Quarter Horse or Arabian. In their own little universes, each might be said to be "average," yet their weight difference might be more than 600 pounds!

The 1,000-pound estimate is good enough for many purposes; for deworming, for example, it's a safe enough ballpark figure because modern deworming drugs are designed with huge margins of safety, making over-dosing a remote risk. (Under-dosing, 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...but that's another article.) But for other situations--such as calculating the amount of feed your horse should eat each day--having a more accurate idea of his true weight is a definite asset.

Weighing your horse isn't a simple matter. Even if you could teach him to stand with all four feet on your bathroom scale, chances are he'd exceed its maximum capacity and cause it to pop all of its springs. Nor are horse-sized scales readily available (they do exist, mostly in the setting of university veterinary hospitals).

You could, of course, use a set of public weight scales designed for measuring truck payloads; these can be found at some feed stores or along highways. With your horse in your trailer, you would park on the scales and get a reading, then return with an empty rig and take the difference. (Unloading your horse is not advised due to the traffic.) I would suggest calling first to see if they can accomodate you.

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

Get Out Your Calculator

Given all of these potential difficulties, it's fortunate that mathematicians have devised a number of relatively simple formulas with which you can estimate your horse's body weight. Taking a few measurements and plugging in the numbers really is all it takes.

The "quick n' dirty" way to do this is to employ a heart girth weight tape, a ridiculously low-tech piece of equipment that can be purchased at nearly any feed store or tack shop for a couple of dollars. (Some companies hand them out free as promotional items.) Such measuring tapes do the calculating for you--they're marked off at intervals which, when the tape is wrapped snugly around your horse's barrel (approximately over his heart, hence "heart girth"), will give you a rough estimate of his body weight. Notice I said "rough estimate." Heart girth weight tapes only take a single measurement of a single portion of your horse's considerable body, so their lack of accuracy is also considerable (and they're completely useless for pregnant mares and close to useless for very fit racehorses). For a quick educated guess, they're fine, but you should expect such a weight tape to be "off" by as much as 100 pounds in either direction.

For a somewhat more accurate calculation, turn to the internet. Several websites have simple "auto-calculators" that will generate your horse's body weight based on your plugging in a couple of measurements (usually, the heart girth plus a measure of your horse's length from point of shoulder to point of buttock). If the idea of crunching numbers intimidates you, this might be the most user-friendly way to go. A simple calculator of this type can be found at http://w1.195.telia.com/~u19503963/index.html/SIDA33.html.

Here's the formula to use without a computer: If A is the measurement in centimeters around your horse's heart girth (over the withers and snugged close behind his elbows, rather than a couple of inches farther back where his saddle girth fits), and B is the measurement (again in centimeters) from the point of his shoulder to the point of his buttock, then you would use this formula:

(A x A x B) divided by 11877

This will yield a body weight in kilograms. To figure pounds from kilograms, take number of kilograms multiplied by 2.205.

So, if your Morgan mare's heart girth measurement (A) is 180 cm, and her length (B) is 140 cm, then (180 x 180 x 140) divided by 11877 = 382 kg (or 842 pounds). There now, that wasn't so bad, was it?

If you prefer to work in inches, use 330 as the dividing factor instead of 11877. So, if the same horse measures 70.92 inches around his heart girth (180 multiplied by 0.394 converts centimeters to inches), and 55.16 inches in length, the calculation would be:

(70.92 x 70.92 x 55.16) divided by 330 = 840.7 pounds

From your final answer, you'll add or subtract a bit according to whether your horse is underweight or overweight. The website provides a link to a page which fully describes what you should look for to determine your horse's body condition score, then directs you to add or subtract anywhere from six to 30 pounds to your solution depending on your estimate of how much flesh he's carrying. Even the interaction between the three areas (neck, back, and hindquarters) is described for you, with the end result being a presumably more accurate, if more time-consuming, final answer.

For every rule there's an exception, of course, and naturally when it comes to horses there are several exceptions. Foals, for example, don't follow the rules for adult horses--and it can be important to monitor a foal's weight to keep track of the rate of weight gain in order to estimate the amount of medication he might need (such as his first deworming). Very small foals, if cooperative, can simply be hoisted in your arms while you stand on a bathroom scale, but they'll very quickly outgrow your ability to lift them. So, use this formula adapted for light horse foals:

[Heart girth (in inches)] - 25.1 divided by 0.07 = body weight in pounds


[Heart girth (in centimeters) - 63.7] divided by 0.38 = body weight in kilograms

These formulas work best on foals one to six weeks of age. For newborns and foals between seven and 12 weeks of age, add 17% to the total obtained by either of the above formulas.

Draft horses also are problematic; measuring them with a heart girth weight tape generally is useless, since most only go up to about 1,300 pounds. The formulas outlined above do work fairly well for heavy horses, but you should bear in mind that they'll probably estimate on the low side, due to a draft's considerable bone.

The Draft Horse Resource website, at www.draftresource.com, has a handy chart for draft horses based on heart girth measurements ranging from 29 7/16 inches (a newborn foal) right up to 96 1/4 inches (click on "My Weight Tape"). The chart is based on the "blocks" of weight found on a Purina heart girth weight tape and extrapolated upward, so do take its accuracy with a grain of salt, but it should give you a reasonable estimate of your heavy horse's true heft.

Calculating Diet Based On Body Weight

Okay, so you finally have an idea of what your horse weighs. Now, how do you apply that information to his feeding program?

Regular readers of this column will recognize this simple rule: Your horse should consume between 1.5% and 3.0% of his body weight every day in total feed. Sticking within these parameters will supply him with sufficient fiber, energy, and nutrients for healthy maintenance metabolism. If he's an easy-keeping pasture potato, lean toward the 1.5% end of the scale; if he's a high-energy, high-performance, high-octane machine, or if for some reason he's severely underweight (post-surgery, for example), he might need the extra calories at the 3.0% end of the formula.

In almost all circumstances, at least 50% of a horse's total diet should be made up of fiber--hay, pasture, or other types of forage (such as pelleted hay, roughage chunks, haylage, beet pulp, or another fiber source). In many cases, fiber can make up 100% of the daily ration; horses only need grain when the energy demands of their jobs exceed what fiber can supply.

So if, for example, you have an Arabian gelding who's currently being used for competitive trail riding at a moderate level (30 and 50 mile rides only, rather than the extreme demands of, say, the Tevis Cup), he might maintain his weight and condition where you'd like it on a diet of 75% fiber and 25% grain. If you've calculated your Arabian's body weight at 850 pounds, that means he'll need to consume between 12.75 pounds and 25.5 pounds of total feed every day. That means between 9.56 and 19.125 pounds would be hay (or another type of forage), and only (approximately) three to six pounds would be grain. Which end of the scale you steer toward is a matter of whether your horse is an "easy keeper" or a "hard keeper," whether the intensity of his work is increasing or decreasing (are you getting him fit, or is it the end of the season and you're letting him down?), and whether his current body condition is acceptable.

As you can see, there's considerable leeway within these parameters, but you can't go too far wrong if you stick within the 1.5% to 3.0% range.

Remember that your horse's weight will fluctuate with the seasons, so plan to do a new calculation of his weight at least every three to six months.


About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More