Calculating a Dog’s Body Mass Index?
The body mass index, or BMI for short, is a quick way to ascertain whether a person is overweight, underweight, or just right. For humans, the BMI is calculated by the formula:
BMI = weightlbs / (heightinches )2 * 703
The resulting value is used to classify the person’s body condition. A score of 16-18.5 is considered underweight, 18.5-25 is considered to be ideal, and greater than 25 is considered overweight.
Although the BMI is probably a better measure than some other measures of body condition, there are many reasons why this single value alone is not a good indication of a person’s body. For example, muscular people will tend to weigh more than non-muscular people and thus, their weight will be higher, which could lead to an inappropriately high BMI value. For an example, my own BMI is ~ 25, which is right at the edge of ‘fat’. Yet, my body fat percentage is usually in single digits 😉 .
Because dogs are quadrupeds, the BMI calculation is not an appropriate metric to describe a dog’s physique. There is an alternative calculation, called the weight to height ratios (WTH), which can be used to assess a dog’s body condition which in turn can be used as an indicator of their ability to perform in different sports and tasks.
Weight to Height Ratio- the Canine BMI
The WTH ratio is calculated by first determining the dog’s weight in pounds. Then the dog’s height is measured from level ground to the withers. For those that don’t know, the withers is the highest point on the dog’s torso, at the base of the neck, between the shoulder blades. Once these two values are available, the WTH is calculated by dividing the weight in lbs. by the height in inches. As an example, a Labrador Retriever that is 75 lbs. and 26 inches high would have a WTH of 75/26 = 2.88.
Like with the human BMI, this measure doesn’t tell us if the dog is fat or muscular, but it could serve as a general indication of whether the dog is overweight. It is certainly a better indication of a dog’s body composition than just a simple weight. Dogs that are on the high end of a Breed’s weight range standard might be so because they are either taller, more muscular, or both.
More importantly, the WTH can provide insight into the type of exercise or training that is best suited for the dog. The higher the WTH ratio, the more stress is put on the body while moving around in space.
Consider the popular sport agility. According to Zink and Van Dyke in Canine Sports and Rehabilitation, a ratio of 2.5 or higher implies that the dog should be trained on softer, forgiving, and non-slip surfaces so as to minimize the joint stress and reduce injury risk. Dogs with a WTH of over 4 are at greater risk for injury and need a carefully structured training program.
Below are indicative WTH values for a select few popular dog breeds.
|Jack Russell Terrier||16||13||1.23|
|Pembroke Welsh Corgi||30||11||2.73|
|Weight in lbs., height in inches.|
|Sources: AKC, Wikipedia|