Knowing your height-to-weight ratio can help you determine if you're at a healthy size. The standard way to calculate this ratio is with the body mass index, or BMI. BMI is a screening tool used in medical settings to estimate if a person has too much body weight for their height. This height-to-weight ratio can be one of many helpful health evaluation tools because it moderately correlates with more precise measures of body fat from a clinic.
The Height-to-Weight Ratio Equation
BMI is usually figured using metric measurements. It's expressed as your weight in kilograms divided by your height in meters squared.
For a person who is 70 kilograms and 170 centimeters, or 1.7 meters tall, the height-to-weight ratio via BMI would be 24.
Use American measurements in the equation by applying a conversion factor of 703. You figure weight in pounds divided by height in inches squared, then multiply it by 703. As an equation, it reads:
BMI = weight in pounds / [height in inches x height in inches] x 703.
A person who weighs 160 pounds and stands 5 foot, 10 inches tall, or 70 inches, would have a BMI of 23.
Many online calculators are available from reputable health organizations if you'd prefer not to do the math yourself.
Interpreting BMI Results
A BMI of 18.5 to 24.9 means your height-to-weight ratio is healthy. A person who registers a BMI of below 18.5 is likely underweight, while a person with a BMI above 24.9 is overweight and a BMI of 30 or higher indicates obesity. Being overweight or obese puts a person at greater risk of certain health conditions, including heart disease, type 2 diabetes and some cancers.
BMI may indicate possible overweight or obesity and the accompanying health risks, but it's not diagnostic and can't affirm whether or not you have a chronic condition. The calculation doesn't take into account your frame size, muscularity, age or gender. If you have a BMI that is outside of the normal range, your doctor may perform additional assessments to determine your risk of disease, including more direct measures of body fat, lifestyle questionnaires, blood pressure checks and blood tests.
BMI Has Limitations
Although BMI can be useful when evaluating much of the population, it can be inaccurate for certain people. Athletes or gym enthusiasts with a muscular build may weigh a lot for their height due to an abundance of muscle. Muscle is dense, but compact, and quite healthy, so a muscular person isn't at risk for developing disease simply because he weighs more due to muscle.
BMI can also mis-categorize people as having a healthy height-to-weight ratio when they actually carry too much body fat. This underestimation of body fat usually occurs in older adults who've lost muscle mass and sedentary people. If your body fat is greater than 20 percent fat as a man, or 30 percent as a woman, you could develop health conditions usually associated with people who are overtly overweight.
A height-to-weight ratio, like BMI, also fails to take into account how a person's weight is distributed. You may register a normal height-to-weight ratio but have too much belly fat. People with a wide waist -- 40 inches or greater as a man or 35 inches or greater as a woman -- have an excessive amount of visceral, or belly, fat. This fat infiltrates the area around internal organs, releasing compounds that raise your risk of chronic disease.
Alternative Ways to Figure Body Fat
Direct measures of body fat, including air displacement, body fat scales and X-ray scans, give you a better idea of your body fat levels. These measurements are available in health clinics and some fitness centers, but they are more invasive, require special equipment and training and are more expensive. Since BMI is more convenient and helpful for much of the population, it's preferred by many providers.
BMI tends to be most accurate for people with higher levels of BMI and body fat. If you fall into the upper range of normal for your BMI, or the lower range of overweight, your doctor may perform additional tests of your body fat levels to determine if you do have too much. Oftentimes your doctor can make further determinations about the accuracy of a BMI value just by doing a physical exam.