Body weight calculators help a healthcare provider determine dosages of medication, approximate renal function and ventilator settings. They can also help a doctor determine if you're at a healthy weight for your height. A variety of equations were derived from various "ideal" height-weight tables, but they are in no way absolute reflections of the health of your size. Body weight calculations can't diagnose your risk of developing conditions resulting from obesity or being overweight, but they can be one of many indications that you need further screenings. Several formulas exist that use centimeters and kilograms, and no one calculation is entirely accurate.
Body Mass Index
Doctors and other healthcare practitioners widely use body mass index, or BMI, as an easy, noninvasive way to estimate body fat levels. The original equation reads: weight in kilograms / [height in meters x height in meters]. To figure the equation in centimeters, divide height in centimeters by 100 to obtain your height in meters. For example, if you weigh 55 kilograms and stand 155 centimeters tall, your height in meters is 155/100 or 1.55. Plug these numbers into the equation: 55 / [1.55 x 1.55] = 23. (ref3)
The resulting number falls into a range on an index to help a doctor evaluate if you're at increased health risk due to your weight. The equation does not give you an exact or ideal weight you should strive for. A BMI below 18.5 indicates you're underweight, while a BMI of 18.5 to 24.9 is considered normal, a BMI of 25 to 29.9 is overweight, and a BMI of 30 or more is obese.
Why BMI Isn't Always Ideal
While BMI can be helpful to evaluate trends of overweight and obesity in a broad population, it can overestimate and underestimate body fat levels in many individuals. A muscular or athletic person may register as overweight according to the index, but they simply have an abundance of muscle rather than an excess of fat. Muscle is more dense than fat, so a person who is quite muscular can weigh more than a person of the same height who has a greater percentage of fat. Excess muscle doesn't present the same health risks as excess fat.
BMI may also put people who are at risk of health complications resulting from too much fat in a category of "normal" or "ideal" weight. In these cases, a person's gross weight seems healthy, but they carry too much fat. More than 20 percent body fat in a man or 30 percent in a woman is excessive, regardless of how much you weigh.
Wasit Circumference in Centimeters
Where you distribute weight, not just how much you weigh, also gives clues as to your health. Waist circumference is a way to tell if have healthy fat distribution. A waist size greater than 88 centimeters in a woman or 102 centimeters in a man indicates an excess of belly fat. Too much of this type of fat greatly increases your risk of inflammation and diseases, such as heart disease and type 2 diabetes.
People who have the "pear-shaped" physique, or carry more weight in their hips and thighs, are usually at less risk of chronic disease than those with abdominal obesity.
French surgeon Paul Broca developed the Broca index in the 19th century as a rough estimate of optimal body size. It's an extremely rough estimate that may apply to some people, but doesn't take into account age or body composition. It also fails to make adjustments for different body shapes, such as people who are stocky or long and lean. The formula isn't used often today because of its questionable accuracy.
The formula requires you just to know your height in centimeters. Men simply subtract 100 from that number to find your ideal weight in kilograms. Women follow the same formula and then take 90 percent of the weight as their ideal size.
For example, a man who stands 180 centimeters tall has an ideal weight of 80 kilograms. A woman who stands 154 centimeters tall performs the formula: [154 - 100] x .9 = 48.6 kilograms.
Equation for Medical Use
An estimation of ideal body weight for the average person helps clinicians provide medicine dosages and other medical interventions that are based on weight. First published by Dr. B.J. Devine, the formula for females reads: 45.5 kilograms + 2.3 kilograms for every inch taller than 5 feet.
An inch is equal to 2.54 centimeters, so to figure in kilograms and centimeters, the formula reads: 45.5 kilograms + 2.3 kilograms for every 2.54 centimeters taller than 152 centimeters.
So, a woman who stands 156 centimeters, or about 5 foot 1 inch, should weigh, according to this formula: 45.5 + 2.3 x 1.57 = 49 kilograms, which is about 108 pounds.
For a man, the formula reads: 50 kilograms + 2.3 kilograms x every inch over 5 feet, but you can again substitute in 2.54 centimeters for every inch. In this case, the formula reads: 50 kilograms + 2.3 kilograms x every 2.54 centimeters over 152 centimeters.
So, a man who stands 180 centimeters, or almost 5 feet 11 inches, should weigh, according to the formula: 50 + 2.3 x 11 = 75.3 kilograms, or 166 pounds.
This equation is an extremely rough estimate of weight that's used in clinical settings. It isn't based on age, body composition or frame size.