Body Mass Index (BMI) Calculator

How to calculate your body mass index

This BMI calculator will let you screen yourself for obesity using the same tool doctors and medical researchers use. If you're not satisfied with the results, take heart: There is no single ideal weight and height for men and women. Ideal weights vary from individual to individual and usually increase with age. For instance, people with muscular builds may be classified as “obese” according to the body mass index, even with a small body fat percentage. A high or “obese” score from the BMI calculator tool is simply a way to tell clinicians to look deeper into a person's health data for evidence of obesity-related health risks and disease.

To use this calculator:

  1. The default BMI of 16.6 shown on the tool is for a 5'5" person who weighs 100 lbs.
  2. Use the switch on top to pick either metric or imperial measurements.
  3. Set the height and weight dials to match your own height and weight.
  4. View you BMI number at the center of the tool and see where you fall on the obesity chart.

BMI values are based on clinical data about averages for these measurements across a wide swath of people.

What is the body mass index?

The body mass index (BMI) is a measurement of choice for many physicians and researchers studying obesity.

The BMI uses a mathematical formula that accounts for both a person's weight and height.

The BMI measurement, however, poses some problems. Not everyone agrees on the cutoff points for "healthy" versus "unhealthy" BMI ranges. BMI also does not provide information on a person's percentage of body fat. However BMI is a useful general guideline and is a good estimator of body fat for most adults 19 and 70 years of age. However, it may not be an accurate measurement of body fat for bodybuilders, certain athletes, and pregnant women.

The BMI equals a person's weight in kilograms divided by height in meters squared (BMI = kg/m2). To calculate the BMI using pounds, divide the weight in pounds by the height in inches squared and multiply the result by 703.

It is important to understand what "healthy weight" means. Healthy weight is defined as a body mass index (BMI) equal to or greater than 19 and less than 25 among all people 20 years of age or over. Generally, obesity is defined as a body mass index (BMI) equal to or greater than 30, which approximates 30 pounds of excess weight.

The World Health Organization uses a classification system using the BMI to define overweight and obesity.

A BMI of 25 to 29.9 is defined as a "pre-obese." A BMI of 30 to 34.99 is defined as "obese class I." A BMI of 35 to 39.99 is defined as "obese class II." A BMI of or greater than 40.00 is defined as "obese class III."

What can’t your BMI tell you?

Assessment of obesity is directed not only at how much fat a person has, but also where that fat is located on the body. The pattern of body fat distribution tends to differ in men and women.

In general, women collect fat in their hips and buttocks, giving their figures a "pear" shape. Men, on the other hand, usually collect fat around the belly, giving them more of an "apple" shape. (This is not a hard and fast rule; some men are pear-shaped and some women become apple-shaped, particularly after menopause.)

Apple-shaped people whose fat is concentrated mostly in the abdomen are more likely to develop many of the health problems associated with obesity. They are at increased health risk because of their fat distribution. While obesity of any kind is a health risk, it is better to be a pear than an apple.

In order to sort the types of fruit, doctors have developed a simple way to determine whether someone is an apple or a pear. The measurement is called waist-to-hip ratio. To find out a person's waist-to-hip ratio

measure the waist at its narrowest point, and then measure the hips at the widest point; divide the waist measurement by the hip measurement. For example, a woman with a 35-inch waist and 46-inch hips would have a waist-to-hip ratio of 0.76 (35 divided by 46 = 0.76).

Women with waist-to-hip ratios of more than 0.8 and men with waist-to-hip ratios of more than 1.0 are "apples."

Another rough way of estimating the amount of a person's abdominal fat is by measuring the waist circumference. Men with a waist circumference of 40 inches or greater and women with a waist circumference of 35 inches or greater are considered to have increased health risks related to obesity.

Are weight-for-height tables useful to determine obesity?

Measuring a person's body fat percentage can be difficult, so other methods are often relied upon to diagnose obesity. Two widely used methods are weight-for-height tables and body mass index (BMI). While both measurements have their limitations, they are reasonable indicators that someone may have a weight problem. The calculations are easy, and no special equipment is required.

Most people are familiar with weight-for-height tables. Although such tables have existed for a long time, in 1943, the Metropolitan Life Insurance Company introduced their table based on policyholders' data to relate weight to disease and mortality. Doctors and nurses (and many others) have used these tables for decades to determine if someone is overweight. The tables usually have a range of acceptable weights for a person of a given height.

One problem with using weight-for-height tables is that doctors disagree over which is the best table to use. Several versions are available. Many have different weight ranges, and some tables account for a person's frame size, age and sex, while other tables do not.

A significant limitation of all weight-for-height tables is that they do not distinguish between excess fat and muscle. A very muscular person may be classified as obese, according to the tables, when he or she in fact is not.

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Body mass index vs. body fat percentage

While the BMI is useful as a screening tool, it's only based on weight and height relative to age, which don't comprise enough data points to tell doctors about your body's composition. If you are a bodybuilder, for instance, you may have an "unhealthy" BMI score, even though you are an athlete with little body fat and are in excellent physical health. Measuring your individual body fat percentage, however, gives your healthcare professional an accurate and individualized reading of your risk for obesity-related disease.

Actually measuring a person's body fat percentage is not easy and is often inaccurate without careful monitoring of the methods, however. The following methods require special equipment, trained personnel, can be costly, and some are only available in certain research facilities.

  • Underwater weighing (hydrostatic weighing): This method weighs a person underwater and then calculates lean body mass (muscle) and body fat. This method is one of the most accurate ones; however, the equipment is costly.
  • BOD POD: The BOD POD is a computerized, egg-shaped chamber. Using the same whole-body measurement principle as hydrostatic weighing, the BOD POD measures a subject's mass and volume, from which their whole-body density is determined. Using this data, body fat and lean muscle mass can then be calculated.
  • DEXA: Dual-energy X-ray absorptiometry (DEXA) measures bone density. It uses X-rays to determine not only the percentage of body fat but also where and how much fat is located in the body.

The following two methods are simple and straightforward:

  • Skin calipers: This method measures the skinfold thickness of the layer of fat just under the skin in several parts of the body with calipers (a metal tool similar to forceps); the results are then used to calculate the percentage of body fat.
  • Bioelectric impedance analysis (BIA): There are two methods of the BIA. One involves standing on a special scale with footpads. A harmless amount of electrical current is sent through the body, and then percentage of body fat is calculated. The other type of BIA involves electrodes that are typically placed on a wrist and an ankle and on the back of the right hand and on the top of the foot. The change in voltage between the electrodes is measured. The person's body fat percentage is then calculated from the results of the BIA. Early on, this method showed variable results. Newer equipment and methods of analysis seem to have improved this method.

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Reviewed on 10/3/2019
References
National Institutes of Health