What is the Body Mass Index or BMI?
The Body Mass Index or BMI is a method of classifying whether an individual is overweight, underweight, obese or normal weight based only on their height and weight and does not use gender specific information. To calculate your BMI see the online BMI calculator, or see the following questions for more BMI Information.

What are the classifications of the Body Mass Index?
A BMI of less than 18.5 is underweight, normal weight is between 18.5 and 25, overweight is between 25 and 30 and obese is greater than 30.

What are the BMI limitations?
The BMI does not make a distinction between fat and muscle. We have compiled a list of athletes who won Gold medals at the Athens 2004 Olympics and would be considered overweight using the BMI scale. See our BMI for athletes section, which shows that all members in the Gold medal winning GB rowing coxless four would be considered overweight using this scale. This highlights a person with a lot of muscle (such as an athlete) may have a BMI in the unhealthy range, but still be healthy and have little risk of developing diabetes or having a heart attack. BMI also may not accurately reflect body fatness in people who are under 5 feet and in older people, who tend to lose muscle mass as they age.

Can the BMI predict my ideal weight?
Yes, see our online ideal weight calculator which also calculates your BMI and estimates the time taken to achieve your ideal weight.

How is the Body Mass Index Calculated?
BMI is found by dividing a person's weight in kilograms by height in meters squared. The mathematical formula is:

BMI = weight (kg) / height squared (m2)

To determine BMI using pounds and inches, multiply your weight in pounds by 704.5,* then divide the result by your height in inches, and divide that result by your height in inches a second time. Alternatively use the online BMI calculator.

Who decided the BMI ranges and how?
In 1995, the World Health Organization recommended a classification for three "grades" overweight using BMI cutoff points of 25, 30, and 40. (see Physical status: The use and interpretation of anthropometry. Report of a WHO Expert Committee. World Health Organization: Geneva, 1995 (WHO Technical Report Series; 854)).
A similar definition was released in a report in June 1998 (see Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. National Institutes of Health, National Heart, Lung, and Blood Institute. June 1998). This report was from an expert panel convened by the National Heart, Lung, and Blood Institute (NHLBI) in cooperation with the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), both part of the National Institutes of Health (NIH).
The panel identified overweight as a BMI > 25 to less than (<)30, and obesity as a BMI > 30. These definitions, widely used by the Federal government and increasingly by the broader medical and scientific communities, are based on evidence that health risks increase more steeply in individuals with a BMI > 25.

What is the relationship between body fat and the Body Mass Index?
BMI does not directly measure body fat percentage. Deurenberg et al. produced a formula to predict body fat from BMI which can be accessed from our body fat and Body Mass Index calculator An alternative to the Deurenberg calculator is the Gallagher body fat calculator. We also have alternative body fat calculators based on body part dimensions and skinfold measurments from calipers.

How accurate are the BMI classifications?
BMI cutoff points are a guide for overweight and obesity and are useful for comparative purposes across populations and over time; however, the health risks associated with overweight and obesity are on a continuum and do not necessarily correspond to rigid cutoff points. For example, an overweight individual with a BMI of 29 does not acquire additional health consequences associated with obesity simply by crossing the BMI threshold > 30. However, health risks generally increase with increasing BMI.

Is the distribution of weight important?
Excess weight, as measured by BMI, is not the only risk to your health. So is the location of fat on your body. If you carry fat mainly around your waist, you are more likely to develop health problems than if you carry fat mainly in your hips and thighs. This is true even if your BMI falls within the normal range. See our waist to hip ratio calculator. Women with a waist measurement of more than 35 inches or men with a waist measurement of more than 40 inches may have a higher disease risk than people with smaller waist measurements because of where their fat lies.

What are the dangers of extra weight?
Extra weight can put you at higher risk for these health problems - type 2 diabetes (high blood sugar), high blood pressure, heart disease and stroke, some types of cancer, sleep apnea (when breathing stops for short periods during sleep), osteoarthritis (wearing away of the joints), gallbladder disease, liver disease and irregular menstrual periods.

Who invented the Body Mass?
The modern Body Mass Index or BMI calculation is based on the work of the Belgian mathematician, Lambert Adolphe Jacques Quetelet who published a study of the average man in 1835 entitled "A Treatise on Man and the Development of His Faculties" ISBN 0820110612. This also explains why the BMI is sometimes referred to as the Quetelet index.

What are the alternatives to the Body Mass Index?
An alternative classification scheme uses the waist to hip ratio (see our online waist to hip ratio calculator) to measure vascular health risk. Other methods estimate body fat percentage and use this value to suggest whether an individual is overweight. These methods range from body fat calculators based on body circumference through to esimates of body fat from calipers using skin folds. More complicated methods include underwater weighing, bioelectrical impedance, and computerized topography.

Are there any units to the Body Mass Index?
Yes, the units are kg/m2.

Which organisations use the Body Mass Index?
The U.S. National Heart, Lung and Blood Institute and National Institutes of Health, and the World Health Organization recommend the body mass index to classify obesity.

Is the body mass index a better predictor than body fat?
Although body mass index is useful because it is easy to measure it has been shown in a recent publication that body fat may be a better method to evaluate obesity (see Ten-year comparison of BMI, body fat, and fitness in the workplace, Am J Ind Med 2006 49(4) 223-230 and Relationship between adiposity and body size reveals limitations of BMI, Am J Phys Anthropol. 2006 129(1) 151-6.)