The BMI Formula to calculate Body Mass Index:
Who decided the Body Descriptions? What is overweight?
by Steven B.Halls, MD FRCPC
A Body Mass Index or bmi score between 25 and 29.9 is “overweight“, and greater than or equal to 30 is “obese“, according to an old BMI definition1. Furthermore a normal bmi was said to be between 18.5 to 24.9. This definition is used by the World Health Organization (WHO) as its international standard3. You can read more about it, (including its problems).
The oldest version of the halls.md body mass index calculator used an even older definition of Overweight. It was based2,8,11 on this table below (for adults). Simply compare your bmi calculation with the table below to see which category you fall into.
|in normal range||19.1-25.8||20.7-26.4|
|very overweight or obese||>32.3||>31.1|
|severely obese||35 – 40|
|morbidly obese||40 – 50|
|super obese||50 – 60|
For children, there was no suitable Body Mass Index (BMI) criteria (until around year 2000), which was a weakness of previous versions of the bmi calculator. (Actually, nearly all other BMI calculators on the internet are still not suitable for use for children.) See below for recent changes!
For older adults, the previous halls.md calculator was using an age adjustment, by adding 1 extra BMI unit per decade of age, starting at age 45, to the criteria shown in the table above7.
New Features, New BMI criteria:
A year 2000 addition to the halls.md body mass index calculator, was a proper method to judge Children’s BMI according to Age, using the CDC criteria. That was a significant improvement and its a unique feature amongst on-line calculators. The thresholds for Adults BMI were set to use the CDC criteria as the default method.
Here is my discussion of how the ‘halls.md v2’ method for judging Adults BMI according to age was developed. It’s interesting, and contains some good science.
Even so, there are still some problems using Body Mass Index (BMI) to determine how overweight you are. Some people with a high percentage of muscle mass can have a high bmi score, which wrongly suggests fatness. Some races, ethnic groups and nationalities (such as those of native american ancestry) have a different body-fat percentage and body composition, so different BMI criteria are needed. This is important medically because some ethnic groups are more at risk of weight related health problems, (such as heart disease, type diabetes, high blood pressure and sleep apnea) than others. I hope to improve the bmi calculator in these areas, in the future.
Weight and Height percentiles
Unlike the Body Mass Index formula, the mathematical formula for height and weight percentiles is an estimate, rather than an exact calculation. But it’s a GOOD thing. A percentile is not the same as a percent. Your percentile rank indicates how your measurement compares to the recent American population. For example, an adult whose weight is at the 50th percentile, is very close to the average weight of the population. An adult at the 90th percentile is quite heavy, and 90% of the population would be lighter.
According to the CDC over half of American adults are overweight (55%)? Apparently so, according to the the NHANES surveys. It is possible to be labelled as overweight, even if your weight is below the 50th percentile. This bothered me, and maybe you too? So I developed my own criteria: the halls.md v2 criteria. (And update in 2015, many years later, I’m thinking of making a version 3 criteria, based on newer information.)
A little history of this website. The oldest version of my BMI calculator was calculating percentiles based on older data. It was like comparing yourself to an “idealized” population from about 35 years ago. Now, the current halls.md calculators are using the most recent data available, from a fatter recent American population. It’s not flattering to Americans in general, but at least there is more room for you, in the middle bmi range of percentiles.
Update in 2015. I made this page showing evidence that as you get older, the more healthy it is to be a little heavier. This does not mean that you have free range to pile on the pounds, but to take a healthy diet and improve your physical fitness by increasing your levels of physical activity. Because we tend to lose lean muscle as we get older and gain body fat, exercise (such as weight-lifting, strength training and cardio workouts) are the most effective way to become healthier.
Height and weight charts for adults and child growth charts are available. They show average weight and height values, as well as the variance within the population, according to age, gender and race/ethnicity.
A collection of scientific articles about Body Mass Index. It’s a bibliography of expert’s publications on BMI.
Other Mathematical Formulas, such as lean body weight
Lean Body Weight (men) = (1.10 x Weight(kg)) – 128 ( Weight2/(100 x Height(m))2)
Lean Body Weight (women) = (1.07 x Weight(kg)) – 148 ( Weight2/(100 x Height(m))2)
Click here to briefly learn about Lean body weight.
Questions and Answers
I have been told that I have a high bmi and fall into the obese category. What should I do?
Well, there is no easy, quick fix solution. Now that you have your bmi calculation you could start looking at weight loss ideas to help you get into the more normal weight range. Remember, the BMI calculator is a screening tool, it is a guide to your weight and health. Regarding calculators however, you could find out your ideal weight for your age, gender and height to give you a realistic goal to work towards. You could also work out your body fat percentage and aim at increasing your muscle mass and decreasing your body fat.
- Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. National Heart, Lung and Blood Institute. June 17, 1998
- Understanding Nutrition, by Whitney and Rolfes. Uses NHANES II classification.
- National Population Health Survey 1996/1997. (Canadian data n=77,403 age & sex stratified.)
- Nutrition Canada, Anthropometry Report 1980 of data from Nutrition-Canada Survey 1971. (n=13,691)
- Pediatric growth charts, from Ross Laboratories. Adapted from Hamill PVV, Drizd TA TA, Johnson CL, Reed RB, Roche AF, Moore WM. Physical growth: National Center for Health statistics percentiles. Am J Clin Nutr 32: 607-629, 1979.
- Rosenthal M, Bain SH, Bush A, Warner JO, Weight/height2.88 as a screening test for obesity or thinness in schoolage children. Eur J Pediatr 1994; 153:876-83.
- Department of Health and Human Services Consensus Conference on Obesity, April,1 1992
- Standards Committee, American Society of Obesity Surgery; Obesity Surgery. Dec. 1997; 7:523
- NHANES III. 1988-1994 data from USA.
- World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation on obesity. World Health Organization. Geneva, 1998.
- McDowell A, Engel A, Massey J, Maurer K. Plan and operation of the second National Health and Nutrition Survey, 1976-80.
- Kuczmarski RJ, Flegal KM. Criteria for definition of overweight in transiation: background and recommendations for the United States. Am J Clin Nutr 2000; 72:1075-1081.
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