Body Mass Index formula and average weight percentile statistics

The Formula for Body Mass Index:

BMI kilograms per meter squared

To calculate your Body Mass Index, take your weight (in kilograms), 
and divide by your height (in meters) squared.

( Need a calculator? Duh.. use the body mass index BMI calculator,
which coincidentally, will convert units from imperial to metric and back)

( or, for an interesting diversion, Click here to learn about Ideal Body Weight formulas. )

Talking Moose


Who decided the Body Descriptions?  What is overweight?

by Steven B.Halls, MD FRCPC

A Body Mass Index between 25 and 29.9 is "overweight", and greater than or equal to 30 is "obese", according to an old definition1. This definition is used by the World Health Organization (WHO) as its international standard3. You can read more about it, (including its problems).

Jessica Jessica
I’ve heard of that. BMI of 25 is called overweight.


The oldest version of the body mass index calculator used an even older definition of Overweight. It was based2,8,11 on this table below (for adults). You can still use this table if you like. Simply calculate your BMI, then refer back to this table.

Adults Women Men
anorexia < 17.5
underweight <19.1 <20.7
in normal range 19.1-25.8 20.7-26.4
marginally overweight 25.8-27.3 26.4-27.8
overweight 27.3-32.3 27.8-31.1
very overweight or obese >32.3 >31.1
severely obese 35 – 40
morbidly obese 40 – 50
super obese 50 – 60

David David
I don’t like that table of numbers.

Billy Billy
I didn’t bother reading it.


For children, there was no suitable 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 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.

Dr. Halls Dr. Halls
The Old old BMI calculator in 1999. Then in year 2000 it became much better, and in year 2014 I have updated the styling, and found new research.


New Features, New BMI criteria:

A year 2000 addition to the 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.

Dr. Halls Dr. Halls
And in year 2014 I changed the default criteria to be “ v2″ which by then had extensive research literature support.

Talking Moose Talking Moose
You were ahead. It took years for mainstream to catch up.


Here is my discussion of how the ‘ 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 BMI to determine overweight. Some very muscular people can have high Body Mass Indexes, which wrongly suggest fatness. Some races, ethnic groups and nationalities have different body fat distribution and body composition, so different BMI criteria are needed. I hope to improve the calculator in these areas, in the future.


Weight and Height percentiles

Unlike the Body Mass Index formula,  the formula for height and weight percentiles is an estimate, rather than an exact calculation.  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.

As you know, people of some nations can be taller than others.  For example, Icelanders tend to be tall and Asians tend to be shorter.  Also, many nations are growing taller over time because of improving nutrition.  Therefore, the percentile calculation used by this calculator is not perfect, and will grow less accurate over time.

Did you know that over half of American adults are overweight (55%)? Apparently so, according to the CDC criteria applied to the NHANES III survey. Thus, it is possible to be labeled as overweight, even if your weight is below the 50th percentile. If this really bothers you and doesn’t seem right for you, try the v2 criteria instead.

Some visitors to this website have noticed that the "percentiles" have changed. Yes, that’s true. The older version of the calculator was calculating percentiles based on older data. It was like comparing yourself to an "idealized" population from about 25 years ago. Now, the current 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 middle ranges of percentiles.

Other resources

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.

Childrens BMI charts for Boys and Girls are available, and so are adults body-mass-index versus Age charts, the Mens BMI chart and Womens BMI chart.

A collection of scientific articles about Body Mass Index. It’s a bibliography of expert’s publications on BMI.

Other 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.



  1. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults.  National Heart, Lung and Blood Institute. June 17, 1998
  2. Understanding Nutrition,  by Whitney and Rolfes.  Uses NHANES II classification.
  3. National Population Health Survey 1996/1997.  (Canadian data n=77,403  age & sex stratified.)
  4. Nutrition Canada, Anthropometry Report 1980 of data from Nutrition-Canada Survey 1971.  (n=13,691)
  5. 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.
  6. 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.
  7. Department of Health and Human Services Consensus Conference on Obesity, April,1 1992
  8. Standards Committee, American Society of Obesity Surgery; Obesity Surgery. Dec. 1997; 7:523
  9. NHANES III.  1988-1994 data from USA.
  10. World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation on obesity. World Health Organization. Geneva, 1998.
  11. McDowell A, Engel A, Massey J, Maurer K. Plan and operation of the second National Health and Nutrition Survey, 1976-80.
  12. 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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