How to Calculate Your Weight and Body Mass Index (BMI); Weight that is higher than what is considered as a healthy weight for a given height is described as overweight or obese. Body Mass Index, or BMI, is used as a screening tool for overweight or obesity.
Definition of Body Mass Index (BMI)
The body mass index (BMI) or Quetelet index is a value derived from the mass (weight) and height of an individual. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from mass in kilograms and height in metres.
The BMI may also be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colours for different BMI categories, and which may use other units of measurement (converted to metric units for the calculation).
The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweight, normal weight, overweight, or obese based on that value.
|BMI calculation and classification in adults|
|Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9.
BMI = 70 kg / (1.75 m2) = 70 / 3.06 = 22.9
Thus, overweight is typically defined as a BMI of 25 to 29.9, and obesity is defined as a BMI of 30 or more, and severe obesity is defined as a BMI of 40 or more.
Table 1: The International Classification of adult underweight, overweight and obesity according to BMI
Source: Adapted from WHO, 1995, WHO, 2000 and WHO 2004.
BMI values are age-independent and the same for both sexes. However, BMI may not correspond to the same degree of fatness in different populations due, in part, to different body proportions. The health risks associated with increasing BMI are continuous and the interpretation of BMI gradings in relation to risk may differ for different populations.
BMI in children and teens
BMI can be calculated the same way for children and teens as it is for adults, but the numbers don’t have the same meaning. This is because the normal amount of body fat changes with age in children and teens, and is different between boys and girls. So for kids, BMI levels that define being normal weight or overweight are based on the child’s age and gender.
To account for this, the US Centers for Disease Control and Prevention (CDC) has developed age- and gender-specific growth charts. These charts are used to translate a BMI number into a percentile based on a child’s sex and age. The percentiles are then used to determine the different weight groups:
- Underweight: less than the 5th percentile
- Normal weight: 5th percentile to less than the 85th percentile
- Overweight: 85th percentile to less than the 95th percentile
- Obese: 95th percentile or higher
An easy way to determine your child’s BMI percentile is to use the CDC’s online BMI percentile calculator at http://apps.nccd.cdc.gov/dnpabmi.
Even in a young person, being overweight or obese can cause health problems. And it may directly increase the risk for certain health problems later in life, including some kinds of cancer. It also increases the chances of being overweight or obese as an adult, as well as the risk of health problems that can come with this.
How good is BMI as an indicator of body fatness?
The correlation between the BMI and body fatness is fairly strong, but even if 2 people have the same BMI, their level of body fatness may differ.
- At the same BMI, women tend to have more body fat than men.
- At the same BMI, Blacks have less body fat than do Whites, and Asians have more body fat than do Whites
- At the same BMI, older people, on average, tend to have more body fat than younger adults.
- At the same BMI, athletes have less body fat than do non-athletes.
The accuracy of BMI as an indicator of body fatness also appears to be higher in persons with higher levels of BMI and body fatness. While, a person with a very high BMI (e.g., 35 kg/m2) is very likely to have high body fat, a relatively high BMI can be the results of either high body fat or high lean body mass (muscle and bone). A trained healthcare provider should perform appropriate health assessments in order to evaluate an individual’s health status and risks.
If an athlete or another person with a lot of muscle has a BMI over 25, is that person still considered to be overweight?
According to the BMI weight status categories, anyone with a BMI between 25 and 29.9 would be classified as overweight and anyone with a BMI over 30 would be classified as obese.
However, athletes may have a high BMI because of increased muscularity rather than increased body fatness. In general, a person who has a high BMI is likely to have body fatness and would be considered to be overweight or obese, but this may not apply to athletes. A trained healthcare provider should perform appropriate health assessments in order to evaluate an individual’s health status and risks.
Weight and Body Mass Index Chart for Calculation
The chart below shows examples of body mass indexes. The figure at which your height corresponds with your weight is your body mass index. To find yours, consult the chart
Body Mass Index (BMI)
|Height (feet, inches)|
Health Consequences of Obesity for Adults
People who are obese are at increased risk for many diseases and health conditions, including the following:
- All causes of death (mortality)
- High blood pressure (Hypertension)
- High LDL cholesterol, low HDL cholesterol, or high levels of triglycerides (Dyslipidemia)
- Type 2 diabetes
- Coronary heart disease
- Gallbladder disease
- Osteoarthritis (a breakdown of cartilage and bone within a joint)
- Sleep apnea and breathing problems
- Chronic inflammation and increased oxidative stress
- Some cancers (endometrial, breast, colon, kidney, gallbladder, and liver)
- Low quality of life
- Mental illness such as clinical depression, anxiety, and other mental disorders
- Body pain and difficulty with physical functioning
Sources and References:
- “BMI Classification”. Global Database on Body Mass Index. World Health Organization. 2006. Retrieved July 27, 2012.
- Malcolm Kendrick (April 12, 2015). “Why being ‘overweight’ means you live longer: The way scientists twist the facts”. http://www.independent.co.uk. Retrieved 12 April 2015.
- “Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies”. The Lancet. 363 (9403): 157–63. 2004. doi:1016/S0140-6736(03)15268-3. ISSN 0140-6736. PMID 14726171.
- Eknoyan, Garabed (2007). “Adolphe Quetelet (1796–1874)—the average man and indices of obesity”. Nephrology Dialysis Transplantation. 23 (1): 47–51. doi:1093/ndt/gfm517. PMID 17890752.
- Commentary: Origins and evolution of body mass index (BMI): continuing sagaBlackburn, H, Jacobs, D Int. J. Epidemiol. (2014) doi: 10.1093/ije/dyu061
- Jeremy Singer-Vine (July 20, 2009). “Beyond BMI: Why doctors won’t stop using an outdated measure for obesity”. com. Retrieved 15 December 2013.
- Keys, Ancel; Fidanza, Flaminio; Karvonen, Martti J.; Kimura, Noboru; Taylor, Henry L. (1972). “Indices of relative weight and obesity”. Journal of Chronic Diseases. 25 (6–7): 329–43. doi:1016/0021-9681(72)90027-6. PMID 4650929.
- “Assessing Your Weight and Health Risk”. National Heart, Lung and Blood Institute. Retrieved 19 December 2014.
- Adapted from WHO, 1995, WHO, 2000 and WHO 2004.