The Mystery of BMI
Written by Libby Norris
Body Mass Index (BMI) is a term you may have seen in articles, at a fitness club or in your doctor's office. You may know that BMI refers to body fat however, knowing what it means and how it's measured can clear up questions you may have.
BMI is used as a measure to screen for weight categories that may lead to health problems. It does not measure body fat directly, but research has shown that BMI correlates with direct measures of body fat, such as underwater weighing, and can be considered an alternative for direct measures of body fat.1,2
BMI is used as a screening measure by fitness and health professionals. When BMI indicates that excess weight may be a health risk, further assessments are typically used to get a broader picture. These additional tests include skinfolds, girth or body measurements, an evaluation of diet and physical activity and family history.
How do you calculate your BMI? You can use either pounds and inches or kilograms and meters with the following formulas. With the metric system, the formula is weight in kilograms divided by height in meters squared.Metric Formula
weight (kg) / [height (m)]2
Sample: Weight = 68 kg, Height = 165 cm (1.65 m), Calculation: 68 ÷ (1.65)2 = 24.98 Using the Empirical formula, BMI calculate BMI by dividing weight in pounds by height in inches squared and multiplying by a conversion factor of 703.
Imperial Formula
weight (lb) / [height (in)]2< x 703
Sample: Weight = 150 lbs, Height = 5'5" (65"), Calculation: [150 ÷ (65)2] x 703 = 24.96
Once you calculate your BMI, the following chart outlines the classifications and potential health risk.
Classification | BMI Category (kg/m 2) | Risk of developing health problems |
| Underweight | < 18.5 | Increased |
| Normal Weight | 18.5 - 24.9 | Least |
| Overweight | 25.0 - 29.9 | Increased |
| Obese class I | 30.0 - 34.9 | High |
| Obese class II | 35.0 - 39.9 | Very high |
| Obese class III> | >= 40.0 | Extremely high |
Note: For persons 65 years and older the 'normal' range may begin slightly above BMI 18.5 and extend into the 'overweight' range. Source: Health Canada. Canadian Guidelines for Body Weight Classification in Adults. Ottawa: Minister of Public Works and Government Services Canada; 2003.
As a standard in Canada, we have a body weight classification system that uses body mass index (BMI) in combination with waist circumference (WC).3
Where body mass index is a measure that refers to body composition overall, the waist circumference is an indicator of abdominal fat. An apple-shaped figure, or excess fat around the waist and upper body, is associated with greater health risk than a pear-shaped figure where fat is located more in the hip and thigh area.3
A waist circumference at or above 102 cm or 40 in. for men, and 88 cm or 35 in. for women, is associated with an increased risk of developing health problems such as diabetes, heart disease and high blood pressure. Because the cut-off points are approximate, waist circumference measurements close to those values should be taken seriously.4
Both body mass index and waist circumference apply to adults over the age of 18 years as Health Canada does not have guidelines for classifying weight or standards for tracking growth in children. Internationally, BMI is often calculated with the same formula as adults but interpreted with different criteria using age and sex percentiles. This is based on data showing that the amount of body fat changes with age and that the amount of body fat differs between girls and boys.5
Although body mass index, as previously noted, does not specifically measure body fat, the correlation is very strong. There is some variation with interpretations based on sex, race and age and includes the following: 6
- At the same BMI, women tend to have more body fat than men.- At the same BMI, older people, on average, tend to have more body fat than younger adults.
- Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness.
How do you use your BMI? Body mass index can be used at home along with weight and body measurements for an overall healthy perspective. It can also be one of the tools you use with your goal setting for additional motivation and measurement.
1. Mei Z, Grummer-Strawn LM, Pietrobelli A, Goulding A, Goran MI, Dietz WH. Validity of body mass index compared with other body-composition screening indexes for the assessment of body fatness in children and adolescents. American Journal of Clinical Nutrition 2002;7597-985.2. Garrow JS and Webster J. Quetelet's index (W/H2) as a measure of fatness. International Journal of Obesity 1985;9:147-153.
3. Health Canada.
4. The 2003 Canadian Guidelines for Body Weight Classification in Adults
5. Cole TJ, Bellizzi MC, Flegal KM & Dietz WH (2000): Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 320, 1240-1243. (kids and obesity)
6. Prentice AM and Jebb SA. Beyond Body Mass Index. Obesity Reviews. 2001 August; 2(3): 141-7.)
