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Body Mass Index (BMI) is a statistical measure to determine the amount of body fat, involving a calculation based on weight and height. The range within which the BMI falls determines whether a person is of a suitable weight, which may otherwise lead to related health problems.
The BMI is a broad measure, and is not without its critics, but it is a straight forward, inexpensive method that can be used by women and men aged 20 and over to assess their weight in relation to the general population.
This is not suitable for pregnant and breastfeeding women. Also note that when interpreting body mass index for children, age and gender need to be taken into account. In any case, BMI should not be used in isolation when assessing an individual's health status.
How to Calculate Body Mass Index
BMI can be calculated using metric units (kilograms and metres) or imperial units (pounds and inches). The metric system is more widely used.
Metric Calculation
Weight in kilograms is divided by height in metres squared.
Weight (kg) / height2 (m)
Here is an example:
Height = (1.58 m), Weight = 54 kg
Calculation: 54 ÷ (1.58)2 = 21.63
Imperial Calculation
Weight in pounds is divided by height in inches squared and multiplied by 703.
Weight (lb) / height2 (in) x 703
Here is an example:
Weight = 119 lbs, Height = 5'2" (62")
Calculation: (119 ÷ (62)2) x 703 = 21.76
If you would like to calculate your BMI easily and quickly, why not use our online body mass index calculator?
Interpretation of Body Mass Index
The range within which a person's BMI falls will help determine whether they are of a healthy weight for their height. The ranges are as follows:
| Weight State | BMI |
| Starvation | < 15 |
| Underweight | < 18.5 |
| Normal weight | 18.5 to 24.9 |
| Overweight | 25 to 29.9 |
| Obesity | 30 to 40 |
| Morbidly obese | 40 > |
In the example given in the calculations above, the BMI value is between 21 and 22, which is normal for a person who weighs 54 kg at 1.58 m. This is an indicator of good health, and there is no need for a person who has a BMI within the normal range to lose or gain weight.
A BMI of 18.5 or under may indicate someone is deliberately restricting their food intake (or adopting other strategies) in order to achieve a desired level of thinness. This is unhealthy, and may lead to health problems such as malnutrition and osteoporosis, which are associated with the eating disorders anorexia and bulimia.
Body mass index is indeed one of the criteria stipulated by the World Health Organisation for a diagnosis of anorexia nervosa, whereby BMI must be 17.5 or less. In some cases the fashion industry is using BMI as an indicator of whether models are too thin for the catwalk.
A person with a BMI of 25 or over is probably overweight, which means they are at a greater risk of heart disease, osteoporosis, diabetes, high blood pressure, stroke, some cancers and other diseases. Steps should be taken to lose weight, including a healthier diet and regular exercise. There is the need to create a calorie deficit (burning more calories than are being consumed).
For someone with a BMI of 30 or over, advice from a health professional about how to lose weight may be beneficial.