Body Mass Index

A person’s BMI measurement can often be found referred to in health and fitness articles and is used by the medical profession as one indicator of health. Very simply your BMI┬áis calculated by taking weight (in pounds or kilograms) and dividing it by height (in feet and inches or metres) squared. At its simplest it provides a quick and reliable test of body fatness for most people and is often used as one part of health screening programs. On a very broad spectrum, without taking into account other factors – a BMI of less than 18.5 shows that a person is underweight; between 18.5 and 24.9 comes within the range of normal; between 25 and 29.9 is overweight; between 30 and 39.9 is obese and over 40 is morbidly, or very dangerously, obese. Other factors need to be taken into consideration, for instance, the older a person is, the higher a healthy BMI can be.

Body Mass Index example

Body Mass for Women

A healthy woman of 5’3″ should have a weight of between 105 and 140 pounds to be considered healthy on the Index. The scale is adjusted between men and women as women’s bodies contain more fat, which is differently distributed than on men’s bodies. BMI increases naturally with age in women and in fact overtakes the average for men, due to the accumulation of fat on women’s breasts and hips. In fact, older women do better when they carry a little extra weight, which diminishes the risks from osteoporosis, making falls less likely to result in broken hips and the subsequent health problems they bring. The BMI scale shouldn’t be used in a normal way during pregnancy, as weight gain is normal and to be expected. With alternative ways of measuring health, such as body fat measurement (see below) women should be aiming for a body fat percentage of below 23.

Body Mass for Men

On the Mass Index scale a male of 5’8″ should weigh between 130 and 165 pounds to be within a healthy weight range. Younger men will have a higher BMI than women, thought to be due to their higher skeletal frame weight. BMI, however, can’t distinguish between fat and lean tissue or different types of fat, so many large and solid-looking athletes and actors, who are considered fit pin-ups, actually come up as being overweight or even obese on the Index. The weight factor of the BMI doesn’t distinguish between fat and muscle, but men who exercise regularly and include weight training in their program will have lean tissue and less fat, but heavier muscle bulk. This makes them healthier and more likely to survive an accident or illness, but the Index will show them as being overweight. It is actually healthier for men to just measure their actual body fat (see below) and aim for a percentage of below 15%.

Body Mass Index for Children

The BMI measurement can be used for children but certain factors have to be taken into account. As children go through growth spurts and at puberty, the ratio of height and weight goes through rapid changes. The amount of body fat between boys and girls at puberty differs widely, so when interpreting results in children – age, gender and developmental stage need to be considered.


The Index has been in use since the 1800’s, but only since 1972 has it been used as a serious measure of body fat percentage. While the BMI is a useful tool for health organisations trying to predict the likelihood of diseases within a general population group, its application to individual cases has come under scrutiny. Its individual application can be flawed and there are now other more reliable options.

BMI studies have generally being carried out across large populations and then researchers have applied the results to individuals without taking into account age, gender, frame size, muscle mass or fat distribution. Fat stored under the skin, known as subcutaneous fat, does not appear to be as dangerous as visceral fat, which is stored in and between the organs. A raw 999 measurement cannot distinguish between the two. This leads to the conclusion that a person with a normal 999 but a high percentage of organ fat will be at high risk of diabetes and heart disease, but an athlete such as a sumo wrestler with a visibly obvious high 999 but little visceral fat will be healthy. Many athletes, especially rugby players and NFL running backs are classified as obese but with their sporty 6-packs and less than 10% body fat are obviously fit and well.

Some Index studies have linked being underweight to a risk of premature death, sometimes an even higher risk than for being overweight. This is where using the BMI can be flawed, as it doesn’t allow for the exclusion of people in the study who are already ill, with diseases such as cancer and emphysema, as well as smokers and dangerous drug users, all of which lead to weight loss. It has not been proven that people with a lower than average BMI die sooner than those who are overweight. Studies also show that people carrying a few extra pounds in later life have diminished health risks. They will also have better outcomes from heart, lung and kidney diseases and be more likely to survive surgery and car crashes.

BMI Chart

Other measurements of healthy body weight

It is much more accurate to just measure body fat. Whatever a person’s Index score is, they are healthier with lower body fat percentages, even when diseases such as diabetes are factored in. Using body fat measurement is the recommended way of gauging fitness used by the American Council of Exercise. A simple way of doing this is with skinfold thickness measurements using calipers.

Another index gaining in popularity is the waist to hip circumference, this gives a clearer indication of the distribution of body fat, which has been shown to be an important indicator of disease risk. People with abdominal fat are at a greater risk of developing heart problems and diabetes, so focusing on losing this weight can have an immediate impact on health now and in the future. Other more extreme measurements of a healthy weight can be made through dual-energy X-Ray absorptiometry, bioelectrical impedance, weighing patients underwater and the dilution of isotopes. These are, however, all very high-tech, need highly trained personnel and consequently, are very expensive.


In the population as a whole, people with high BMI are more likely to have health related life issues. Only a slight increase in an Index score shows a significant increase in cancers, and being obese by the age of 40 gives a significantly increased risk of dying early, as scores over 25 are linked to heart disease and hypertension. It is important, however, to realise that when losing weight, it needs to be focused on shifting areas of fat, such as around the abdomen, rather than losing muscle bulk and tone which will have no impact on overall health, and may even cause disease. In conclusion, BMI is a useful tool, but only one in the overall gauge of a person’s health. For statisticians it is perfect, but for individuals it should be approached with caution.