BMI – Body Mass Index and BMI Calculator

BMI – Body Mass Index and BMI Calculator

Body Mass Index (BMI) is a way to measure body fat levels. It was invented between 1830 and 1850 by Adolphe Quetelet, and so it is sometimes referred to as the Quetelet scale.

Why is BMI important?

BMI is an important and useful way to measure someone’s weight. It helps to standardise what is ‘healthy’ across the population.

Issues arise when trying to state what is a healthy body weight and what isn’t. Someone who is 1.9m tall is not going to have the same healthy body weight as a person who is 1.4m tall; they will simply weigh more by being taller.

BMI takes this into account, and judges bodyweight against height, giving a fair indication as to whether the person is a healthy weight or not.

This makes it more significant, applicable, and accurate for each individual person, means it can be used to set target weights to aim for, and highlights if there is a high risk of a health problem as a result of being over or under the ideal weight.

How accurate is BMI?

As useful as BMI is, it is still limited by some factors:

  • Being pregnant
  • Having a muscular body
  • Being under the age of 18

Obviously, pregnancy changes the body in a lot of ways, and during pregnancy, BMI readings will not be very accurate or reflect overall health.

In the case of those with very muscular or athletic bodies, BMI may give a skewed result. The equations involved in generating the BMI figure do not differentiate between body mass types. This means that muscle gets treated the same way as fat, and so those with a lot of muscle, even if they have a low amount of body fat, will still score high on the BMI scale, indicating higher health risks when the reality is probably the opposite.

Age can also make a big difference. People under the age of 18 will likely still be growing, and their body masses compared to their heights can be completely different, subject to all sorts of variable to the BMI calculations. In the case of younger people, it might be more relevant to use another system, such as a waist size measurement to get an idea of health and body mass.

How to calculate BMI

BMI is calculated via a fairly simple equation, in which you input your weight and height.

There are two methods of working out the BMI of a person, and which one you use depends on what measurements you have gotten, whether they are metric or imperial.


Take the weight of the person (in kilograms), and divide it by their height (in metres) multiplied by itself.

Written out, it looks like this:

BMI (kg/m2) = mass (kg) / height (m)2

For an example, take a 21-year-old male with a height of 1.77 metres, and a weight of 78kg. The equation would look like this:

78 / 1.772 = 24.89kg/m2

This man therefore has a BMI of 24.89.


The basics behind the imperial equation is very similar to that of the metric, but it involves a conversion.

Take the weight of the person (in pounds), and multiply it by 703. Divide this result by their height (in inches) multiplied by itself.

Written out, it looks like this:

BMI (kg/m2) = (mass (lbs) x 703) / height (in)2

As an example, take a 20 year old female, who is 5ft 9in (69in) and weighs 11st 4lbs (158lbs).

(158 x 703) / 692 = 111,074 / 4,761 = 23.32

This woman, therefore, has a BMI of 23.32.

BMI results explained

The above equations will give you a figure which will indicate body mass, and determine whether the person is underweight, overweight, obese, or at a healthy weight. This figure is in a unit called kg/m2.

The NHS consider a BMI of 18.5 to 24.9 to be healthy. Under 18.5 is considered underweight.

A BMI of 25 and above is more than the ideal weight for the person’s height, and is the overweight category.

30 and above is considered obese, and anything over 40 is considered morbidly obese.

These figures are called the thresholds, or the cut-offs. The cut-offs are fairly universal for adults, but can change in some cases.

BMI cut-offs for children

BMI scales for a child are different to that for an adult. The results and interpretation is skewed by them not being fully developed.

Children and teenagers can vary in both height and natural body fat levels. In order to be able to have a reliable figure for comparison, you have to have a wide sample to compare them to.

Put simply, there have been studies undertaken which collected data on a wide sample of children, collecting their height and weight levels, split by gender and organised by age.

The result of a child’s BMI can then be compared in a percentile to their peers, usually done by a health professional.

Generally, anything above the 85th percentile is considered overweight, with anything over the 95th percentile considered as obese. Anything below the 5th percentile is considered underweight.

BMI cut-offs for people from Asia

Although BMI gives a standard against which we can see whether a person should be considered a healthy weight or not, the points of risk are not so easily discernible.

Race and heritage can affect how likely someone is to develop a health condition when they are at a certain weight.

People of Asian heritage are more likely to develop conditions such as type 2 diabetes at a lower BMI than a person of European or American descent.

The World Health Organisation (WHO) compared data of race and of BMI, and found that people of Asian heritage were more likely to suffer a health condition at a lower BMI. However, they could not specify where a more race-specific cut-off point would go, on the BMI scale.

Alternatives to BMI

The accuracy of BMI is often contested. Luckily, it is not the only way to get an indication of overall weight and health risks.

Waist measurement

One way to judge is a simple measurement around the waist using a tape measure. A circumference measurement around the waist, if above 88cm (35in) for a woman and 102cm (40in) for a man for a man, can indicate whether someone is at a high risk of health complications.

Waist to hip ratio

Expanding on the waist measurement, the ratio of waist to hips can give a more relative indication of potential health issues.

Take the measurement of the waist circumference, and divide it by the circumference around the widest part of the hip.

Men Women
0.90 0.80 Healthy Weight
0.90 to 0.99 0.80 to 0.84 Overweight
1.0+ 0.85+ Obese

Body fat measuring

A third – and impractical for most – method of measuring body fat is to have a health professional individually measure each part of the body with callipers.

Measurements are taken at the wrist, biceps, shoulder blade and waist, to name a few, and the measurements are combined and put through several formulas before being charted against the patient’s height, age, and sex.

This is a more in-depth look at body fat, but isn’t something that you can measure yourself at home.

Health concerns

Being overweight can cause a huge increase in the risk of developing an illness, or illnesses, of some description.

The Study to Help Improve Early Evaluation and Management of Risk Factors Leading to Diabetes (SHIELD), which was undertaken in 2004, and the National Health and Nutrition Examination Survey (NHANES) which was carried out better 1999 and 2002, found that a higher BMI reading was linked with a higher risk of developing type 2 diabetes mellitus, hypertension, and dyslipidemia.

All of these can lead to serious medical complications, including one of the major and most well-known consequences of being overweight: heart disease. The strain that the extra weight on the body, combined with the fat that can build up in the blood vessels, can lead to some serious health problems and even death.

Therefore, a standard by which to judge the risk of developing these conditions can not only help to prevent people from suffering them, but also help alleviate the health service by preventing issues before they arise.

What can I do about my BMI?

If a person’s BMI is above the healthy range, it is not the end of the world. Weight loss and exercise can help to reduce body fat levels, and return a person to the healthy range.

In general, the idea behind losing weight is to “eat less and move more.” A lower calorie intake, and more exercise, will shift the balance and help people to begin to lose the weight that has built up. A ‘calorie deficit’ is where more calories are being burned than are being consumed, and so will result in weight loss.

Of course, nothing is ever quite that simple. Calories are just one of many factors which can contribute to weight gain, or weight loss, and so the type of diet that is undertaken has to be considered. Should you be going low-carb, for example? How will you balance this against exercise?

The one important point to remember is to make sure that weight loss is done in a controlled and healthy way.

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