Body mass index, or BMI, is one of the most common ways of grouping people by their body weight. It’s also one of the most criticised, and not without reason: a “healthy” BMI can hide an unhealthy body composition, while a high one can be perfectly healthy. Here’s what BMI is actually measuring, how it’s used, and when it’s worth paying attention to.
Jump to:
- What is BMI?
- What are the BMI categories?
- Why is BMI so widely used?
- What are the limitations of BMI?
- What can you use instead of BMI?
- Is BMI useful at a population level?
- Frequently asked questions
What is BMI?
BMI was first developed in the mid-1800s by Adolphe Quetelet, a polymath (someone with expertise across several scientific fields) who also founded the science of anthropometry, the study of human physical shape and size. It was originally called the Quetelet Index, but became commonly known as the body mass index in the 1970s thanks to its use by the nutrition researcher Ancel Keys.
BMI is a way of measuring someone’s body weight in relation to their height. You can calculate it yourself by dividing your weight in kilograms by your height in metres squared. For example, someone who weighs 75kg and is 178cm tall would calculate 75 divided by 1.78 squared, giving a BMI of 23.7.
What are the BMI categories?
BMI on its own doesn’t tell us much, so scientists have created BMI categories to group people according to how far their weight is from the norm for their height.
The World Health Organization has designated the normal BMI range as 18.5 to 24.9.1 Below 18.5 is considered underweight, with below 16 classed as severe underweight. At the other end of the scale, a BMI of 25 to 29.9 is considered overweight, and 30 or over is classified as obese. Within obesity, there are further classifications: class 1 (a BMI of 30 to 34.9), class 2 (35 to 39.9) and class 3 (40 or more).
This naming is arguably a problem with the BMI classification system rather than BMI itself. Nobody wants to be told they’re overweight or obese, so changing the category names to something more neutral, such as numbers, might feel less unpleasant. For example, a normal BMI could be called class zero, underweight could be BMI class minus one, and overweight could be BMI class plus one. That said, the naming system probably isn’t going to change any time soon.
It’s worth remembering that these categories are simply medical terms used to group people by weight in relation to height.
Why is BMI so widely used?
Science loves categorising things, because it helps us understand concepts better, and we can observe certain health trends in relation to different BMI categories. The further someone’s BMI is from the normal range in either direction, whether over or underweight, the greater their risk of health problems tends to be.
This is where BMI can have some use: as a quick and easy screening tool in medicine for identifying when someone’s weight category might put them at risk of a particular health problem. The word “risk” matters here, because in science we can only talk in terms of risk, not certainty. Someone whose BMI is further from the norm has a greater statistical chance of health issues, but this is by no means definite in every case. There are always exceptions.
BMI is also very widely used simply because it’s easy to measure. You only need someone’s height and weight, with no fancy equipment required, so it’s quick and cheap to work out, which matters in medical practices seeing a lot of patients in a day. In a general sense, BMI does have some relation to certain health conditions or the risk of developing them, which is part of why it remains such a popular index.
What are the limitations of BMI?
BMI is not a way of diagnosing how much body fat someone has, and it isn’t a good way of categorising someone as healthy or unhealthy. This is because BMI doesn’t tell us anything about a person’s level of body fat, their body fat distribution (where they store body fat on their body, which has a major impact on health), or their muscle mass.
BMI doesn’t reflect muscle and body fat levels
Within the normal BMI category, you’ll find people with normal levels of muscle and body fat, but also people with very low muscle and high body fat. This is sometimes called “skinny fat” in popular culture, but in research it’s known as sarcopenic obesity, combining sarcopenia (low muscle mass) and obesity (higher body fat). This is an unhealthy type of body composition, but because it falls within the normal BMI category, it’s often missed and overlooked.
On the other hand, many athletes have high muscle and low body fat, but because muscle is heavy, they’re often categorised as overweight, or even obese. Looking at their BMI alone would suggest a higher risk of health problems, when in fact they may be exceptionally healthy, as many athletes are.
It’s also possible to have a lot of muscle but also carry a fair amount of body fat. And there’s a point beyond which having lots of muscle won’t protect you from the negative effects of excess body fat.
Metabolically healthy obesity
Having a higher level of body fat doesn’t necessarily mean someone is less healthy. Some people have what’s known as metabolically healthy obesity: their BMI falls into the obese category and they carry high levels of body fat, but they don’t have the metabolic issues often associated with obesity, such as high blood sugar, insulin resistance, high LDL (“bad”) cholesterol or low HDL (“good”) cholesterol.
That said, people with metabolically healthy obesity aren’t very common, and are still at a higher risk of developing health issues in the future.
Weight stigma
Weight stigma is when people are discriminated against, in thought or action, because of their weight. Categorising people based exclusively on their weight, body fat or BMI can contribute to this.
For example, if someone classified as overweight or obese goes to the doctor with a health complaint, some doctors may put the issue down to their body weight and advise them to lose weight, without further investigation. This may not be the most helpful way to treat a patient, and it risks missing another reason behind their health issue if the doctor hasn’t looked beyond body weight.
Weight stigma might even discourage someone from seeking medical care in the future, out of fear of discrimination based on their size, meaning they may miss out on care they actually need.
What can you use instead of BMI?
BMI on its own isn’t particularly useful for determining whether someone is healthy. So what could we use to support it?
One easy measure to add alongside BMI is waist-to-hip ratio or waist-to-height ratio, which gives a better idea of where and how someone is storing body fat. This matters because storing a lot of body fat around the waist, known as abdominal obesity, is linked to a higher risk of a wide range of health conditions.
A more advanced option is DEXA (dual energy X-ray absorptiometry), a technique that uses X-rays to measure body composition. It gives accurate measurements of muscle mass, body fat percentage, and even where fat is stored, such as around the abdomen or organs (known as visceral fat), which can have negative health effects.
DEXA may be one of the best ways to assess someone’s body fat and muscle levels, but it’s also expensive and only available in places with an expensive DEXA scanner, such as large hospitals or universities. This brings us back to why BMI remains so popular: it’s quick, cheap and easy to measure, which is why it’s still widely used as an initial screening tool.
Is BMI useful at a population level?
While BMI may not give us much useful information about an individual’s health, it can be genuinely useful for looking at trends across populations. For example, the average BMI in places like Europe and the US has risen steadily since the 1970s, along with average waist circumference. This isn’t because people are generally gaining more muscle, so the rise in BMI is more likely due to an increase in body fat.
Research also shows that both very high and very low BMIs are associated with a greater risk of dying from disease in general.
Putting this together, BMI is a useful tool for looking at how body weight relates to health at a population level over time, but it is by no means a complete tool, and it isn’t very useful for building a full picture of an individual’s health. It’s also worth remembering that having a very low BMI carries its own increased risk of sickness and death, so there’s likely a “sweet spot” for BMI that can vary considerably between people.
Frequently asked questions
What is a healthy BMI range?
The World Health Organization considers a BMI between 18.5 and 24.9 to be within the normal range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Can BMI measure body fat?
No. BMI only measures body weight in relation to height, and doesn’t tell you anything about someone’s actual level of body fat, where that fat is stored, or their muscle mass.
Why do some athletes have a high BMI?
Muscle is heavy, so athletes with high muscle mass and low body fat are often categorised as overweight or even obese by BMI, even though they may be exceptionally healthy.
What is metabolically healthy obesity?
It’s when someone’s BMI falls into the obese category and they carry high levels of body fat, but don’t have the metabolic issues often associated with obesity, such as high blood sugar or unfavourable cholesterol levels. It’s uncommon, and still carries a higher future health risk.
What are good alternatives to BMI?
Waist-to-hip ratio or waist-to-height ratio can give a better idea of where body fat is stored. DEXA scanning is more advanced and accurately measures muscle mass and body fat, though it’s expensive and less widely available.
Is a low BMI a health risk?
Yes. Both very high and very low BMIs are associated with a greater risk of health problems, so a very low BMI is not a sign of good health.
Take home message
BMI is a quick, cheap and easy way to estimate body weight in relation to height, and it remains useful for tracking health trends across populations over time.
However, it says nothing about muscle mass, body fat levels or where fat is stored, which means it can miss unhealthy body compositions in people with a “normal” BMI, and misclassify muscular, healthy people as overweight or obese.
For a fuller picture of individual health, measures like waist-to-hip ratio, waist-to-height ratio or DEXA scanning can add valuable context that BMI alone can’t provide.
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- World Health Organization. (n.d.). Nutrition Landscape Information System (NLiS). Retrieved September 17, 2026, from https://apps.who.int/nutrition/landscape/help.aspx?menu=0&helpid=420
