Health professionals have been using body mass index (BMI) to help decide whether people are overweight or underweight for over 100 years. However, BMI has several flaws.

BMI is derived from a simple math formula created in the 1830s by Lambert Adolphe Jacques Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist.

Researchers in population studies, doctors, personal trainers, and others use the BMI in their work.

However, BMI has some important flaws. For example, it does not measure overall fat or lean tissue (muscle) content.

It aims to estimate whether a person has a healthy weight by dividing their weight in kilograms (kg) by their height in meters (m) squared.

## The BMI formula

The current BMI calculation divides a person’s weight by their height to the power of 2, or squared:

BMI = weight (kg) / height2

According to most criteria accepted around the world:

• a BMI of 18.49 or below means a person is underweight
• a BMI of 18.5–24.99 means they are of normal weight
• a BMI of 25–29.99 means they are overweight
• a BMI of 30–39.99 or more means they are obese
• a BMI of 40 or more means they are morbidly obese

If you want to find out what your BMI is, use our BMI calculator.

## Is it too simple?

When Quetelet devised the BMI formula, there were no computers, calculators, or electronic devices, so he developed a simple system.

Now, some people argue, we have technology that can help us add some complexity to the calculation. After all, people are three-dimensional, not two-dimensional, and healthy bodies grow in different shapes and sizes.

In 2013, Nick Trefethen, a mathematician from Oxford University in the United Kingdom, wrote a letter to The Economist in which he questioned the usefulness of the current BMI formula, calling it a “bizarre measure.”

Trefethen argued that the formula leads to confusion and misinformation. The height term, he says, divides the weight by too much when people are short and by too little when they are tall.

The result is that short people think they are thinner than they really are, while tall people think they are fatter than they are.

He recommends a “new BMI calculator” that:

1. multiplies the weight by 1.3 for metric measures (kg), or by 5,734 for imperial measures (lb)
2. divides the weight by height to the power of 2.5, instead of 2, or squared

BMI (metric) = 1.3 x weight (kg) / height (m)2.5

or

BMI (imperial) = 5,734 x weight (lb) / height (in)2.5

You can try it here.

Trefethen points out that any calculation that assigns one number to a person will not be perfect. Humans are too complex to be described by a single figure. However, he believes this new calculation gives a closer approximation to the reality of human shape and size.

### Muscle or fat?

One problem with BMI is that it does not distinguish between muscle and fat.

Consider that a person who does no exercise; is 1.83 m, or 6 feet, tall; and weighs 92 kg, or 203 pounds (lb), would have a BMI of 27.

An Olympic athlete who is 1.83 m, or 6 feet, tall and weighs 96 kg, or 211 lb, would have a BMI of 28.

According to this, the athlete is more “overweight” than the person who does no exercise. However, muscle is about 18 percent more dense than fat, so this is clearly not true.

Still, Trefethen points out that if muscle is 18 percent denser than fat, a person who exercised enough to convert 10 percent of their fat into muscle would still increase their BMI by just 1.8 percent. The BMI would still not represent the increase in fitness.

Some people have proposed other options for assessing whether a person is overweight.

### Waist circumference and waist-to-height ratio

One suggestion is to combine BMI with waist circumference (WC) for a more accurate measurement.

However, some scientists argue that waist-to-height ratio might be more appropriate than BMI alone or BMI with WC, as research has proven it to be a predictor of cardiometabolic health.

Researchers have suggested that people keep their waist circumference to less than half their height to maximize health and life expectancy.

A person with fat around the abdomen has a higher risk of heart disease and metabolic disorders, because the fat affects the internal organs such as the liver, heart, and kidneys. Fat around the hips and thighs may be less risky.

### Measuring body fat

Another option is to measure body fat. Men and women need different amounts of fat.

For a man, 2–5% fat is essential, 2–24% fat is considered healthy, and more than 25% classifies as obesity.

For a woman, 10–13% fat is essential, 10–31% fat is healthy, and more than 32% classifies as obesity.

Studies have suggested that measuring fat gives a more accurate view of health and health risks, but getting an accurate measurement is not easy.

Methods include:

• calipers
• air displacement plethysmography
• near-infrared interactance
• dual energy X-ray absorptiometry (DXA)

However, researchers still need to do more work before these methods will become as easy as using a BMI calculator.

## Does BMI misclassify people?

People often assume that someone whose BMI says they are overweight or obese will be unhealthy, while someone with a normal BMI will be healthy. However, research published in 2016 suggested that this was incorrect for 75 million Americans.

Researchers found that 54 million Americans had been categorized as overweight or obese, but cardiometabolic measures showed they were healthy. Another 21 million had been categorized as “normal” in terms of BMI but were unhealthy.

Other scientists, however, have suggested that although some people may appear to be overweight but healthy, their higher weight puts them at higher risk of certain diseases as they get older. A research article published in 2019 concludes that, overall, people with obesity have a higher risk of developing metabolic syndrome than those without.

## The ideal healthy weight

A healthy weight is hard to exactly figure out. One size does not fit all.

Factors that affect healthy weight include:

• general health status
• height
• muscle-to-fat ratio
• bone density
• body type
• sex
• age

## Summary

BMI is useful when studying populations and trends, and it can give a rough idea of a person’s health and weight status. For now, it is probably the best guide we have.

It may be more important to follow a healthy lifestyle with a balanced diet and regular exercise to prevent gaining excess weight in the first place.