Body mass index: What is it and is BMI an accurate measure of your weight?

When it comes to checking your weight, one of the most common measurements is body mass index, or BMI. It’s often used in hospitals and other health settings as a quick and easy way of seeing whether someone is considered a ‘healthy’ weight or not.

However, BMI is not without its limitations, leading many of us to question if body mass index is accurate or not. As we explain in our guide to body composition, BMI is something of a blunt instrument. It uses a basic calculation to get an answer, without necessarily considering the variety of factors that can potentially lead to a misleading result.

So, is it time for us to stop giving so much weight (excuse the pun) to BMI?

BMI explained

Body mass index uses your height and weight to show whether your weight is healthy. To calculate your BMI, divide your weight in kilograms by your height in metres squared (BMI = kg/m2). The resulting number is your BMI, which will place you in one of the following categories:

• Below 18.5: Underweight
• 18.5-24.9: Normal Weight
• 25.0-29.9: Pre-obesity (overweight)
• 30.0 or above: Obese

Body mass index was originally developed by a Belgian named Adolphe Quetelet in 1832. The Quetelet Index, as it was originally known, was renamed body mass index, or BMI, in 1972.

BMI was identified as correlating strongly with risk indicators of disease. Generally, as BMI rises above ‘normal’ levels, the risk of certain health conditions also increases. This includes type 2 diabetes, coronary heart disease, certain types of cancer, and stroke.

Is BMI accurate?

The World Health Organisation says this about BMI:

"During the 1970s...researchers noticed that BMI appeared to be a good proxy for adiposity and overweight related problems. Like any other measure it is not perfect because it is only dependant on height and weight and it does not take into consideration different levels of adiposity based on age, physical activity levels and sex. 

For this reason, it is expected that it overestimates adiposity in some cases and underestimates it in others.’

In other words, while BMI does appear to have some correlation with overall health, and the risk of developing other health conditions, there is recognition that it isn’t perfect.

We spoke with Sophie Thompson, specialist dietitian at the Princess Grace hospital, part of HCA Healthcare UK.

She says: "BMI is quite a complex topic. You've got to not look at just height and weight, but look at other factors like their gender, age, things like that. Also, you've got to have accurate measurements.

"When you're weighing people, is there fluid on the body, are you measuring their height properly, and you have to make adjustments [to the calculation] for amputees, for example".

We’ll explain this in more detail below, but body mass index doesn’t necessarily consider other factors that can influence whether you are healthy. As a result, it can give misleading results for some people.

The same BMI doesn't mean the same body composition

As an example, two people who are 165cm tall (5ft 5in) and weigh 63kg (9st 13lb) would have a BMI of 23.1 – and would generally be regarded as having a ‘healthy’ BMI (we’ll explain the ranges below). However, those two people could have very different body composition.

Thompson explains: "Some people might have a low BMI but have lots of visceral fat, which is around the organs. You can't see that, BMI doesn't doesn't distinguish between visceral fat and subcutaneous fat, which is less harmful.

"Someone might class themselves as overweight but actually haven't got much visceral fat at all. And someone who is a healthy weight might have lots of visceral fat.

"It also doesn't distinguish fat-free elements like your muscle mass. So big rugby lads who have loads of muscle might come out as obese but actually, they've got lots of fat free mass."

Comparing two people, one might be very athletic and have a high muscle mass and low percentage of body fat. The other, though, could have extremely low muscle mass and high levels of body fat. But BMI is unable to differentiate between the two. In some respects, asking “Is BMI accurate?” is almost the wrong question.

If you do the correct calculation or use one of the many available online calculators, then you will get an accurate BMI. You can also check this BMI chart from the Heart of England NHS Foundation Trust.

The more pertinent question, really, is whether BMI can give you enough information to accurately measure your body composition. And for some people, the answer will be no.

What factors influence BMI?

BMI is a useful tool for screening for weight problems, but it isn’t a perfect measure of health. BMI does not, for instance, take into account body composition – so muscle mass, bone density, or body fat distribution aren’t measured.

People with a high BMI may be healthy, and people with a low BMI may be unhealthy. This is often what’s being described when you hear the term ‘skinny fat’.

There are several factors that can affect BMI accuracy, including:

• Age: BMI is less accurate in older adults, as they tend to have higher levels of body fat than younger adults with the same BMI.
• Sex: Men and women have different body compositions, so BMI may be less accurate in women. According to the Centers for Disease Control and Prevention, women will (on average) have greater amounts of total body fat than men with an equivalent BMI.
• Body shape: Muscular individuals can potentially be classed as overweight or obese, even with a low body fat percentage. Similarly, as we age, we often lose muscle mass due to a process called sarcopenia. BMI cannot take this into account.
• Ethnicity: BMI may be less reliable in people of Black, Asian and other minority ethnic groups. According to the NHS, people from these groups with a BMI of 23 or above are considered at increased risk of developing some long-term health conditions (as opposed to a BMI of 25 or higher).
• For example, research suggests that BMI tends to underestimate health risks in those of Asian descent.

Why is BMI still used?

Given all the issues with BMI, you’d be forgiven for asking why it still gets used. Put simply, it’s one of the fastest and cheapest ways of screening for potential weight issues in individuals. While it won’t definitively state whether someone is at elevated risk of a specific disease, it can be an indicator that this could be a problem.

BMI is arguably at its most useful when used alongside other forms of testing, to build a complete picture of someone’s health.

"BMI is still useful, when used with other measurements like measuring their waist, grip strength, or mid-upper arm circumference, for example" says Thompson.

And the good news is that there are other methods of checking your weight and body composition, to help complement the BMI readings.

Alternatives to BMI

Other ways to measure body composition apart from body mass index

A simple way to measure body composition is by standing on a set of body composition scales, which use a small electrical current to estimate your body composition. Many of the best smart scales include metrics such as fat mass and muscle mass. Some even claim to measure visceral fat. They’ll often measure your BMI as well.

There are still question marks around how accurate smart scales really are. But if you want a straightforward way to track changes in your body composition over time, these can be helpful.

Thompson agrees that multiple BMI measurements over time are more useful than a single, standalone measurement: "More than anything, it's a ballpark, and it's numerical, which does make it easy to track. You can track it, you can compare it. You can see the trend. It's a good kind of universal screening tool. And with those measurements then you can monitor the trends, if you want someone to gain or lose weight".

Alternatively, you can calculate your waist to height ratio, which can show if you have excess fat around your middle. To do this, measure your waist and divide it by your height (waist in cm/height in cm). You can use inches if you prefer – just make sure you use the same units for both measurements. If your waist to height ratio is above 0.5, this may indicate increased risk of certain health conditions.

Is BMI still useful?

BMI undoubtedly has some limitations. Even so, it can still be a useful tool for screening for potential health problems, especially when used in conjunction with other measurements. This approach can provide a more complete picture of your overall health.

Above all, staying active – for example, through cardio exercise or strength training – and eating a healthy diet incorporating all the food groups, will be significant factors in your overall health, regardless of where you sit on a BMI chart. And as always, if you have any concerns about your weight or your health in general, speak to your doctor.