BMI explained: uses, limits and what to measure instead

BMI has been used for decades to assess whether someone's weight falls within a healthy range. But a BMI result doesn't tell you everything about your health. It doesn't distinguish between muscle and fat, for example, or show where fat is carried around the body.

This means two people with the same BMI can have very different body compositions and health profiles.

Understanding what BMI can and cannot tell you can help you put your result into context.

What is BMI and how is it calculated?

BMI is calculated using your weight and height. You divide your weight in kilograms by your height in metres squared.

For most adults, BMI is used to place people into the following categories:

These categories are useful as a general screening tool, but they don't directly measure body fat or tell you where fat is stored.

The NHS explains that BMI is just one measure of health and should be considered alongside other factors.

How accurate is BMI?

BMI can be useful for identifying whether someone may be underweight, overweight or living with obesity. However, it has limitations when used to assess an individual.

The biggest limitation is that BMI cannot distinguish between muscle and fat.

For example, someone who has a lot of muscle may have a BMI in the overweight or obesity range despite having relatively little body fat. On the other hand, someone can have a BMI within the healthy range while carrying a higher proportion of body fat.

This is why your BMI shouldn't be treated as a definitive measure of your health or body composition.

Why doesn't BMI tell you how much body fat you have?

BMI is based on your total weight rather than what that weight is made up of.

Muscle is denser than fat, so someone who does a lot of strength training or has naturally high muscle mass may weigh more for their height without necessarily having excess body fat.

The opposite can also happen. Someone with a healthy BMI may have a relatively high amount of body fat and lower muscle mass.

Looking at body composition alongside BMI can therefore give you more useful information about what your weight actually consists of.

Why does where you carry fat matter?

BMI doesn't tell you where fat is stored.

Fat around the abdomen, particularly fat stored deeper inside the body around the organs, is associated with a higher risk of conditions such as type 2 diabetes and cardiovascular disease.

This means two people with the same BMI can have different health risks depending on their body composition and where they carry fat.

One simple way to look at central fat is your waist-to-height ratio. The NHS recommends aiming to keep your waist measurement below half your height.

What is waist-to-height ratio?

Waist-to-height ratio compares your waist measurement with your height.

You calculate it by dividing your waist measurement by your height, using the same units for both. For example, if your waist is 80cm and your height is 170cm:

80 ÷ 170 = 0.47

A result below 0.5 is generally considered a useful guide for a healthy waist-to-height ratio. The NHS recommends keeping your waist measurement to less than half your height for adults who are able to use the measure.

It can be particularly useful alongside BMI because it provides information about where you're carrying weight.

Does BMI vary by ethnicity?

Yes. BMI does not give exactly the same indication of health risk for everyone.

Some ethnic groups have a higher risk of conditions such as type 2 diabetes and cardiovascular disease at lower BMI levels than the general population.

For this reason, the NHS uses lower BMI thresholds for overweight and obesity for some people from Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds.

This is an important reminder that BMI should not be interpreted in isolation. Your ethnic background, along with factors such as age, sex, family history and other health measurements, can affect how your result should be understood.

Does age or sex affect BMI?

BMI doesn't take your age, sex or body composition into account.

As we get older, it is normal to lose some muscle mass and experience changes in body fat. This means two people with the same BMI but different ages can have different body compositions.

Men and women can also have different patterns of body fat and muscle mass at the same BMI.

These differences are another reason why BMI is best viewed as one measurement rather than a complete picture of your health.

Is BMI useless?

No. BMI is still a useful screening tool. It's quick, simple and doesn't require specialist equipment. It can help identify people who may benefit from a closer look at their weight and associated health risks.

The problem is using BMI as though it tells you everything about an individual person's health.

A more complete assessment can look at BMI alongside measures such as waist-to-height ratio, body fat percentage, muscle mass and visceral fat.

What should you measure instead of BMI?

You don't necessarily need to replace BMI with another measure. Looking at several measures together can give you more useful context.

• Body fat percentage tells you what proportion of your total body weight is made up of fat. This can help put your BMI into context, particularly if you have a higher level of muscle mass.
• Muscle mass gives you an indication of how much of your body is made up of muscle. Looking at muscle alongside body fat can help provide a clearer picture than weight or BMI alone.
• Visceral fat is fat stored around your internal organs. Higher levels are associated with an increased risk of conditions including type 2 diabetes and cardiovascular disease. A body composition assessment can provide an estimate of visceral fat alongside other measurements.
• Waist-to-height ratio is a simple way to assess whether you are carrying more weight around your middle. The NHS recommends keeping your waist measurement below half your height.

Together, these measurements can provide more context than BMI alone.

What is a body composition scan?

A body composition scan looks at what your body is made up of rather than simply measuring your total weight.

Bluecrest's body composition scan provides measurements including body fat, muscle mass, visceral fat, bone mass and hydration. The scan uses a safe electrical signal to estimate the different components of your body and is included in Bluecrest's full body health check packages.

Because different measurements tell you different things, looking at them together can provide more context than BMI alone.

Go beyond BMI with Bluecrest Wellness

BMI can be a useful starting point, but it can't show how much of your weight comes from fat or muscle, or where that fat is stored.

A Bluecrest Body Composition Scan looks beyond BMI, providing measurements including body fat, muscle mass, visceral fat, bone mass and hydration. It's included as part of Bluecrest's full body health check packages.

Your results are presented in a personalised report, helping you understand your measurements and identify anything you may want to discuss with a healthcare professional.

Explore the Body Composition Scan.

Need help? Call free on 0800 652 2183 to speak to one of our team.

This article is for general health information purposes and does not constitute medical advice. If you have concerns about your BMI, weight or health assessment results, please speak with a healthcare professional.