BMI Breakdown: A Simple Guide to Understanding Your Body Mass Index

Your body mass index (BMI) is a useful tool for assessing if your weight is in a healthy range. But how exactly does it work, and what are its limitations? This article breaks down the key things to know about BMI.

Calculating Your BMI

Calculating your BMI is actually quite simple. It involves dividing your weight in kilograms by your height in metres squared. So you’ll need to know both your current weight and height to determine your BMI.

BMI = weight (kg) ÷ height (m)²

Many online BMI calculators can do this maths for you quickly. Just plug in your weight and height, and it will calculate your BMI value. Children’s BMI uses the same basic calculation, but the result is interpreted using age- and sex-specific centiles because their bodies are still developing.

You can calculate your BMI using the NHS BMI calculator. Separate results and guidance are provided for adults and children.

BMI Ranges

Once you’ve calculated your BMI, you can compare it against standard adult BMI ranges to see if you fall into the healthy, underweight, overweight or obese categories:

• Below 18.5 – Underweight
• 18.5 to 24.9 – Healthy weight
• 25 to 29.9 – Overweight
• 30 and over – Obesity

However, factors like age and ethnicity can affect how these ranges should be interpreted. A healthcare professional can interpret your BMI in the context of your overall health.

For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, NICE recommends lower adult BMI thresholds because health risks can occur at a lower BMI. For these groups, a BMI of 23 to 27.4 is classified as overweight, and 27.5 or above as obesity.

These adult ranges should not be applied to children. Children’s BMI is assessed using age- and sex-specific centiles. See the NHS BMI calculator for children and teenagers.

Health Risks of High BMI

Having a high BMI can be associated with an increased risk of conditions like type 2 diabetes, heart disease, stroke, arthritis and breathing problems. Carrying excess fat specifically around the abdomen is also linked to health problems, even if BMI is technically in the healthy range.

So BMI alone does not give a complete picture of health. But it can serve as a red flag to prompt further evaluation and lifestyle changes to minimise disease risk if needed.

Limitations of BMI

While BMI is a convenient measurement, it does have some limitations to be aware of:

• It does not distinguish between body fat and muscle. People with more lean muscle mass, like athletes, may register as overweight by BMI even with low body fat.
• It does not fully account for age, sex and ethnic differences that influence how BMI relates to health risks.
• It fails to consider where body fat is distributed. Abdominal fat is generally associated with greater health risks than fat stored elsewhere.

BMI should also be interpreted with particular care in adults aged 65 and over, taking account of existing health conditions and muscle mass. Standard adult BMI classifications are not intended for assessing weight changes during pregnancy.

Supplemental Measurements

Because of these limitations, healthcare providers may recommend additional measurements to assess weight status for certain individuals:

• Waist circumference – Measure your waist midway between the bottom of your ribs and the top of your hips, usually just above your belly button. Waist circumference helps assess abdominal fat, which BMI alone cannot identify.
• Waist-to-height ratio – Divide your waist measurement by your height, using the same units for both. A ratio of 0.5 to 0.59 indicates increased health risk, while 0.6 or above indicates further increased risk. A useful general aim is to keep your waist measurement to less than half your height.

For example, someone who is 170 cm tall would generally aim for a waist measurement below 85 cm.

NICE recommends using waist-to-height ratio alongside BMI when assessing central adiposity in adults with a BMI below 35. This measurement is not suitable for everyone, including during pregnancy, and should not be used on its own to diagnose a health condition.

You can check your result using the NHS waist-to-height ratio calculator.

The Bottom Line

While BMI has some shortcomings, it still provides a useful initial benchmark for the health of your weight, especially when used as part of an overall health assessment. Tracking BMI over time can also help monitor weight changes. Just remember that it does not give the whole picture. Further evaluation may be needed based on your individual health profile and risk factors.

For more detailed information about how BMI and waist measurements should be assessed, see NICE guideline NG246: Identifying and Assessing Overweight, Obesity and Central Adiposity.

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