Understanding BMI: What It Tells You — and What It Does Not

Body mass index — BMI — is one of the most commonly used tools in weight management. It is calculated by dividing your weight in kilograms by the square of your height in metres. A BMI of 18.5 to 24.9 is considered healthy; 25 to 29.9 is overweight; 30 and above is obese.

Why is BMI so widely used?

BMI is quick, free, and requires no equipment beyond a scale and a tape measure. At a population level, it correlates reasonably well with health risk. For these reasons, it has become a standard screening tool in NHS and private healthcare settings.

Where does BMI fall short?

BMI does not distinguish between fat and muscle. A highly muscular person may have a BMI in the overweight range despite having very little body fat. Conversely, someone with a normal BMI may carry a high proportion of visceral fat — the metabolically active fat stored around the organs — which carries significant health risk.

BMI also does not account for where fat is distributed. Abdominal fat is more strongly linked to cardiovascular disease, type 2 diabetes, and metabolic syndrome than fat stored elsewhere. Two people with identical BMIs can have very different health profiles.

Additionally, BMI thresholds were developed largely from studies of white European populations. Research suggests that people of South Asian, East Asian, and Black African heritage may face increased health risks at lower BMI values.

What do doctors use alongside BMI?

• Waist circumference — a waist measurement above 88 cm in women or 102 cm in men indicates increased cardiovascular risk
• Waist-to-height ratio — keeping your waist circumference to less than half your height is a useful rule of thumb
• Blood tests — fasting glucose, HbA1c, lipid profile, and liver function tests give a fuller picture of metabolic health
• Blood pressure — hypertension is closely linked to excess weight and visceral fat

What does this mean for you?

BMI is a useful starting point, but it is not the whole story. A doctor-led assessment looks beyond the number on the scale to understand your individual health risks and goals.

For a thorough, personalised assessment, speak to the Weight Loss Wales team.