What is BMI and What Does Your Score Actually Mean?

BMI — Body Mass Index — is one of the most widely used health screening tools in the world. Doctors, insurers, and health organisations use it daily, yet many people don't fully understand what their number actually means, how it's calculated, or where its limitations lie. This guide covers all of it.

What is BMI?

Body Mass Index is a numerical value calculated from a person's height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and was originally designed as a statistical tool for studying populations — not as a clinical measure for individuals. Despite this origin, it became the standard screening tool used by healthcare providers worldwide.

BMI gives a rough estimate of body fatness by comparing weight to height. It's not a direct measurement of body fat, but it correlates well enough with more precise measurements (like DEXA scans) to be useful as a quick screening tool.

How is BMI calculated?

There are two versions of the formula depending on whether you're using metric or imperial measurements:

BMI categories

The World Health Organisation defines four standard BMI categories for adults:

BMI range	Category	General health implication
Below 18.5	Underweight	May indicate nutritional deficiency or underlying health issue
18.5 – 24.9	Normal weight	Associated with lowest health risk for most people
25.0 – 29.9	Overweight	Increased risk of some health conditions
30.0 and above	Obese	Significantly increased risk of cardiovascular disease, diabetes, and other conditions

These ranges apply to adults aged 18 and over. Children and teenagers use different age- and sex-specific BMI charts because their body composition changes significantly as they grow.

What your BMI does and doesn't tell you

BMI is a useful starting point but it has real limitations that are worth understanding.

What BMI tells you

• A rough estimate of whether your weight is in a healthy range for your height
• A simple risk screening tool — BMI outside the normal range is associated with higher health risks at a population level
• A consistent, comparable number that's easy to track over time

What BMI doesn't tell you

• Body composition — BMI cannot distinguish between fat and muscle. A very muscular athlete may have a high BMI without excess body fat
• Fat distribution — where fat is stored matters as much as how much. Belly fat (visceral fat) carries higher health risks than fat stored in the hips and thighs
• Bone density — heavier bones can push BMI higher without indicating excess fat
• Age-related changes — older adults tend to have more body fat at the same BMI as younger adults
• Sex differences — women typically have more body fat than men at the same BMI
• Ethnicity — research suggests that people of Asian descent face higher health risks at lower BMI values than the standard categories suggest

BMI and health risk

At a population level, BMI does correlate with health outcomes. People with BMIs in the overweight and obese ranges have statistically higher rates of type 2 diabetes, cardiovascular disease, high blood pressure, certain cancers, and joint problems. People who are underweight have higher risks of nutritional deficiencies, bone density loss, and immune system issues.

However, these are population-level statistics. An individual with a BMI of 27 may be perfectly healthy, while someone with a BMI of 22 could have underlying health issues that BMI cannot detect.

Better measurements to use alongside BMI

Healthcare professionals increasingly use BMI alongside other measurements for a more complete picture:

• Waist circumference — a waist measurement above 94cm (37in) for men or 80cm (31.5in) for women indicates increased cardiovascular risk
• Waist-to-hip ratio — compares waist size to hip size as a measure of fat distribution
• Body fat percentage — measured by DEXA scan, skinfold calipers, or bioelectrical impedance; gives a direct measure of fat vs lean mass
• Blood pressure, cholesterol, and blood sugar — these metabolic markers give direct evidence of health risk that BMI cannot

Tips for moving toward a healthier BMI

• Focus on sustainable lifestyle changes rather than rapid weight loss
• Combine regular physical activity with a balanced, nutritious diet
• Strength training builds muscle, which improves body composition even when BMI changes slowly
• Track trends over time rather than obsessing over a single number
• Consult a doctor or registered dietitian before making significant dietary changes