Body Mass Index is calculated from two measurements — weight in kilograms divided by height in metres squared — and is used by healthcare systems worldwide as a quick screen for weight-related health risk. Understanding what the number means, and where its limits lie, helps you use it sensibly rather than over-interpret it.
The BMI categories
- **Below 18.5:** Underweight
- **18.5–24.9:** Healthy weight
- **25.0–29.9:** Overweight
- **30.0–34.9:** Obese class I
- **35.0–39.9:** Obese class II
- **40.0 and above:** Obese class III (severe)
What BMI predicts — and doesn't
BMI predicts population-level risk well: across large groups, higher BMI is consistently associated with greater risk of type 2 diabetes, cardiovascular disease, certain cancers and joint disease. This population-level correlation is why it remains so widely used — it's cheap, quick and reproducible.
At the individual level, however, BMI is a rough tool. It doesn't distinguish between fat and muscle — a heavily muscled athlete can have a BMI in the overweight or even obese range with healthy body fat percentage. It also doesn't indicate where fat is distributed, which matters enormously: visceral fat (around the abdominal organs) carries far greater metabolic risk than subcutaneous fat (under the skin).
Why ethnicity matters
BMI cut-offs were developed primarily from European and North American populations. People of South, East and South-East Asian descent tend to have higher body fat percentage and greater metabolic risk at lower BMI values. Major health organisations now recommend lower cut-off points for these populations:
- **Overweight for South Asian, East Asian and South-East Asian adults:** BMI above 23.0
- **Obese:** BMI above 27.5
Better measures to use alongside BMI
Waist circumference and waist-to-height ratio are stronger predictors of metabolic disease risk than BMI alone, because they capture abdominal fat directly.
- **Waist circumference:** above 88 cm (35 in) for women and above 102 cm (40 in) for men signals elevated metabolic risk
- **Waist-to-height ratio:** keeping waist circumference below half your height is a simple rule of thumb supported by evidence across ethnic groups
- **Body fat percentage:** measured by DEXA, bioelectrical impedance or skinfold callipers — gives a direct fat mass reading that BMI can't provide
BMI and children
For children and teenagers, BMI is plotted on age- and sex-specific growth charts rather than against fixed thresholds. A child's BMI percentile is interpreted in the context of their growth trajectory, not as an absolute number.
Using BMI sensibly
BMI is a useful starting point — not a verdict. A BMI in the overweight range combined with normal waist circumference, healthy blood pressure, good lipid panel results and no metabolic markers may be entirely benign. Conversely, a 'healthy' BMI with high waist circumference and a rising HbA1c is a clearer risk signal.
The most useful thing you can do with your BMI is track it over time alongside other health metrics — weight trends, waist circumference, blood pressure and lab results. That longitudinal picture, rather than any single number, is what guides meaningful health conversations.