Understanding Your BMI: What the Numbers Mean (and Where They Fall Short)
16 May 2026 · 4 min read
Body Mass Index is the most widely used screening tool for weight-related health risk — you will encounter it at GP surgeries, occupational health checks, and insurance assessments worldwide. It is also one of the most routinely misunderstood numbers in medicine. This guide explains how to read your BMI correctly, and where the number genuinely falls short.
The formula
BMI is calculated as weight in kilograms divided by height in metres squared: BMI = kg ÷ m². Take someone who weighs 75 kg and stands 1.75 m tall: 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5. That single number places them squarely in the normal range. The formula's appeal is that it requires only two measurements, no equipment beyond a scale and a tape measure, and produces a consistent result regardless of who performs it.
The four categories
The WHO classification divides BMI into four bands:
- Underweight — below 18.5. Associated with nutritional deficiency, immune compromise, and in severe cases, organ damage. Clinical assessment is recommended.
- Normal weight — 18.5 to 24.9. At a population level, this range correlates with the lowest risk of weight-related chronic disease.
- Overweight — 25 to 29.9. Elevated risk of type 2 diabetes, hypertension, and cardiovascular disease, particularly when combined with other risk factors.
- Obese — 30 and above. Significantly increased risk across multiple disease categories. Obesity itself is subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+) for clinical purposes.
These thresholds were established from population epidemiology — they describe risk distributions across large groups, not certainties for any individual.
BMI's biggest blind spot
The formula cannot distinguish between muscle and fat. A kilogram of lean muscle tissue and a kilogram of visceral fat weigh the same; BMI treats them identically. A professional rugby player at 90 kg and 1.80 m tall has a BMI of 27.8 — technically "overweight" — while likely having single-digit body fat and exceptional cardiovascular fitness. Their BMI classification is, for clinical purposes, meaningless.
The reverse problem is less obvious but more dangerous: normal-weight obesity. A person can sit within the normal BMI range (18.5–24.9) while carrying excessive visceral fat — the metabolically active fat that accumulates around internal organs and drives insulin resistance, inflammation, and cardiovascular risk. BMI will give them a clean score while their metabolic health is genuinely compromised.
Ethnicity and adjusted thresholds
The standard BMI thresholds were derived primarily from European population data. Research has since established that South Asian populations — including people of Indian, Pakistani, Bangladeshi, and Sri Lankan heritage — experience elevated rates of type 2 diabetes and cardiovascular disease at significantly lower BMI values. Many clinicians and health guidelines now apply an adjusted threshold of 23 as the boundary between normal and overweight for South Asian patients, and 27.5 for the obese category. If you have South Asian heritage, the standard categories may understate your risk.
A better companion: waist-to-height ratio
Waist-to-height ratio is increasingly recommended as a complement to BMI because it directly captures central adiposity — the fat distribution pattern most strongly linked to metabolic disease. The rule of thumb is simple: your waist circumference should be less than half your height. If your waist exceeds that threshold, your risk is elevated regardless of what your BMI says. Measure your waist at the midpoint between your lowest rib and your hip bone, after a normal exhale.
When to seek medical advice
BMI is a screening tool, not a diagnosis. That said, certain values warrant a conversation with a doctor: a BMI below 17.5 (associated with clinical underweight), above 35 (where comorbidity risk rises sharply), or a significant unexplained change in either direction over a short period. The number is a prompt to investigate, not a verdict to live with.
It is a useful starting point — a piece of information, not an identity.