| Category | BMI Range | Health Implications |
|---|---|---|
| Underweight | Below 18.5 | May indicate malnutrition, bone density concerns, or other health issues |
| Normal weight | 18.5 – 24.9 | Associated with lowest risk of weight-related health conditions |
| Overweight | 25 – 29.9 | Increased risk of cardiovascular disease, type 2 diabetes, hypertension |
| Obese (Class I) | 30 – 34.9 | High risk; often linked to metabolic syndrome and sleep apnoea |
| Obese (Class II) | 35 – 39.9 | Very high risk; significant impact on quality of life and longevity |
| Obese (Class III) | 40 and above | Severely increased risk across multiple organ systems |
How It's Calculated
BMI is weight (kg) divided by height squared (m²). In imperial units it's the same formula with a conversion factor of 703 applied to lbs and inches.
History
Devised by Belgian mathematician Adolphe Quetelet in the 1830s, BMI was later adopted by health organisations as a simple population-level screening metric.
Asian BMI Thresholds
For people of South and East Asian descent, the WHO recommends lower cut-off points — "overweight" at 23 and "obese" at 27.5 — due to differing body composition patterns.
Children & Teens
For those under 18, BMI is interpreted against age- and sex-specific growth charts (BMI-for-age percentiles) rather than fixed adult cut-offs.
⚠️ Limitations of BMI
- Does not distinguish between fat mass and muscle mass — athletes often score as overweight
- Ignores fat distribution; visceral (abdominal) fat is more dangerous than subcutaneous fat
- Not validated equally across all ethnic backgrounds
- Does not account for age-related changes in body composition
- Provides no information about fitness level, cardiovascular health, or metabolic markers
- Should always be used alongside waist circumference, blood tests, and clinical assessment
Move Daily
Aim for 150 minutes of moderate aerobic activity per week — brisk walking counts.
Eat Whole Foods
Prioritise vegetables, legumes, lean protein, and whole grains over ultra-processed options.
Sleep 7–9 Hours
Poor sleep disrupts hunger hormones (ghrelin & leptin), increasing appetite and cravings.
Stay Hydrated
Drinking adequate water supports metabolism and can reduce unnecessary snacking.
Manage Stress
Chronic stress elevates cortisol, which promotes fat storage — especially around the abdomen.
See a Professional
Your GP or a dietitian can provide personalised guidance that goes well beyond BMI alone.