The BMI formula — two versions
Body Mass Index is calculated by dividing your weight by the square of your height. There are two versions depending on your unit system:
Metric: BMI = weight in kg ÷ height in metres²
Example: A person who is 5'9" (69 inches) and weighs 175 lbs has a BMI of (175 × 703) ÷ 69² = 123,025 ÷ 4,761 = 25.8 — just into the "overweight" category.
The WHO BMI categories
- Under 18.5: Underweight — increased risk of nutritional deficiency and bone loss
- 18.5–24.9: Normal weight — associated with lowest health risk
- 25.0–29.9: Overweight — moderately increased risk of metabolic disease
- 30.0–34.9: Obese Class I — substantially increased risk
- 35.0–39.9: Obese Class II — severely increased risk
- 40+: Obese Class III (morbidly obese) — very severely increased risk
BMI thresholds differ for Asian populations
The WHO recognises that people of East and South Asian descent tend to develop metabolic complications at lower BMI values. The World Health Organization recommends lower cut-off points for Asian populations:
- Overweight threshold: 23.0 instead of 25.0
- Obese threshold: 27.5 instead of 30.0
Many health authorities in Singapore, Hong Kong, Japan, China, and South Korea apply these modified thresholds in clinical practice.
What BMI does not measure
BMI is a population screening tool, not a clinical diagnostic measure. It is blind to several factors that matter enormously for actual health:
- Muscle mass: A highly muscular person can have a BMI of 28 and be extremely healthy. This is why athletes and bodybuilders routinely fall into the "overweight" category.
- Fat distribution: Visceral fat (fat around organs) is far more dangerous than subcutaneous fat (fat under skin). Two people with identical BMIs can have very different health risks based on where fat is stored.
- Bone density: Heavier bones can push BMI into the overweight range without any excess fat.
- Age and sex: For the same BMI, older adults and women typically carry more body fat than younger adults and men.
Better measures to use alongside BMI
Waist circumference: Waist above 88cm (35in) for women or 102cm (40in) for men signals elevated metabolic risk regardless of BMI.
Waist-to-height ratio: Divide your waist in cm by your height in cm. Values above 0.5 indicate excess central adiposity in most populations.
Body fat percentage: Measured via DEXA scan, BIA scales, or skinfold calipers. Healthy ranges: women 21–31%, men 14–24%.
What to do if your BMI is above 25
A BMI above 25 is a signal to investigate further — not a verdict. For most people, bringing BMI into the healthy range involves two evidence-based interventions: a moderate calorie deficit (250–500 calories below TDEE) and regular physical activity (150+ minutes of moderate-intensity exercise weekly). The NHS, CDC, and AHA all recommend focusing on sustainable lifestyle changes rather than crash diets for long-term results.
If your BMI is above 35, speak with your GP or primary care physician before starting an intense weight loss programme. At BMI 40+, bariatric surgery may be discussed as a medically appropriate option in some cases.
BMI during pregnancy and for children
Pregnancy: BMI is not used to track weight during pregnancy. Use the pregnancy weight gain calculator, which is based on IOM guidelines accounting for your pre-pregnancy BMI.
Children: BMI for children (ages 2–19) uses age-and-sex-specific percentile charts, not the adult cut-off values. A child's BMI should only be interpreted by a paediatrician using the appropriate growth chart.