Understanding your BMI result
The World Health Organization defines the following BMI classification for adults aged 20 and over. It applies to both men and women.
| BMI (kg/m²) | WHO classification |
|---|---|
| Below 16.0 | Severe thinness |
| 16.0 – 16.9 | Moderate thinness |
| 17.0 – 18.4 | Underweight (mild thinness) |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight (pre-obesity) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
- NHS ethnicity adjustment: for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, overweight is defined from BMI 23 and the obesity action threshold from BMI 27.5.
- BMI categories for children and teenagers use age- and sex-specific percentile charts (for example the WHO or CDC growth references), not the adult thresholds above.
- In adults over 65, some research (summarized by Mayo Clinic and others) suggests a BMI slightly above the normal range is not necessarily associated with higher mortality; interpretation should be individualized by a clinician.
What is BMI?
Body Mass Index (BMI) is a person's weight in kilograms divided by the square of their height in metres (kg/m²). It was developed as a simple population-level screening indicator of weight status and is used worldwide by health agencies including the World Health Organization (WHO) and the UK National Health Service (NHS).
BMI is a screening tool, not a diagnostic measure. It does not distinguish between fat mass and muscle mass, and it does not measure where fat is stored on the body. Clinicians commonly interpret BMI alongside waist circumference, waist-to-hip ratio, and other assessments. Athletes with high muscle mass may have a high BMI without excess body fat, and older adults may have a BMI in the normal range while carrying excess fat.
Research summarized by the NHS shows that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds face higher cardiometabolic risk at lower BMI values. For these groups, the overweight threshold is commonly lowered from 25 to 23, and the obesity action threshold from 30 to 27.5.
How to use this BMI calculator
- Enter your weight. Use the Metric/Imperial toggle to switch between kilograms and pounds.
- Enter your height in centimetres, or in feet and inches in imperial mode.
- If you have a South Asian, Chinese or other Asian background, select that option to apply the NHS-adjusted thresholds.
- Read your BMI, its WHO classification, and the healthy weight range for your height — results update instantly.
The formula behind BMI
BMI divides body weight by height squared. The same formula underlies both unit systems; the imperial form multiplies by 703 to convert pounds and inches to metric equivalents.
Common mistakes
- Applying adult BMI thresholds to children or teenagers, who need age- and sex-specific percentile charts instead of fixed cut-offs.
- Treating BMI as a direct measure of body fat — it does not distinguish fat mass from muscle mass.
- Overlooking the NHS ethnicity-adjusted thresholds for South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, which lower the overweight and obesity cut-offs.
- Relying on BMI alone for athletes or very muscular individuals, where it commonly overestimates health risk.
- Assuming BMI reflects fat distribution — two people with the same BMI can carry very different amounts of abdominal (visceral) fat.
Frequently asked questions
What is a healthy BMI?
For most adults, a healthy BMI is 18.5 to 24.9 kg/m² according to the World Health Organization. For adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, the NHS advises that health risks increase from BMI 23, so a healthy BMI for these groups is 18.5 to 22.9.
Is BMI accurate for athletes?
BMI does not distinguish muscle from fat, so muscular athletes are often classified as overweight or obese despite low body fat. For athletic builds, measurements such as body-fat percentage, waist circumference or waist-to-hip ratio give a more meaningful picture.
How is BMI calculated?
BMI equals weight in kilograms divided by height in metres squared (kg/m²). In imperial units, BMI equals 703 times weight in pounds divided by height in inches squared. A 70 kg adult who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
Why do BMI thresholds differ by ethnicity?
Studies cited by the NHS and WHO show that at the same BMI, people of Asian backgrounds tend to carry more body fat and face higher risk of type 2 diabetes and cardiovascular disease. The commonly used adjusted thresholds are 23 for overweight and 27.5 for obesity-level risk.
Does BMI apply to children?
Not directly. For children and teenagers aged 2–19, BMI is calculated the same way but interpreted using age- and sex-specific percentiles from growth references such as the WHO or CDC charts, because healthy body composition changes with age and differs between boys and girls.
What are the limitations of BMI?
BMI does not measure body-fat percentage, fat distribution, muscle mass, bone density or metabolic health. Two people with identical BMI can have very different health profiles. Health agencies recommend using BMI as one screening indicator alongside waist measurements, lifestyle factors and clinical assessment.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894 (2000) — BMI classification.
- NHS. Body mass index (BMI) — including adjusted thresholds for Asian, Middle Eastern, Black African and African-Caribbean adults. nhs.uk.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. The Lancet 2004; 363(9403): 157–163.
- Centers for Disease Control and Prevention (CDC). About Adult BMI and BMI percentile charts for children and teens. cdc.gov.
- Winter JE et al. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr 2014; 99(4): 875–890.