General
Body Mass Index (BMI)
Weight-for-height screening measure.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
Body mass index (BMI) is a screening measure of weight relative to height, calculated as weight in kilograms divided by height in metres squared (kg/m²). It provides a quick, standardised way to categorise adults as underweight, healthy weight, overweight or living with obesity. It is widely used in primary care and public health because it requires only two simple measurements. BMI does not, however, distinguish muscle from fat or describe fat distribution.
Interpreting the result
In adults, a BMI below 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight and 30 or above indicates obesity, with further sub-categories at higher values. These thresholds are population screening cut-offs and should be interpreted alongside other factors. For some South Asian and other higher-risk ethnic groups, lower thresholds for overweight and obesity are recommended because cardiometabolic risk rises at a lower BMI.
Worked example
A person weighing 80 kg and 1.75 m tall has a BMI of 80 / (1.75 × 1.75) ≈ 26.1 kg/m². This falls in the overweight category and would prompt a broader assessment of cardiometabolic risk and lifestyle.
Pearls / pitfalls
- BMI does not distinguish muscle from fat, so it can misclassify very muscular individuals as overweight.
- It does not capture fat distribution; waist circumference adds information about central adiposity and risk.
- Lower thresholds apply for some South Asian and other ethnic groups, who face higher cardiometabolic risk at a given BMI.
- Standard adult cut-offs do not apply to children, who require age- and sex-specific centile charts.
Evidence & validation
BMI categories follow the World Health Organization classification for adults and are used by NICE, which also recommends lower thresholds for some ethnic groups in obesity assessment.
Frequently asked questions
What is a healthy BMI?
For most adults a BMI of 18.5–24.9 kg/m² is considered healthy. Below 18.5 is underweight, 25–29.9 is overweight and 30 or above indicates obesity.
Is BMI accurate for everyone?
No. BMI does not distinguish muscle from fat or describe fat distribution, so it can mislead in very muscular people and does not apply to children. It is a screening tool, not a diagnosis.
Why are thresholds lower for some ethnic groups?
People of South Asian and some other backgrounds develop cardiometabolic risk at a lower BMI, so lower overweight and obesity thresholds are recommended for them.
Should waist measurement be used as well?
Often yes. Waist circumference reflects central fat, which carries particular cardiometabolic risk, and adds useful information that BMI alone does not provide.
Can BMI be used in children?
Not with adult cut-offs. Children and young people require age- and sex-specific BMI centile charts rather than the fixed adult categories.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. 2000.
- NICE CG189: Obesity: identification, assessment and management. 2014 (updated 2023).
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