Body Mass Index (BMI)
Calculates BMI from weight and height. WHO obesity classification.
How to use & interpret
BMI (weight in kg ÷ height in m²) is a quick population-level screen for underweight and excess weight. Standard adult bands: <18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, ≥30 obese (with obesity sub-classes I–III).
In the UK, NICE advises lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family origin (e.g. overweight from 23 and obese from 27.5), because cardiometabolic risk rises at a lower BMI. BMI does not distinguish fat from muscle or describe fat distribution, so pair it with waist circumference (or waist-to-height ratio) and clinical assessment.
Score interpretation
BMI < 18.5 kg/m²: Underweight.
→ Nutritional assessment. Exclude malnutrition, malignancy, eating disorder.
BMI 18.5–24.9 kg/m²: Normal healthy weight.
→ Maintain healthy weight with balanced diet and regular exercise.
BMI 25–29.9 kg/m²: Overweight.
→ Lifestyle modification: calorie reduction + 150 min exercise/week. Target 5–10% weight loss.
BMI 30–34.9 kg/m²: Obese Class I.
→ Weight management referral. Screen for comorbidities. Consider pharmacotherapy (semaglutide/orlistat).
BMI 35–39.9 kg/m²: Obese Class II.
→ Specialist obesity service. Consider GLP-1 agonist. Bariatric surgery if comorbidities.
BMI ≥40 kg/m²: Morbid obesity.
→ Bariatric surgery assessment. Document high anaesthetic risk.
Interpretation bands for the BMI. Apply clinical judgement and local guidance.
Frequently asked questions
Is BMI valid for athletes or older adults?
Less so. Very muscular people can have a high BMI with low body fat, and in older adults muscle loss can mask excess fat. Use waist measures and clinical judgement alongside BMI.
References
- WHO. Physical status: the use and interpretation of anthropometry. 1995.
Related
Curated clinical cross-links plus same-class fallbacks.
- Sepsis Screening and Sepsis Six · UK Sepsis Trust; NICE NG51; Surviving Sepsis Campaign 2021
- Unintentional Weight Loss Workup · NICE NG12; BSG
- Chronic Fatigue Workup · NICE NG206; BMJ Best Practice
- Lymphadenopathy Workup · NICE NG12; BMJ Best Practice
- Pre-op Medical Clearance · NICE NG45; ESC 2022
- Secondary Hypertension Workup · NICE NG136; ESH 2023
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.