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BMI Calculator UK
Calculate your Body Mass Index using the WHO 2000 classification — the standard used by NHS England. Supports metric (kg/cm) and imperial (lbs/ft) units.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is a simple screening measurement that compares your weight to your height to estimate whether you fall into an underweight, healthy, overweight, or obese category. The NHS and World Health Organisation use BMI as a first-line assessment to identify adults who may be at increased risk of type 2 diabetes, cardiovascular disease, and certain cancers. A BMI between 18.5 and 24.9 is considered healthy for most adults, though clinicians also factor in age, ethnicity, muscle mass, and waist circumference before making treatment decisions. This free calculator uses the same WHO 2000 classification system used by NHS England and accepts both metric and imperial measurements. Your data is processed entirely in your browser — nothing is stored or sent to any server.
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WHO BMI Classification
| BMI Range | Classification | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, immune suppression |
| 18.5 – 24.9 | Healthy Weight | Lowest risk in this range |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese (Class I) | High risk — T2 diabetes, cardiovascular disease |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Severe Obesity (Class III) | Extremely high risk — morbid obesity |
Source: WHO (2000). Obesity: Preventing and Managing the Global Epidemic. Used by NHS England and NICE guidelines.
Important
BMI is a population-level screening tool. It does not diagnose obesity or any health condition. It does not account for muscle mass, ethnicity, age, or sex differences in body composition. South Asian, Chinese, and Black African populations may experience health risks at lower BMI thresholds. Always consult a healthcare professional for a full assessment. Not suitable for children, pregnant women, or people under 18. If your BMI is below 16 or above 40, speak to your GP as a matter of priority or contact NHS 111 for guidance.
Clinically reviewed by Tahir Hameed, Pharmacist Independent Prescriber (GPhC: 2210908) — Last reviewed: April 2026
Frequently Asked Questions
What is a healthy BMI for adults in the UK?
According to the NHS and WHO guidelines, a healthy BMI for adults is between 18.5 and 24.9. However, BMI thresholds may differ for certain ethnic groups — for example, South Asian populations may face increased health risks at a BMI of 23 or above.
Is BMI an accurate measure of health?
BMI is a useful screening tool but has limitations. It does not distinguish between muscle and fat mass, so athletes may show a high BMI despite being healthy. It also doesn't account for fat distribution — waist circumference is often a better indicator of visceral fat risk. BMI is best used alongside other health markers.
Can I use this BMI calculator if I'm under 18?
No. Adult BMI categories do not apply to children and teenagers, as their body composition changes with age and growth. The NHS uses BMI percentile charts (centile charts) for under-18s, which account for age and sex. Speak to your GP or school nurse for a child BMI assessment.
What should I do if my BMI is over 30?
A BMI of 30 or above is classified as obese by the WHO. NICE recommends discussing weight management options with your GP, which may include dietary advice, structured exercise programmes, and in some cases, prescription weight loss medication. Even a 5–10% reduction in body weight can significantly lower health risks.
Does this tool store my data?
No. All calculations are performed entirely in your browser using JavaScript. No personal data — including your height, weight, or BMI result — is sent to any server, stored in any database, or shared with any third party. Your privacy is fully protected.
